FOUS30 KWBC 130904
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
504 AM EDT Thu Aug 13 2026

Day 3  Valid 12Z Sat Aug 15 2026 - 12Z Sun Aug 16 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE FRONT RANGE
AND HIGH PLAINS OF COLORADO AND WESTERN KANSAS AND NEBRASKA, AS
WELL AS PARTS OF THE UPPER MIDWEST...

...Wisconsin to Ohio...
The favorable corridor for potential training convection shifts a
bit northeastward on Saturday into IL/WI and northern IN/OH as the
upper ridge to the south slowly shifts eastward and expands north.
Much of the same ingredients are in place from prior days...with
shortwave energy riding along the periphery of the ridge, an upper
jet to the north and strong low level moisture transport into the
northward lifting boundary. We do see a bit stronger forcing
develop by this time and an area of low pressure to the
north...which should help kick things eastward a bit more than
previous days. Nonetheless, the strong low level moisture transport
is supportive of some backbuilding near the warm front. Again we
see some latitudinal differences in the guidance...with our
preference continuing to stay on the southern edge of the model
spread. This aligns with typical biases, along with the better
performing AIFS and AIGFS, and the 00z RRFS (the only CAM that
goes this far out). So while some adjustments in the risk area are
likely, the corridor from northern IL into northern OH seems most
favored at this time.

...High Plains...
The wet pattern should persist across eastern CO into western
KS/NE. Scattered convection within a high PW and unstable airmass
will result in some cell mergers and heavy rainfall rates. Some of
these areas will likely be more hydrologically sensitive by this
time as well, with previous days rainfall increasing soil saturation.

Chenard
 

Day 4 and Day 5  Valid 12Z Sun Aug 16 2026 - 12Z Tue Aug 18 2026

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL ON SUNDAY FOR
PORTIONS OF THE COLORADO FRONT RANGES AS WELL AS FOR THE MID-ATLANTIC...

...Colorado Front Ranges and High Plains...

The pattern of the previous days will continue into Sunday across
the High Plains. A large ridge covering much of the southern tier
of states finds its northwestern periphery over this region.
Another shortwave driving northeastward along the ridge interface
will spark yet another round of storms Sunday afternoon and
evening. This will be supported by an incredibly persistent pool of
anomalous moisture stuck over the area, likely due to the daily
rounds of storms that reinforce the atmospheric moisture in place.
Further, easterly flow into the mountains from off the Plains is
continually reinforced, including into Sunday. At the moment, the
guidance is not as aggressive on the rainfall amounts across
Colorado on Sunday as compared to previous days. This means Slight
level impacts will be more reliant on favorable antecedent
conditions across this area from previous days. There remains some
uncertainty specifically with how far south the storms will form.
Any offset of storm formation on Sunday compared to prior days will
greatly mitigate the flooding threat. Thus, there remains some
uncertainty with the location of the Slight. However, given the
persistent and unmoving pattern, there is enough confidence to
continue the Slight for yet another consecutive day.

...Mid-Atlantic...

A pair of fronts emanating from a low up over Quebec on Sunday will
sweep across the Mid-Atlantic. The warm front will come first,
advecting a plume of deep tropical moisture into the Mid-Atlantic.
PWATs will increase to over 2 inches in most areas with the warm
frontal passage. The front will move across this region from the
morning through the early afternoon. Widespread shower and
thunderstorm activity will be close on its heels. A cold front
tries to push across the area, but is likely to get hung up over
northern Pennsylvania Sunday night. This will keep much of the area
in the soupy warm and muggy air mass. A secondary low forms along
the fronts Sunday night, which will invigorate another round of
showers and storms likely persisting well into Sunday night.
Southwesterly flow aloft will support the storms' backbuilding and
persisting in this pattern as more moisture is advected into the
region ahead of the cold front. Urban and small stream flooding
will be the primary threats with these storms.

The Slight Risk area covers the mountainous regions of western
Pennsylvania and West Virginia which were both hit hard by recent
heavy rains, are prone to flash flooding due to the terrain, as
well as the urban area of Pittsburgh. The eastern part of the
Slight Risk area covers the I-95 corridor region from Wilmington,
DE through New York City. Of course, urban concerns will be a large
factor in any flooding concerns, as well as the increased danger
from the likely nighttime convection affecting the area.

...Day 5/Monday...

A series of Marginal Risks were drawn from New Mexico and Colorado
for continued afternoon convection (albeit on the down trend from
prior days), a MCS affecting portions of the upper Midwest and
northern Plains, continued WNW flow of deep moisture into the
mountains of Kentucky, West Virginia, and southwest Virginia, and
finally the approach of the same cold front that brings heavy rains
on Sunday to areas of the Mid-Atlantic further north brings rounds
of storms Monday to the Tidewater of Virginia and eastern North
Carolina. The Plains and coastal features are both quite transient,
and are likely to adjust in location and intensity over the coming
days. Meanwhile the Appalachians and High Plains areas have more
uncertainty on rainfall amounts over areas that have both been
hard-hit with heavy rains.

Wegman
$$
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