Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Sun Mar 26 2023
 
Day 1
Valid 12Z Sun Mar 26 2023 - 12Z Mon Mar 27 2023 

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF 
CENTRAL ALABAMA AND GEORGIA...

There has been quite a notable flip-flop in all of the guidance as 
pertains to the weather expected to unfold across portions of the 
Southeast today. A stalled out front remains in place across the 
Slight and Marginal Risk areas from Louisiana to southern North 
Carolina. This front is already serving as a focus for convection 
from central MS through far western GA. The convective activity is 
only expected to increase for the rest of the predawn hours and 
into the morning. 30-40 kts of southwesterly 850 mb flow is 
drawing plentiful Gulf moisture northward into the front. 
Meanwhile, a pair of subtle disturbances are expected to track 
eastward along the front, causing local increases in both 
convective coverage and severity. This is probably the largest 
change from 24 hours ago, when we had eliminated even a Marginal 
risk of flash flooding! These disturbances acting to organize the 
convection along the front now will be quite effective at taking 
advantage of as much as 2,500 J/kg of MUCAPE and somewhat unusual 
levels of atmospheric moisture at 2 sigma above normal and PWATs 
exceeding 1.5 inches. 

Uncertainty remains above average given the rainfall event is 
already beginning, as to where the heaviest rain is expected to 
fall. While most of the CAMs have highlighted the Slight Risk 
area, the primary area of uncertainty is how a subtle upper level 
disturbance interacts with the ongoing convection and associated 
cold pools this evening. The wave will track well north and west 
of the area, but nonetheless will have enough impact on the 
ongoing thunderstorms to result in a local increase in storm 
coverage and intensity. Some of the CAMs suggest this will result 
in the heaviest convection displaced well to the north of the 
surface front, highlighting Birmingham and Atlanta, whereas others 
think the primary forcing will stay with the front further south, 
resulting simply in an area of strong thunderstorms as the latest 
salvo in the wave train. The impacts from this difference would be 
how severe the likely ongoing flooding can get if the stronger 
storms track over the same areas, or if they're sufficiently 
displaced to the north to result in fewer impacts (despite the 
lower FFGs due to yesterday's rain across northern AL). If the 
former scenario pans out where the storms stay with the main 
surface front, then there's some potential for extreme rainfall 
totals exceeding 6 inches (as depicted in the 00Z HRRR primarily), 
and the potential need for an additional upgrade to a Moderate 
Risk. 

As regards training storms, the potential is very high, and in 
fact is already happening in the Slight risk area of eastern 
AL/western GA along the front. These storms will continue training 
at least through the morning hours, at which point the overall 
convective activity will wane a bit with the typical weakening of 
the LLJ. Convection then increases again with the approach of the 
second wave in the late afternoon and evening hours. For all 
convection today, the motion of the storms will be 
east-northeastward, which is parallel to the surface front that is 
not expected to move much during the day. Thus, the flow orienting 
parallel to the front supports continued training through the day. 
Already, currently ongoing severe thunderstorm activity is 
producing local rainfall rates up to 1.5 inches per hour, and that 
intensity of convection is expected to continue over the next 
several hours, and again late this afternoon and evening. 

For the surrounding Marginal Risk, expansions were made in all 
directions with this morning's update. Some convection has already 
developed in northwestern Alabama and central and southern 
Mississippi as of this writing, and continued scattered convective 
development is likely through the day in both areas. The increased 
coverage and intensity required an eastward expansion of the 
Marginal Risk to the Carolina coast, as all of the convection will 
track along this corridor (albeit likely in a somewhat weaker 
state than areas further west), and therefore training convection 
once again will be the dominant factor favoring development of 
flash flooding, particularly if those stronger storms move over 
urbanized areas.

...Potential for Future Risk Changes...

Depending on storm coverage through the morning, and if guidance 
remains consistent on continued training storms in much of the 
Slight Risk area late this afternoon and this evening, a Moderate 
Risk may be needed.

Wegman
 
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt