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A Description of the Mid-Atlantic Numerical Weather Forecast System

Model Configuration: Real-time numerical weather forecasts are produced by the fifth generation of mesoscale, nonhydrostatic, nested-grid model (i.e., MM5V3.2). The MM5 is a community model that was originally developed at the PennState University and National Center for Atmospheric Research. Currently, the model forecasts are made on three nested domains with a grid size of 36 km in the outer domain, a grid size of 12 km in the next inner domain and a grid size of 4 km in the inner-most domain. The inner domain uses a finer resolution terrain data, as shown in the Mid-Atlantic maps, and it covers the states of Maryland, Delaware, New Jersey, Connecticut, Pennsylvania, Ohio, Virginia, West Virginia, and most of North Carolina, Kentucky, New York, and Massachusetts. The inner-most domain uses even a finer resolution terrain data, as shown in the high resolution Maryland maps, and it covers all of Maryland, the Distric of Columbia, Delaware, and parts of New Jersey, Pennsylvania, Virginia and West Virginia. The model utilizes a terrain-following (sigma) coordinate with 24 layers and a radiative upper-boundary condition at 50 hPa. Other key physical parameterizations include:

• The Blackadar high-resolution planetary boundary layer parameterization;
• Explicit cloud physics with simple ice microphysics (no mixed-phase processes); and
• The Kain-Fritsch cumulus parameterization.

Numerical Forecasts: Currently, the MM5V3.2 is run once a day for 48 hour forecasts, on the Department's newly purchased DEC-Alpha 8400's 4-processor workstation. The model is initialized with the analyses of the National Center for Environmental Prediction (Eta Model). A 48-hour forecast, including the initialization, requires about 10 hours to complete.

Future Improvements: The model is under continuing improvements. We plan to make the following changes in the near future:

• Include a 12 hour pre-forecast spin-up period using dynamic nudging;
• Incorporate an atmosphere-land interaction scheme; and
• Integrate the model twice a day.

Interpreting Wind Barbs: In meteorology, winds are named for the direction from which they are coming from (a northwest wind is coming from the northwest), while wind symbol generally point in the direction in which they are going to. "Wind barbs" as they are seen on our maps, are plotted in the standard meterololog ical convention. Unless the map you are looking at specifically says "Wind vectors ", the wind barbs represent the wind direction and speed. The feathers are always to the right of the arm and are 10 knots for every long feather, 5 knots for a short feather and 50 knots for a flag...so that if a wind barb showed one flag, 2 long and one short feather, that would represent a windspeed of 75 knots.