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The System for Convection Analysis
and Nowcasting (SCAN)


SEVERE WEATHER EVENT IN PROGRESS: What does the NWS FORECASTER need to know?Severe thunderstorms are racing towards Washington D.C. from Pennsylvania. Some are stronger than others. Some may be producing large hail. Some may be producing tornadoes. The FORECASTER needs to know QUICKLY WHICH of the MANY STORMS are the MOST DANGEROUS and consider issuing a NWS SEVERE THUNDERSTORM or TORNADO WARNING. In the MODERNIZED NWS, forecasters will use the SCAN software on AWIPS to help them make warning decisions.

for Washington D.C.- Baltimore NWS FORECAST OFFICE in Sterling, Virginia. The display shows an ACTUAL SEVERE THUNDERSTORM EVENT that effected the WASHINGTON D.C. METRO AREA last summer on July 21, 1998. The display shows NEXRAD radar echoes with individual thunderstorms indicated by the colored circles. The STRONGEST STORMS are indicated by the RED CIRCLES. The individual storms are RANKED from STRONGEST toWEAKEST in the THUNDERSTORM TABLE. The STRONGEST STORM (M0) is placed in the TOP ROW of the table. The THUNDERSTORM STRENGTH parameters are shown in the COLUMNS of the table. The values are colored by severity. Red is most severe. Yellow is less severe. White is least severe. Five parameters are colored RED for storm M0, showing why is it the strongest STORM. In particular, storm M0 has a HIGH PROBABILITY OF HAIL, 80%.

The THUNDERSTORM TABLE shows the forecaster WHICH STORMS are most dangerous and WHY. The THUNDERSTORM TABLE helps the FORECASTER to make a WISE and QUICK DECISION on whether or not to issue at NWS SEVERE THUNDERSTORM or TORNADO WARNING.


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National Weather Service
Office of Science and Technology
Meteorological Development Laboratory
Last Modified:
January 14, 2004
Page Author: Iris Boon 
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