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< Day 1 Outlook Valid Through 12Z Today
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WPC Day 1 Excessive Rainfall Outlook Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 0820 UTC Sun Mar 26, 2023
Valid: 12 UTC Mar 26, 2023 - 12 UTC Mar 27, 2023
Forecast Discussion
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
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