4. Forecast Applications
Table of Contents
- Introduction
- Boundary Layer
- Convection
- Air Mass Thunderstorms
- Severe Thunderstorms
- Convective Wind Gusts
- Microbursts: Wet vs. Dry
- Hail
- Forecasting Hail Occurrence
- Forecast Exercise: Qualitative Skew-T Assessment
- Hail/No Hail Nomogram
- Forecast Exercise: Hail/No Hail Nomogram
- Forecast Exercise: Hail/No Hail Nomogram 2
- Fawbush-Miller Method for Determining Maximum Hail Size
- Forecast Exercise: Hail Size - 1
- Forecast Exercise: Hail Size - 2
- Forecast Exercise: Hail Size - 3
- Summary
- Heavy Precip./Flash Floods
- Forecasting Heavy Precipitation and Flash Floods
- Forecasting Flash Floods
- Atmospheric Conditions Leading to Heavy Precipitation
- Example 1: Central U.S.
- Example 2: Mid-Atlantic States (east of the Appalachian Mountains)
- Example 3: Front Range of the Rocky Mountains
- Example 4: Tropical Islands During the Cool Season
- Forecast Exercise
- Winter Weather
- Precipitation Type
- Introduction and Relevant Parameters
- 0000 UTC Day 1
- 0000 UTC Day 1 (Hour 0)
- 1200 UTC Day 1 (Hour 12)
- 12-hour Forecast valid 0000 UTC Day 2 (Hour 24)
- Observations: 0000 UTC Day 2 (Hour 24)
- 12-hour Forecast valid 1200 UTC Day 2 (Hour 36)
- Observations: 1200 UTC Day 2 (Hour 36)
- 12-hour Forecast valid 0000 UTC Day 3 (Hour 48)
- Observations: 0000 UTC Day 3 (Hour 48)
- Summary
- Precipitation Type
- Turbulence
- Icing
Introduction

There are a wide range of forecasting problems that lend themselves to the use of the plotted skew-T diagram and associated atmospheric parameters in real-world applications. These include, but are not limited to:
- assessment of convective potential and severity
- forecasting wind shear, turbulence, and icing hazards to aircraft
- forecasting temperatures and precipitation type
This section builds upon the concepts of basic skew-T interpretation, evaluation of parameters, and stability assessment by applying the Interactive Skew-T to several forecast problems dealing with a variety of phenomena.
Boundary Layer
Boundary Layer Maximum Temperature
Boundary Layer » Maximum Temperature Forecasting Maximum Temperature, U.S. Air Force Method
There are several ways to forecast maximum temperature from a single sounding. The U.S. Air Force developed a simple method to make such forecasts, depending on sky conditions. The method works for locations fairly near sea level, away from water, and not expecting an air mass change. We examine this technique below.
If no inversion is present between 4,000 and 6,000 feet (1200-1825 m) above the surface:
- From the morning sounding (1200 UTC in North America), find the temperature at 850 hPa (T850).
- Clear/scattered clouds: Proceed from T850 dry adiabatically to the surface to get TMAX.
- Broken clouds/overcast: Proceed from T850 moist adiabatically to the surface to get TMAX.
If an inversion is present between 4,000 and 6,000 feet (1200-1825 m) above the surface:
- Find the temperature (T) at the warmest point (top) of the inversion.
- From that point, proceed dry adiabatically to the surface.
- The resulting temperature at the surface is TMAX.
Boundary Layer » Maximum Temperature Question

The 1200 UTC sounding above is at an inland location over the southeastern U.S. in early October. Winds aloft are very light, and the location is underneath a large scale ridge aloft. Scattered clouds are being reported in the surface observation. Using the plotted sounding, predict the afternoon maximum temperature.
Enter your answer in the text box below, then click Done.
Boundary Layer » Maximum Temperature Geographic Limitations
Note that this simple, straightforward method does have its limitations and therefore must be applied with care.
Generally the technique will not work at locations near a large body of water.
For higher elevations such as the High Plains of North America or mountain areas, the 850 hPa reference level may be at or below the surface elevation. In these cases, 700 hPa or some other level may be more appropriate, depending on local elevation. It is best to consult or develop a local climatology of soundings in order to determine the appropriate reference level for a given season or station elevation.
Boundary Layer » Maximum Temperature Short-term Variations
Over wet soil, surface heating may be limited.
This method is not applicable when an air mass change is anticipated.
If the sounding will be modified by some dynamic topographic influence (for example, a downslope wind) this method will not work very well.
Boundary Layer » Maximum Temperature Seasonal Variations
There are also seasonal variations that must be taken into account when using this method. At elevations that are within a few hundred feet (100 m) of sea level, 850 hPa is an appropriate reference level in the spring or fall.
In the summer, with higher sun angles and greater solar heating, the reference level may be as high as 700-750 hPa.
In winter, with relatively low sun angles and less solar heating, you may choose a reference level at or below 900 hPa.
Boundary Layer » Maximum Temperature Other Methods
Finally, there are many other potential methods for determining maximum temperature from a single sounding, but we won't go into them here. Often, by using your knowledge of the weather situation to step beyond these simple methods, you may be able come up with other ways to use a morning sounding to predict the maximum temperature. For instance, if afternoon convection is anticipated, the temperature is likely to level off at a maximum value as convective clouds begin to form. So one could easily see the utility of convective temperature in coming up with a maximum temperature forecast in this situation.
Convection
Convection Air Mass Thunderstorms
Convection » Air Mass Thunderstorms Introduction
Air mass thunderstorms refer to cumulonimbus clouds that develop from ordinary convection; that is, convection due to warming of the boundary layer. The air mass thunderstorm has three stages: the cumulus stage, the mature stage, and the dissipating stage.
- Cumulus stage: Updrafts prevail and one or more towering cumulus will form. Air is lifted to its level of free convection level of free convection and continues to rise. Precipitation may form in the upper portion of the cloud but is unlikely in the sub-cloud layer.
- Mature stage: The mature stage is characterized by updrafts and downdrafts and rainfall. Air parcels reach the equilibrium levelequilibrium level. Updrafts may extend through the depth of the troposphere. An anvil begins to form at the upper levels, and precipitation occurs below the cloud. Evaporative cooling at low-levels forms a cold pool and a gust front that spreads out from the cloud and leads to uplift of warm, moist, unstable air, creating new cumulus clouds.
- Dissipating stage: This stage is characterized by downdrafts. Convective rainfall diminishes while stratiform rainfall may continue from the anvil. The gust front advances away from the storm and inhibits air from being lifted into the storm.
Convection » Air Mass Thunderstorms Forecasting Air Mass Thunderstorms

Air mass thunderstorms develop when synoptic forcing is weak but the boundary layer is moist and relatively warm. Vertical wind shear is usually weak, and parcel lifting is due primarily to buoyant energy, which can be estimated from the convective available potential energy (CAPE)convective available potential energy (CAPE). The environmental lapse rate must be conditionally unstableconditionally unstable and the air must have sufficient moisture that rising air parcels will saturate and continue rising beyond their level of free convectionlevel of free convection. A capping inversion may be present during the early morning and can delay initiation until the boundary layer is warmed to its convective temperatureconvective temperature. Recall that the convective temperature is the surface temperature that must be reached to start the formation of convective clouds by surface heating.
In some instances, the environmental lapse rate may be more unstable and the vertical wind shear may be stronger. In these cases, the convection may no longer be of the ordinary air mass-type, and can develop, instead, into severe thunderstormssevere thunderstorms, accompanied by hail, strong winds and, in rare instances, tornadoes. Sounding parameters for assessing the potential for severe convection are discussed in the Severe Thunderstorms section.
Convection » Air Mass Thunderstorms Skew-T Parameters

Below are a few parameters that are helpful in diagnosing the potential for air mass thunderstorms.
- Convective Temperature
- Convective Temperature
- Instability:
- Buoyant Energy:
- Thunderstorm Potential and Moisture Content:
- Vertical Wind Shear:
Convection » Air Mass Thunderstorms Convective Forecast Exercise: Task 1
Convection » Air Mass Thunderstorms Convective Forecast Exercise: Task 2
Convection » Air Mass Thunderstorms Convective Forecast Exercise: Task 3
Convection Severe Thunderstorms
Convection » Severe Thunderstorms Introduction

Severe thunderstorms are thunderstorms that produce at least one of the following:
- In the U.S.:
- Damaging winds: 93 kph (50 kts, 58 mph) or greater
- Large hail: 2.5 cm (1 inch) diameter or greater
- A tornado
- In Canada:
- Damaging winds: 90 kph (48 kts, 56 mph) or greater
- Large hail: 2 cm (¾ inch) diameter or greater
- Heavy rain: 50 mm (2 in) or more per hour (Alberta to Southern Quebec), 25 mm (1 in) or more per hour (Pacific, Northern, and Maritime provinces)*
- A tornado
The environments that produce severe thunderstorms are marked by either high CAPE, strong shear, or both. The vertical distribution of the shear (e.g., shear depth, strength) influences the organization and evolution of severe thunderstorms. Examination of certain skew-T parameters is essential but not sufficient for forecasting severe thunderstorms. The skew-T parameters should always be used in combination with other information (surface and upper air analyses, NWP output, etc.) to make a full assessment of severe weather potential.
Skew-T Parameters and Convective Indices
Instability
- Convective available potential energy (CAPE)
- Convective available potential energy (CAPE)
- Convective inhibition (CIN)
- Convective inhibition (CIN)
- Lifted index (LI)
- Lifted index (LI)
- Showalter stability index (SSI)
- Showalter stability index (SSI)
- Total Totals index (TT)
- Total Totals index (TT)
Vertical Wind Shear
Vertical wind shear is a very important parameter to evaluate when assessing the potential for severe thunderstorms as well as forecasting the storm type, evolution, and movement. As the magnitude of the 0-6 km shear increases, the storm characteristics evolve from short-lived ordinary cells to multi-cell, with supercells favored for shears > 25 m/s.
In environments favorable to severe storm development, wind shear vectors commonly turn clockwise, depicting veering of the winds over the lowest couple of kilometers. For cases of strong shear magnitude, a clockwise-turning hodograph favors a more sustained, long-lived storm with a cyclonically rotating updraft.
For a complete description of how to use a hodograph in diagnosis of severe thunderstorms, see the module: Using Hodographs.
For more on wind shear and convective storms, see the module: Shear and Convective Storms .
Combinations of Instability and Wind Shear
Convection » Severe Thunderstorms Severe Weather Sounding Types - Type I

This sounding is characterized by a moist, fairly well-mixed layer of at least 100-150 hPa depth, separated from a dry layer above by a capping inversion. Lapse rates above the cap are typically nearly dry adiabatic. Often referred to as "loaded gun" soundings, they usually have surface Td>10°C and 850 hPa Td>8°C. Type I soundings usually exhibit high values for CAPE and TT, low values for LI and SSI, and are commonly observed in the Great Plains of the U.S. during the spring severe weather season.
Convection » Severe Thunderstorms Type II Sounding

This sounding type is common in the tropics but observed at times over much of the U.S. east of the Rocky Mountains, especially in the summertime over the Gulf Coast and Southeast. It is characterized by a deep, moist, conditionally unstable layer with relative humidities >60% from the surface up to 7 km AGL. There is no capping stable layer so that widespread convection is typically observed.
Convection » Severe Thunderstorms Type III Sounding

This sounding type is similar to Type 2, except 10-15°C cooler. It is commonly observed near cold-core, upper-level troughs and cyclones.
Convection » Severe Thunderstorms Type IV Sounding (Inverted-V)

This sounding type is characterized by a relatively dry, well-mixed lower layer, with RH increasing with height, giving the appearance of an "inverted V." It is commonly observed during the summer season over the High Plains of the U.S. and the mountain and plateau regions of the Western U.S. It is typically associated with high-based thunderstorms with vigorous, evaporatively driven downdrafts and microbursts.
Convection » Severe Thunderstorms Sounding Type Question

This sounding was taken 0000 UTC 25 April 2003 at Jackson, MS (JAN). Severe weather occurred within a few hours of this sounding.
How would you classify this sounding?
Convection » Severe Thunderstorms Forecast Exercise: Question 1
To answer this question and others that follow, you first need to open the Interactive Skew-T with this sounding.
Based on the 1200 UTC May 3 sounding for Norman, Oklahoma (OUN) (in the central U.S.), we will attempt to assess the potential for severe weather for the afternoon and evening period (around 0000 UTC).
Upon a general inspection of the sounding, what are the prospects of severe
weather in the vicinity of OUN later in the day?
(Choose the best answer.)
Convection » Severe Thunderstorms Forecast Exercise: Question 2
Convection » Severe Thunderstorms Forecast Exercise: Question 3
Convection » Severe Thunderstorms What Happened?

On 3 May 1999, a major tornado outbreak occurred in parts of Oklahoma and Kansas, with 10 tornadic supercells and a total of 74 tornadoes reported. At least 4 of the tornadoes were F4 or F5 intensity.
When we compare the afternoon sounding (0000 UTC 4 May) with the morning sounding (1200 UTC 3 May), we see that significant warming of up to 4°C from the surface to 600 hPa took place during the day. The vertical wind shear also increased. Also note some moistening of the boundary layer occurred. The 0000 UTC sounding was taken when a supercell thunderstorm producing an F5 tornado was located about 40 km west-southwest of the launch location.
Reference: May 3, 1999 Oklahoma/Kansas Tornado Outbreak
Convection Convective Wind Gusts
Convection » Convective Wind Gusts Introduction

There are a number of methods available by which to forecast convective wind gusts. Here we will focus our attention on two empirical methods that have been used for over 30 years by the U.S. Air Force Weather Agency (AFWA). In developing these methods, forecasters relied on evidence that most convective wind gusts at the surface appear to result from downdraft air originating in the lower portion of the thunderstorm. Both of the methods discussed here attempt to account for temperature differentials associated with evaporatively cooled, convective downdrafts to provide a realistic prediction of convective wind gusts. The forecast direction is determined by the mean layer wind direction 10,000-14,000 ft (3.0-4.3 km) above the surface.
Convection » Convective Wind Gusts AFWA T1 Method
The T1 Method for determining the speed of the maximum convective wind gust is as follows:
- Compute the dry stability index T1. Depending on the inversion characteristics, T1 will be
computed in one of two ways:
- With an inversion, the top of which is no more than 150-200 hPa above the surface, go up a moist adiabat from the surface to 600 hPa and note the temperature. Subtract the 600-hPa dry bulb temperature from this value to get T1.
- With no inversion or an inversion more than 200 hPa above the surface, compute maximum surface temperature, TMAX, by going dry adiabatically from the 850-hPa temperature to the surface. From this surface temperature, proceed moist adiabatically up to 600 hPa. Subtract the 600-hPa dry bulb temperature from this value to get T1.
- Compute the maximum wind gust V1, using the formula V1 = 13 * sqrt (T1). This will yield the speed of maximum wind gust in knots. (Note: Multiply this result by 1.15 to convert to mph, by 1.85 for kph, or by 0.51 for m/s)
Estimate the direction of the mean wind in the layer between 10,000 ft (3 km) and 14,000 ft (4.3 km) AGL. This will be the forecast wind direction of the computed gust.
Convection » Convective Wind Gusts Forecast Exercise: T1 Method

Using this morning sounding, use the T1 methodT1 method to forecast the maximum wind gust speed and direction associated with anticipated afternoon convection. You can open the Interactive Skew-T with this sounding.
Step 1: Determine T1.
Enter your answer in the text box below, then click Done.
Convection » Convective Wind Gusts AFWA T2 Method
Compared to the T1 Method, the T2 method more completely accounts for the effects of the convective downdraft temperature. The T2 Method for determining the speed of the maximum convective wind gust is as follows:
- Determine T1 and TMAX as outlined on the previous page.
- From the wet-bulb zero temperature, go down a moist adiabat to the surface. This temperature is subtracted from the TMAX to get T2.
- Determine the mean wind speed in knots in the lowest 5,000 feet (1500 m). Divide the mean wind speed by 3. This is V.
- Compute the maximum wind gust from this formula: V2 = V + 13 * sqrt [(T1 + T2) / 2]
As in the T1 method, estimate the direction of the mean wind in the layer between 10,000 and 14,000 ft (3-4.3 km) AGL. This will be the forecast wind direction of the computed gust.
Convection » Convective Wind Gusts Forecast Exercise: T2 Method

