Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 PM EDT Mon May 10 2021
 
Day 2
Valid 12Z Tue May 11 2021 - 12Z Wed May 12 2021 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF 
THE MIDDLE GULF COAST REGION TO EASTERN AND SOUTH TEXAS...

21z Update...Areas along the Gulf are still on track to see heavy 
rainfall amounts associated with the frontal boundary. Latest 
guidance has shifted the axis of heaviest amounts a bit further 
eastward through portions of central MS/AL with upwards of 1 to 2 
inches during this time frame with pockets of 2.5+ inches. These 
areas are still sitting at 150 to 600 percent of normal of 
rainfall over the past week. With this being said, have extended 
the Marginal Risk area further eastward for MS and southward 
through southern portions of AL per request of MOB CWA. Have also 
extended the Slight Risk area a bit further eastward for these 
areas as well as southward through southern TX with latest QPF 
guidance showing heavy amounts of 2 to 4 inches for this area.   

Chiari/Campbell

Previous Discussion...

Model agreement has gotten better in terms of the speed and 
placement of the front approaching from the north...so only a 
minor expansion was made around the western and southern/eastern 
periphery of the previously-issued risk area.  

The southward moving front over Texas and the central Gulf states 
will slow its progression during this period which results in the 
boundary becoming oriented from west-to-east. With very moist air 
streaming in from the east/southeast over boundary, convection 
will build westward and support training cells along and north of 
this boundary. The signal remains for moderate to heavy rainfall 
across parts of the area, although the GFS had backed off the 
areal coverage and maximum rainfall amounts somewhat from 09/18Z.  
Even so, am not inclined to make significant changes either to the 
QPF or the Excessive Rainfall Outlook given the anomalous amount 
of moisture in place (maximum anomalies in excess of 2 standard 
deviations above climatology) which is sustained by continued 
influx of moisture on low-level southeasterly winds and enough 
instability (1000-1500 J/kg MLCAPE) to result in cells being able 
to produce downpours.

Bann
 
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt