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
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1140 AM EDT Fri May 27 2022
Valid 16Z Fri May 27 2022 - 12Z Sat May 28 2022
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE EASTERN
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
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 rates...as 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: www.wpc.ncep.noaa.gov/qpf/94epoints.txt