Compute maximum wind gust using the T2 methodT2 method.
- Recall from the previous page that TMAX = 23°C and T1 = 12°C.
- Open the Interactive Skew-T with this sounding.
- Open the table of pressure vs. height in a standard atmosphere
Step 1: Determine T2.
Enter your answer in the text box below, then click Done.
Convection » Convective Wind Gusts Forecast Exercise: Direction of the Maximum Wind Gust

Determine the wind direction of maximum wind gust.
It may help to open the table of pressure vs. height in a standard atmosphere.
Enter your answer in the text box below, then click Done.
Convection Microbursts: Wet vs. Dry
Convection » Microbursts: Wet vs. Dry Introduction

Microbursts are defined as severe downdrafts that do not exceed 4 km in horizontal extent. Aircraft that encounter a microburst may suddenly lose airspeed and experience an associated loss of lift, with potentially catastrophic results. Forecasters need to recognize some of the common sounding characteristics associated with microbursts, so that aviation interests can be alerted when their potential exists.
The downdrafts associated with microbursts are driven by negative buoyancy due to evaporative cooling. Once these strong downdrafts reach the surface, they spread horizontally, producing hazardous wind shear. Microbursts fall into two basic categories: those associated with precipitating thunderstorms, known as wet microbursts, and those observed in thunderstorms with little or no precipitation, known as dry microbursts.
Convection » Microbursts: Wet vs. Dry Wet Microbursts

Here we see a typical wet microburst sounding. You can open the Interactive Skew-T with this sounding.
Wet microburst soundings are characterized by moist, nearly saturated conditions at low levels and drier conditions aloft. These soundings often appear similar to the capital "Y" severe weather soundings, but generally have higher precipitable water values. The dry air aloft gets entrained into convective downdrafts and enhances evaporative cooling, which increases negative buoyancy and strengthens the downdrafts. These microbursts occur most frequently over the southeastern United States and are often accompanied by heavy precipitation.
Convection » Microbursts: Wet vs. Dry Dry Microbursts

Here we see a typical sounding associated with dry microbursts. You can open the Interactive Skew-T with this sounding.
The lower troposphere is relatively dry, with moist, sometimes saturated conditions at mid levels. Due to the distinct appearance of their temperature and dewpoint curves, we sometimes refer to dry microburst soundings as "inverted-V" soundings. The CCL is high and therefore the convection typically has a high base, with precipitation evaporating into the drier layer in the lower troposphere. The evaporative cooling in turn contributes to negative buoyancy fueling strong downdrafts. These downdrafts spread horizontally once they reach the surface. Often no precipitation actually reaches the surface. Only a ring of dust below the virga may signify the presence of a dry microburst.
Convection » Microbursts: Wet vs. Dry Forecasting Microburst Wind Gusts
Several indices have been developed to predict convective wind gusts. In the Convective Wind Gusts section, we demonstrated two methods from the U.S. Air Force. Here, we introduce WINDEX (McCann 1994), which was developed as a direct result of microburst research.
WINDEX (WI) is computed as follows:
where:
WI = maximum wind gust (knots)
HM = height of the melting level (km)
RQ = QL/12 but the value for RQ may not exceed 1
QL = mean mixing ratio in lowest 1 kilometer (g/km)
QM = mixing ratio at melting level (g/km)
= Lapse rate from melting level to surface (°C/km)
Convection » Microbursts: Wet vs. Dry Microburst Forecasting - Question 1

Here is a morning (1200 UTC) sounding on a day in which you expect afternoon convection. You can open the Interactive Skew-T with this sounding.
Determine the type of microburst activity and the associated wind gusts that might occur with afternoon convection.
Based upon the morning sounding, what type of convection might you expect?
(Choose the best answer.)
Convection » Microbursts: Wet vs. Dry Microburst Forecasting - Question 2

What type of microbursts would you expect?
(Select the best answer.)
Convection » Microbursts: Wet vs. Dry Microburst Forecasting - Question 3

To answer this question, you may wish to examine the following:
- WINDEX calculator
- Interactive Skew-T with this sounding
- Table of pressure vs. height in a standard atmosphere
Now, make a forecast of the maximum wind gusts associated with microburst activity.
First, determine the values of the variable in the WINDEX equation:
Enter your answers in the text boxes below, then click Done.
Convection Hail
Convection » Hail Forecasting Hail Occurrence

Hail occurrence is relatively rare with routine air-mass thunderstorms, but is more common with multicell and supercell thunderstorms. Several factors come into play when attempting to forecast hail occurrence. Important considerations that can be evaluated on the skew-T include the following:
- Equilibrium level (EL)
- Equilibrium level (EL)
- Freezing Level
- Freezing level
- Wet-bulb zero level
- Wet-bulb zero level
- Convective condensation level (CCL)
- Convective condensation level (CCL)
- CAPE
- CAPE
- Vertical wind shear (see module Using Hodographs.)
The higher the EL in a sounding, the greater the chance of hail, which tends to be favored in taller thunderstorms. Nearly two-thirds of all storms with radar echo tops over 50,000 feet (15.2 km) report hail at the surface.
An evaluation of the freezing and wet-bulb zero levels is also important. Lower zero levels favor hail because a greater depth of the thunderstorm has sub-freezing temperatures where hail production can occur. As a result, hail is more likely to occur in thunderstorms with freezing levels lower than 12,000 feet (3.7 km).
The higher the CAPE, the higher the updraft velocity and greater potential for more and larger hail.
Soundings exhibiting significant vertical wind shear indicate thunderstorms may become tilted, with updraft/downdraft separation that can promote enhanced intensity and longevity of updrafts.
Finally, elevation plays a key role. For instance, in North America hail occurs most frequently to the lee of the Rocky Mountains from Colorado and Wyoming in the United States northward to Alberta, Canada. In this region surface elevations are 4000-6000 feet (1.2-1.8 km) MSL so the climatological freezing level is closer to the surface.
Convection » Hail Forecast Exercise: Qualitative Skew-T Assessment

