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Day 2 Outlook >
WPC Day 1 Excessive Rainfall Outlook
Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 1540 UTC Fri May 27, 2022
Valid: 16 UTC May 27, 2022 - 12 UTC May 28, 2022
Day 1 Excessive Rainfall Forecast
Forecast Discussion
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1140 AM EDT Fri May 27 2022
Day 1
Valid 16Z Fri May 27 2022 - 12Z Sat May 28 2022 


16Z Update: Initial line of showers/storms moving through 
Mid-Atlantic this morning will continue tracking north/northeast. 
Higher PWs and unidirectional flow supports some efficient rain 
producing storms with the potential for training and 
isolated/scattered instances of flash flooding. Redevelopment to 
the west along the cold front later this afternoon/evening is 
likely and could bring an additional round of storms across a 
similar area, so the Slight Risk was maintained for this update.  
Slow moving convection along stationary boundary and near/north of 
the closed low could result in pockets of higher rain totals. The 
slow storm motions may lead to an isolated flash flood risk and 
MPD 260 in effect through 21Z goes into more detail for that 
particular risk.


---previous discussion---
Much of the eastern U.S. will see areas of showers and 
thunderstorms today ahead of the eastward moving large scale 
trough and embedded well defined mid/upper level low. A Slight 
risk was maintained for portions of the Mid Atlantic from northern 
NC into eastern PA. From a flash flood areal coverage perspective 
this is probably on the lower end of the Slight risk range 
(15-40%), however localized swaths of flash flooding do appear 
probable. The environment will be conducive to efficient rainfall, 
with PWs forecast above the climatological 90th percentile, a deep 
warm layer, and high deep layer RH. Thus we should be able to 
realize some pretty heavy rainfall rates with convection across 
this area...main question will be the duration of this rainfall. 
Convection is expected to move progressively off to the 
east...however mainly unidirectional flow does support some linear 
south/north training segments through the day. There is also the 
likelihood of multiple convective rounds over some locations, with 
strong synoptic forcing lagging the initial convective line and 
supporting additional development in its wake. The combination of 
efficient rainfall, small scale training segments, and multiple 
convective rounds...should support a localized to scattered flash 
flood risk today within the Slight risk region. The risk is 
greatest across urban areas and/or locations where any training 
segments materialize (resulting in swaths of 2-3" of rainfall).

The potential is there for localized higher rainfall totals south 
of the Slight risk area from southern NC into the FL Panhandle. 
Better instability supports pockets of 3-5" of rainfall 
here...although amounts of this magnitude should stay pretty 
localized in nature. That combined with the higher FFG over this 
region suggests more of a localized flash flood risk which is 
covered by a Marginal risk.

A Marginal risk extends westward across portions of the OH valley 
into the southern Great Lakes. Slow moving convection near and 
north of the closed mid level low should support localized areas 
of heavy rainfall across this corridor (similar to what occurred 
north of the low Thursday). Isolated flash flooding is possible.

Training convection near a stationary front is possible this 
afternoon/evening across portions of northern NY into northern New 
England. This signal is strongest over portions of northern ME 
within the ~22z-03z time frame tonight. The 00z HREF indicates 
moderate probabilities of 1" an hour instability should 
be just enough to take advantage of near record PWs for late May 
to support these rates. Three hour FFG over the area is ~1.5" and 
the 00z HREF indicates about a 40% chance of exceeding this total. 
However given the average to below average soil moisture and 
streamflows, and rainfall rates probably not exceeding 1" in an 
hour by all that much, the resultant flash flood risk is probably 
still pretty localized in nature. Thus will maintain a Marginal 
risk over this region.

Day 1 threat area:

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