FOUS30 KWBC 180812
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
412 AM EDT Sun Aug 18 2024

Day 1
Valid 12Z Sun Aug 18 2024 - 12Z Mon Aug 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID ATLANTIC...

...Mid Atlantic and Northeast...
An approaching cold front will bring another day of scattered to
numerous showers and thunderstorms to portions of the Mid Atlantic
and Northeast today into tonight. Aloft, the broad upper trough
will slowly advance eastward into the eastern Great Lakes and
central Appalachians. Ahead of it, southwesterly flow will allow
for warm and moist air to continue lifting into the region and will
be characterized by dewpoints well into the 70s and PWs above 1.5"
with locally higher values exceeding 1.75", which is around 2-2.5
standard deviations above normal. With peak heating, instability of
between 1500-2000 J/kg is forecast to develop. 

The 00Z guidance continues to key on the urban corridor from near
Washington, DC to near NYC metro for the greatest QPF and
probabilities for intense rain rates today into tonight. Several 
clusters of thunderstorms are likely to develop (some this morning) and
with the mean flow out of the southwest near parallel to the storm
motions, some enhanced training or backbuilding will be possible, 
particularly over eastern PA, northern New Jersey, and into 
southern New York. Here, the HREF neighborhood probabilities are 
high for 3" totals (above 60 percent) and show a slight signal for 
5" (15-20 percent). Rain rates of 2-3"/hr will be possible given 
the PW environment. Given the proximity and likelihood for this to 
occur over the more urbanized corridor, scattered instances of 
flash flooding will be possible, some of which could be locally 
significant.

Elsewhere in the region outside of the Slight Risk, thunderstorm 
activity is expected to be more isolated in nature, but given the
environmental ingredients could still result in localized flash 
flooding. This includes much of PA and NY states where there is 
still a decent signal in the HREF probabilities for 2-3" isolated 
totals. Another area of concern is further south from eastern VA into
the Piedmont and coastal regions of the Carolinas where the 
combination of locally heavy rainfall over more sensitive/saturated
ground conditions could lead to additional flash flooding 
concerns. 

...Southwest and Intermountain West...
Impressive and deep monsoonal moisture will continue to stream
around the western periphery of the monsoon ridge and combine with
daytime instability to produce scattered to widespread showers and
thunderstorms from AZ as far north as WY and ID. Convection will
be capable of producing hourly totals up to 0.75" at times,
particularly across portions of northern AZ and southern Utah where
guidance continues to show potential for locally higher rainfall
amounts. 

...South Florida...
Another day of scattered, slow-moving thunderstorms are expected
across the region where rain rates may approach 2-3"/hr at times.
This is due to the lingering surface trough in the vicinity
focusing the higher PWs. The consensus among the 00Z guidance is
that this should largely stay south of the major metro areas, but
will be close enough to continue the Marginal Risk for portions of
South Florida and the upper Keys. 

...Ozarks Region...
Shortwave energy rounding the top of the upper ridge will quickly
drop through the Ozarks region late in the period
(this evening/overnight). With a modest low level jet forming in
response and a decent moisture influx, a narrow strip of locally
heavy rainfall is likely to develop in the region. Given the
northwest flow aloft and storm motions, some backbuilding is
possible. While confidence in these scenarios isn't very high, 
some of the outlook area has seen recent heavy rainfall and the 
potential for a narrow/localized area of 1-3" is possible based on 
the 00Z hi-res guidance and HREF neighborhood probabilities. This 
could lead to a few instances of flash flooding, mainly in the 
06Z-12Z period early Monday morning.

Taylor


Day 2
Valid 12Z Mon Aug 19 2024 - 12Z Tue Aug 20 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE NORTHEAST U.S. AND SOUTHWEST INTO THE INTERMOUNTAIN WEST...

...Northeast...
The upper trough will make a final pass across the Mid Atlantic to
Northeast on Monday with the cold front expected to move across
the coastal areas during the late morning into the evening hours.
With peak heating and sufficient destabilization, vigorous
convective elements are expected to develop again, mainly from the
coastal Mid Atlantic through much of eastern New York and portions
of New England. Storm motions should be fairly progressive, but a
few storms could repeat/train due to merging/colliding cells and
storm totals could approach 1-2" in spots leading to flash
flooding.

...Southwest/Intermountain West... 
By Monday, the bulk of the anomalous moisture is expected to have 
lifted northward through the Four Corners region and into the 
Intermountain West/Rockies. With another shortwave trough embedded 
in the flow moving through, this should be sufficient to produce 
another day of isolated to scattered thunderstorms, most likely 
over portions of northern Utah, Colorado, and into western WY, far 
eastern Idaho. Flash flooding will be possible, particularly for 
the most sensitive locations like areas near recent burn scars.

Taylor


Day 3
Valid 12Z Tue Aug 20 2024 - 12Z Wed Aug 21 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST U.S...

The upper ridge over the Southern U.S. is expected to build/grow 
and expand westward into more of the Southwest U.S. on Tuesday. 
This will likely limit the extent and coverage of monsoon 
thunderstorm activity across the region, but with PWs still 
elevated across Arizona and the expectation for at least a modest 
amount of instability, isolated thunderstorms are possible through
the late morning into evening hours. If they do develop, these 
storms would pose an isolated flash flood risk for the typical 
vulnerable locations like areas near burn scars, dry washes, and 
slot canyons.

Taylor
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