This skew-T diagram shows the morning sounding from a location in the Midwest of the U.S. in late spring. You can open the Interactive Skew-T with this sounding.
Evaluate this sounding to assess the potential for hail-producing thunderstorms later in the day.
As described on the previous page, to properly evaluate the sounding, we need to determine the following parameters:
- Equilibrium level (EL)
- Equilibrium level (EL)
- Freezing Level
- Freezing level
- Wet-bulb zero level
- Wet-bulb zero level
- Convective condensation level (CCL)
- Convective condensation level (CCL)
- CAPE
- CAPE
- Vertical wind shear (see module Using Hodographs)
Given the above sounding-derived parameters, write a short synopsis explaining the potential for hail later in the day. When you are satisfied with your response, click Expert Opinion for feedback.
Convection » Hail Hail/No Hail Nomogram
This figure plots the occurrence of hail versus no hail for 70 thunderstorm cases in the Midwest U.S. The horizontal position of each point shows the height of the freezing level. Hail is more likely to occur if the freezing level is relatively low. Thus, a position further to the right favors hail.

The vertical position of each point shows the cloud depth ratio, which is the ratio of the depth of the cloud above freezing to the depth of the cloud below freezing. A deep cloud layer that is below freezing will maximize the potential for hail formation and growth. Thus, a lower position favors hail.
Overall, positions in the lower-right corner indicate hail is likely, while positions in the upper-left indicate hail is unlikely.
The method outlined above considers the CCL (cloud base), the freezing level, and the equilibrium level (cloud top).
Procedure:
- Subtract the freezing level from the CCL. This represents the depth of the cloud from its base up to the freezing level. Call this a (hPa)
- Subtract the EL from the freezing level. This represents the depth of the cloud from the freezing level up to its top. Call this b (hPa)
- Determine the cloud depth ratio a/b.
- On the nomogram, find the cloud depth ratio on the vertical axis and the freezing level on the horizontal axis and plot the point. For points that plot above the diagonal line, hail is unlikely, and vice versa.
Convection » Hail Forecast Exercise: Hail/No Hail Nomogram

To answer the next question, you may want to open one or both of the following:
Consider the morning sounding examined previously. Determine the cloud depth ratio and the freezing level.
Convection » Hail Forecast Exercise: Hail/No Hail Nomogram 2

Using the freezing level and cloud depth ratio values previously determined, click on the Hail / No Hail Nomogram to determine whether hail is likely to occur later in the day.
Freezing level = 696 hPa
Cloud depth ratio = 0.44
Try again.
Plotting the freezing level / cloud depth ratio point (696 hPa, 0.44) on the nomogram, the point falls into the category where hail is likely, consistent with our earlier findings about hail potential.
Note that this diagram only applies to hailstorms over the Midwestern U.S., but could easily be modified using cases from other regions. Also note the considerable scatter in the data used to determine the hail / no hail line. One should not be quick to dismiss hail from your forecast based entirely on use to this nomogram, particularly for cases that plot close to the line that separates the hail and no hail fields.
Convection » Hail Fawbush-Miller Method for Determining Maximum Hail Size
Having now determined that hail is likely to be observed in thunderstorms later in the day, we proceed to try and estimate the maximum hail size. There are several empirically-derived, hail-size forecast algorithms; we will focus on one here. The Fawbush-Miller method (Miller 1972) is a tried-and-true method that has been used by U.S. Air Force forecasters since the 1950s.

Procedure:
- Identify the CCL and the isobar where the sounding temperature crosses the -5°C isotherm. We will refer to this isobar as -5I
- From the CCL, proceed up the moist adiabat to where it intersects the -5I. This is point A.
- From -5°C at the -5I level (Point B), proceed down a dry adiabat to the level of the CCL. This is point C.
- Take the temperature difference A-B. On the nomogram, this is the base.
- Take the temperature difference C-B. On the nomogram, this is the altitude.
- Apply the Fawbush-Miller nomogram 1 to determine the maximum hail size.
- Check to see if the wet-bulb zero height is above 10,500 feet (3.2 km or ~670 hPa). If so, modify the maximum hail size from nomogram 1 using Fawbush-Miller nomogram 2.
In this procedure, the temperature difference A-B serves as a substitute for lower cloud buoyancy and hence, vertical velocity. The difference C-B serves as a substitute for cloud depth below the -5°C level.
The second nomogram provides a modification to the maximum hail size forecast to account for higher wet-bulb zero levels that are typically observed with summertime thunderstorms over the southeast U.S.
Convection » Hail Forecast Exercise: Hail Size - 1

To answer the next question, you may want to open the Interactive Skew-T with this sounding.
Now let's apply the hail size nomogram.
Consider the morning sounding examined previously.
Determine the following quantities:
Enter the values for each property, then click Done.
Convection » Hail Forecast Exercise: Hail Size - 2

Using the base and altitude values determined on the previous page, click on the nomogram to determine the hail size that is likely to occur.
Base = A-B = 4
Altitude = C-B = 29
Try again.
Convection » Hail Forecast Exercise: Hail Size - 3

To answer the next question, you may want to open one or both of the following:
Convection » Hail Summary
We used a hypothetical Midwest U.S. morning sounding to assess the potential for hail in thunderstorms later in the day. Our initial analysis revealed that hail was a good possibility and this was confirmed by an empirically-derived forecast diagram. We also used the Fawbush-Miller method to forecast maximum hail size of about 1 inch (2.5 cm). Since this is equal to the threshold hail diameter (1 inch, 2.5 cm) for severe thunderstorms in the U.S., we would want to alert our aircraft operations of the potential for severe thunderstorms and damaging hail this afternoon.
Convection Heavy Precip./Flash Floods
Convection » Heavy Precip./Flash Floods Forecasting Heavy Precipitation and Flash Floods

Flash floods are the most deadly of thunderstorm hazards in the United States. While several definitions of flash floods exist, in this module a flash flood refers to heavy precipitation that causes rapid rise of water in creeks or small rivers and in which fast moving run-off overwhelms the drainage system.
Convection » Heavy Precip./Flash Floods Forecasting Flash Floods
Flash floods generally occur where the precipitation rate is heaviest for the longest period. Flash flood-producing systems include: strong multicellular storms, supercells, mesoscale convective systems, and tropical disturbances (waves, depressions, and cyclones).
Flash floods will occur when a rain-producing system becomes quasi-stationary over a drainage basin or when successive convective systems pass over the same drainage area. New convective development is influenced strongly by the outflow boundary produced from existing (and previous) cells. Regeneration or "training" of cells in the same location is associated with a slow-moving low-level boundary, a low-level jet, upper-level divergence, and weak vertical wind shear.
This radar animation shows a persistent heavy rain event over an area just east of the Rocky Mountains in Fort Collins, Colorado, that produced a large flash flood.
Convection » Heavy Precip./Flash Floods Atmospheric Conditions Leading to Heavy Precipitation
The rainfall rate at a particular point is proportional to the magnitude of the upward, vertical moisture flux. This means rising air should have substantial water vapor content, and the ascent rate should be large. The atmosphere must be conditionally unstable, and there must be a mechanism by which the warm, moist air will rise to its level of free convection.
Skew-T Parameters:
- Instability:
- Temperature profile:
- At or very close to moist adiabatic, especially in the lower half of the troposphere
- At or very close to moist adiabatic, especially in the lower half of the troposphere
- Low-level moisture and precipitation efficiency:
- High values of K index (usually greater than 35)
- High values of K index (usually greater than 35)
- Deep warm cloud layer (from LCL to melting level (freezing level) greater than 4 km)
- Deep warm cloud layer (from LCL to melting level (freezing level) greater than 4 km)
- Precipitable Water exceeding 150% of average (for instance, for most of the continental U.S., greater than 1.75 inches (4.5 cm) precipitable water represents a water-loaded sounding, while less than 0.75 inches (1.9 cm) represents a fairly dry sounding)
- Wind profileWind profile:
- Low-level jet: winds at 850 hPa exceeding 25 kt (29 mph, 46 kph, 13 m/s) and at 700 hPa exceeding 30 kt (35 mph, 56 kph, 15 m/s)
- Weak wind shear in the mid-troposphere (and sometimes light and variable winds)
Convection » Heavy Precip./Flash Floods Example 1: Central U.S.
This sounding from the central Plains of the U.S. (Norman, Oklahoma) in August of 2003 is typical of a sounding associated with heavy precipitation in the central U.S.

Skew-T Parameters:
- Instability:
- Low-level moisture and precipitation efficiency:
- Wind profileWind profile:
- Weak low-level jet: winds at 700 hPa = 22 kt
- Weak wind shear in the mid-troposphere
Convection » Heavy Precip./Flash Floods Example 2: Mid-Atlantic States (east of the Appalachian Mountains)
Flash floods can also occur where moist low-level air encounters elevated terrain. For example, the sounding below is from east-central Virgina in the eastern U.S. The Appalachian Mountains lie to the west, primarily oriented north-south. Moist, lower tropospheric easterly flow in this area is perpendicular to the Appalachian foothills and is forced upward by the higher elevations.

Skew-T Parameters:
- Instability:
- Low-level moisture and precipitation efficiency:
- Wind profileWind profile:
- Low-level jet: 850 hPa winds = 40 kt
- Weak wind shear between 600 and 500 hPa
Convection » Heavy Precip./Flash Floods Example 3: Front Range of the Rocky Mountains in the U.S.
Flash floods also occur where cells regenerate and track along the mountains. The skew-T parameters do not need to exceed the thresholds used for flash floods at lower elevation because orographic lifting becomes very important. When forecasting in areas with significant terrain, examine the lower-tropospheric wind direction relative to the terrain to determine if moisture convergence and lifting will be focused in a small area.
Some skew-T parameters may not be applicable in mountains because the pressure levels at which they are calculated are below ground. For some parameters, calculations can be made at different pressure levels. For example, K index can be calculated using the temperature at 800 hPa instead of 850 hPa.

Skew-T Parameters:
- Instability:
- Temperature profile:
- Close to moist adiabatic through the troposphere
- Close to moist adiabatic through the troposphere
- Low-level moisture and precipitation efficiency:
- Wind profileWind profile:
- Low-level jet: 600 - 500 hPa winds are > 20 kt, approximately 10 kt greater than the layers above and below
- Weak wind shear between 500 and 350 hPa
Convection » Heavy Precip./Flash Floods Example 4: Tropical Islands During the Cool Season
During the cool season or transition months, tropical islands can be affected by heavy rainfall and flash floods due to upper-level troughs, cold fronts, and surface pre-frontal troughs.
In these areas, the skew-T parameters do not need to exceed the thresholds used for flash floods in the mid-latitudes because of the abundant moisture and increased probability of warm rain processes.

