High Resolution WRF Forecasts for 3/28/09 Event

Experimental, high resolution forecasts are being produced at the National Severe Storms Laboratory to determine the operational value of model simulations that are “cloud permitting” and able to explicitly forecast clouds and hydrometeors.  Although these forecasts are often at long range for a particular event and may have errors in exact timing or specific location of an event, they can be useful if they produce storms of the correct “convective mode”.  Furthermore, these simulations are capable of simulating details at a finer scale than many current operational forecast models.  For the most recent event (3/28), the NSSL experimental forecasts suggested that discrete cells would initially form along a cold front that trailed into western Alabama.

Radar reflectivity simulated by the NSSL WRF 4-km forecast, valid at 2000 UTC, March 28 2009.

Radar reflectivity simulated by the NSSL WRF 4-km forecast, valid at 2000 UTC, March 28 2009.

Simulation of land surface processes are key to producing an accurate forecast of convective initiation, as fluxes of heat and moisture within the boundary layer modify the characteristics of lifted parcels.  The high resolution forecast suggests some pooling of moisture along the boundary.  In this case, simulated surface dew points were a few degrees too high.  Advanced land surface parameterizations, such as the NASA Goddard Land Information System, in conjunction with accurate sea surface temperatures from MODIS, are being explored within SPoRT to test improvements in convective forecasts throughout the Central Plains and the Florida peninsula.

Surface dew point (moisture) at the same time, displaying the forecast cold front as a wind shift from south to southwest in western Alabama.

Surface dew point (moisture) at the same time, displaying the forecast cold front as a wind shift from south to southwest in western Alabama.

Storms finally initiated within the region around 2200 UTC and produced several reports of hail, as well as two category EF-1 tornadoes within the Huntsville, AL WFO county warning area.  Although the model forecast produced convection a few hours in advance of the actual onset, these simulations show promise for future forecast improvement as satellite data and improved parameterizations contribute to a more accurate initial condition and representation of the Earth’s physical processes.

Base radar reflectivity from the National Weather Service Doppler Radar in Hytop, AL at approximately 2200 UTC.  Image courtesy of UCAR.

Base radar reflectivity from the National Weather Service Doppler Radar in Hytop, AL at approximately 2200 UTC. Image courtesy of UCAR.

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