When forecasting in the tropics, examine the upper-level synoptic environment and lower-tropospheric wind direction relative to the terrain to determine where moisture convergence and lifting will be focused over a small region.
Air masses with tropical characteristics can cause flooding events even in mid-latitudes.
Convection » Heavy Precip./Flash Floods Forecast Exercise
Winter Weather
Winter Weather Precipitation Type
Winter Weather » Precipitation Type Introduction and Relevant Parameters
Introduction
Forecasting precipitation type is one of the major operational challenges during the cool season. A change of as little as 1°C in surface temperature can result in a major ice or snow storm, as opposed to a rather benign rain event. Analysis of a skew-T and associated precipitation type parameters can yield important clues for forecasting precipitation type, as illustrated in the following exercise.
Relevant Parameters:
Winter Weather » Precipitation Type 0000 UTC Day 1
As shown in the animation, a slow-moving winter cyclone is moving toward your area from the southwest, and is expected to pass just south of your location in the next few days. [Note: Your "Forecast Area" is denoted by an "X" in the animation.] A variety of precipitation types are being observed with this system. You are expecting to be in the overrunning precipitation region of the cyclone, with precipitation beginning in the next 12 hours. In this scenario, you will be using observed and model forecast soundings plotted on a skew-T to diagnose and forecast precipitation type during the next 48 hours.
Winter Weather » Precipitation Type 0000 UTC Day 1 (Hour 0)

This sounding shows a relatively warm (T > 0°C) layer extending from the surface at 1000 hPa up to 733 hPa. A nearly isothermal layer exists between 950 and 800 hPa. Winds near the surface are from the NE, veering across the warm frontal zone to ESE at 850 hPa, and to SW above the isothermal layer. The surface temperature/dewpoint is +4.7°C / -10.0°C, yielding a wet-bulb temperature of 0°C and a relative humidity of 34%, and the sounding is unsaturated throughout. The 1000-500 hPa thickness is 5449 m.
Task 1: Predict what type of precipitation will be falling 12 hours from now. (To assist with this question and others that follow, you can open the Interactive Skew-T with this sounding).
Winter Weather » Precipitation Type 1200 UTC Day 1 (Hour 12)
Winter Weather » Precipitation Type 12-hour Forecast valid 0000 UTC Day 2 (Hour 24)
Winter Weather » Precipitation Type Observations: 0000 UTC Day 2 (Hour 24)

The observed sounding is consistent with the model forecast sounding and shows saturated conditions in the lower and middle troposphere, with a 60 hPa deep warm, above-freezing layer near 800 hPa. As the warm air arrived aloft, the snow began mixing with ice pellets about 6 hours ago. Light ice pellets are being observed at the surface. There is a 4-inch (10 cm) accumulation of snow and sleet on grassy areas, and the roadways are covered with patches of ice and slush.
Winter Weather » Precipitation Type 12-hour Forecast valid 1200 UTC Day 2 (Hour 36)
Winter Weather » Precipitation Type Observations: 1200 UTC Day 2 (Hour 36)

Once again, the model sounding did a good job forecasting. Like this observed sounding, it shows a shallow subfreezing layer near the surface. Moderate freezing rain is being observed, and there is significant icing of trees, roadways, and power lines. There is concern for the potential of significant ice accumulation during the next 12 hours.
Winter Weather » Precipitation Type 12-hour Forecast valid 0000 UTC Day 3 (Hour 48)
Winter Weather » Precipitation Type Observations: 0000 UTC Day 3 (Hour 48)

The model forecast sounding continues to accurately depict the observed sounding conditions. Surface temperatures quickly warmed to above freezing by 1400 UTC, changing the precipitation to rain. At 0000 UTC on Day 3, light rain is being observed and significant melting of the ice has occurred.
Winter Weather » Precipitation Type Summary
There are a number of parameters that may be assessed to help forecast precipitation type at a given location. These include thickness; wet-bulb temperature; existence, strength, and depth of a warm layer (T>0°C) above the surface; depth of a cold layer (T<0°C) near the surface; and surface temperature, just to name a few. At times, these parameters may produce disagreement, emphasizing how important it is to evaluate the entire sounding (either observed or from a forecast model) to adequately assess and forecast winter precipitation type.
Turbulence
Turbulence Introduction
Turbulence » Introduction Introduction
Turbulence results from eddies or fluctuations in wind speed or direction. This figure shows streamlines associated with a vertically propagating mountain wave. Also note the low-level rotor circulation downstream of the mountains. Both the vertically propagating wave and the rotor circulation create turbulence.
When turbulence is encountered, aircraft typically experience an irregular series of sharp bumps, which may impact their stability and motion. The degree of impact depends on aircraft characteristics (size, airspeed, etc.) as well as the intensity of the turbulence encountered. The turbulence intensity is primarily determined by horizontal and vertical wind shear and atmospheric stability in the flight layer, characteristics that can be assessed via a skew-T.
This section will briefly examine the forecast methodology for using a skew-T to predict turbulence for aviation interests. For more information on mountain waves, see the COMET module: Mountain Waves and Downslope Winds.
Turbulence » Introduction Intensity of Turbulence

There are four different levels of turbulence intensity defined by the aviation community:
1. Extreme turbulence:
Violent bumping of the aircraft, occasionally resulting in significant structural damage and
loss of control.
Relatively rare occurrence, fortunately, and generally confined to the lee
of mountain ranges at relatively low levels in the vicinity of a rotor cloud (see photo), when
the cross-barrier wind component is greater than 50 knots, or in severe thunderstorms associated
with squall lines.
2. Severe turbulence:
Sudden large changes in aircraft altitude, airspeed, and attitude, occasionally leading to
temporary loss of control.
May be found in mountain waves where the cross-barrier wind component is 25-50 knots.
In these cases, turbulence may occur from ridgetop level to a few thousand feet (1000 m) above,
for up to 50 miles (80 km) to the lee of the range. If the cross-barrier component exceeds 50 knots,
the severe turbulence layer be as deep as 5000 feet (1525 m), extending up to 150 miles (240 km) to the
lee of the range. Severe turbulence can also occur near thunderstorms and in strong horizontal and
vertical wind shear environments, extending above and below, as well as up to 100 miles (160 km)
poleward of an upper-level jet stream core.
3. Moderate turbulence:
Sudden, frequent bumpiness of the aircraft but little or no variations in aircraft altitude or attitude.
4. Light turbulence:
Light bumpiness, little more than a nuisance to aircraft operators.
Turbulence Thermal Turbulence
Turbulence » Thermal Turbulence Introduction
Thermal turbulence is caused by solar heating at the earth's surface which heats the lower atmosphere resulting in uneven convective currents and turbulence.
Turbulence » Thermal Turbulence Skew-T Procedure
In the absence of significant air mass changes during the warm season, a single-sounding method may be employed to infer turbulence in convective clouds. The so-called "Eastern Airlines Method" is an empirical method developed by comparing pilot reports of turbulence with analyzed soundings.
Procedure:
- Find the convective condensation level (CCL)convective condensation level (CCL)
- Proceed upward from the CCL along a moist adiabat to 400 hPa and determine the parcel temperature (T')
- Subtract the observed 400 hPa temperature (T400) from T' and call the result Delta-T.
Delta-T = T' - T400. - Determine turbulence intensity from the table below:
Delta-T (°C) | Turbulence Intensity |
0 - 3 | Light |
4 - 6 | Moderate |
7 - 9 | Severe |
>9 | Extreme |
Turbulence » Thermal Turbulence Question
Icing
Icing Icing Forecasting
Icing » Icing Forecasting Forecasting Icing Using the Skew-T

Icing is a significant hazard to aircraft. Ice accumulations on an aircraft's wings and air frame can lead to added weight, loss of lift, power reduction, and other issues severely compromising airworthiness. Therefore, forecasters need to recognize conditions that might be favorable for icing.
Icing » Icing Forecasting Forecasting Icing Occurrence

In making an icing forecast, one must evaluate observed and model-derived soundings to look for factors favorable for ice formation. These include:
- Existence of clouds, including coverage, type and depth, always taking into account local effects
- Subfreezing areas or layers where there are clouds and, possibly, precipitation
Over the years, empirical forecast methods and various forecasting heuristics related to icing have been developed. These studies have found icing is likely:
- if the dewpoint depression in a layer is less than 2°C for temperatures of 0 to -7°C.
- if the dewpoint depression in a layer is less than 3°C for temperatures of -8 to -15°C.
Icing tends to be less likely at temperatures below -15°C. For instance, for temperatures of -16 to -22°C with dewpoint depressions greater than 1°C, there is a 90% probability that icing will not occur, and for temperatures colder than -22°C, there is a 90% likelihood that icing will not occur no matter the dewpoint depression.
Icing » Icing Forecasting Icing Type

Empirical studies have shown the type of icing depends on the ambient temperature as well as the types of clouds (vertical motions) in the environment.
- Rime Icing occurs at temperatures colder than -15°C but can also occur at warmer temperatures in stratiform clouds.
- Clear Icing occurs at temperatures between 0 and -8°C, in cumulus clouds, and/or freezing precipitation.
- Mixed Icing (rime and clear) occurs in cumulus clouds at temperatures of -9 to -15°C.
Therefore, to evaluate the potential for icing from a skew-T plot, forecasters need to determine the thickness of the cloud/moist layer and the potential for supercooled conditions by examining the temperature, dewpoint depression, and lapse rate within the sounding.
Icing » Icing Forecasting Icing Example 1

Consider this sounding to assess the potential for icing. (To assist with this question and others that follow, you can open the Interactive Skew-T with this sounding).
In what layer would you expect clouds?
(Choose the best answer.)
Icing » Icing Forecasting Icing Example 1 (continued)

(If you haven't done so already, you can open the Interactive Skew-T with this sounding).
In what layer would you expect icing?
(Choose the best answer.)
Icing » Icing Forecasting Icing Example 2

Consider this sounding to assess the potential for icing. (To assist with this question and others that follow, you can open the Interactive Skew-T with this sounding).
In what layer would you expect clouds?
(Choose the best answer.)
Icing » Icing Forecasting Icing Example 2 (continued)

(If you haven't done so already, you can open the Interactive Skew-T with this sounding).
In what layer would you expect icing?
(Choose the best answer.)
Icing » Icing Forecasting Closing Remarks

It should be noted that assessment of icing potential from a skew-T plot, while providing valuable information, can not give a complete picture of icing potential. For instance, the above methodology has not addressed issues regarding the intensity of icing. Empirical studies suggest light icing tends to occur with slightly cold or neutral advection. Moderate icing tends to occur with strong cold advection, or in environments where building cumulus clouds are expected. With that in mind, a single sounding should always be used in combination with observed or model forecast temperature and wind fields to get a more complete assessment of temperature advection. And in fact, it is always preferable to combine skew-T sounding analysis with analyses of observational and model output fields when producing an icing forecast.
COMET modules
For a more complete treatment of aviation icing, see the following COMET modules: