FOUS30 KWBC 170745
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Sat Aug 17 2024

Day 1
Valid 12Z Sat Aug 17 2024 - 12Z Sun Aug 18 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE
SOUTHWEST INCLUDING THE SLOT CANYON REGION OF UTAH AND PORTIONS OF
THE MID ATLANTIC...

...Southwest U.S...
An active convective day is in store for portions of the Southwest
U.S. as the deep monsoon moisture surges northward through the
region. PWs are forecast to reach nearly +2 to +3 standard
deviations above normal, with values as high as 1.25" into Idaho
and locally 1.5"+ across Arizona and southern Utah. Aloft,
shortwave energy will be moving through the flow, providing greater
forcing for ascent. Finally, with daytime heating and favorable
lapse rates, upwards of 1500-2000 J/kg of instability is expected
to develop. All of this points to robust thunderstorms capable of
producing intense rain rates (localized up to 1" in an hour) over
the more sensitive regions of the Southwest U.S. including the 
Slot Canyon region of Utah. HREF neighborhood probabilities are 
moderate (50%) for localized 2" totals over southern Utah and even 
show a slight signal (15-20%) for 3" totals. 

No significant changes were needed to the Slight Risk area and the
rest of the risk areas were adjusted minimally to the latest
guidance. The most vulnerable locations will be the slot canyons,
normally dry washes, and areas near recent burn scars for
potentially localized significant flash flooding. 


...Eastern Great Lakes, Mid-Atlantic, and into the Mid-Mississippi
Valley...

The slow moving upper trough will continue to make a progression to
the east today, primarily affecting areas from the eastern Great
Lakes through the central Appalachians and Mid Atlantic region.
Fairly decent forcing for ascent provided by the approaching height
falls and upper divergence, which will provide the best support for
convection across the Mid Atlantic later today. Further west,
convection along the Appalachians/Ohio Valley will be tied to the
advancing cold front. Moisture profiles are sufficient but not
overly impressive and instability is expected to be enough to
produce scattered to numerous showers and thunderstorms, though
storm organization is generally expected to be weak and overall
progressive as well. This should limit the scope of any flash
flooding concerns. 

With that said, there are signals for some pockets of higher rain
totals, one of which is across the Mid Atlantic where some of the
00Z hi-res guidance paints some hefty QPF totals, although spotty
in places. This drives the HREF probs for totals for 3" above 50
percent across northern VA through eastern/east-central PA. 
Confidence in this is lower with the various scenarios is 
conditional based on how earlier/morning activity plays out. 
However, given the run to run continuity of the HREF signals and 
potential for at least a couple rounds of heavier rains, a Slight 
Risk for mainly urban flash flooding was introduced from portions 
of MD (near Washington, DC) through eastern PA. 

Farther south along the stalled boundary, some training of
convection is possible as the mean flow becomes more parallel to
the expected storm motions, particularly across portions of the
Tennessee Valley southward. Despite the decent HREF signal for
localized/pockets of higher rain totals, generally high FFG and
drier soils should keep the flash flood threat at the Marginal
level. 


...Pacific Northwest...
An impressive upper level trough will amplify into a closed low off
the northern CA coast, dropping 500mb heights to less than -3 sigma
this afternoon. Downstream of this low, pinched flow will 
transition into broad divergence over the Pacific and Interior 
Northwest, surging moisture and ascent into the region. PWs are 
progged to reach above 1 inch, and possibly above 1.25" in parts of
the Cascades, reaching nearly +3 sigma from climatology. As the 
upper low pivots slowly northeast, this moisture fetch will be 
persistent into the region, and IVT values peak around 350 kg/m/s, 
reflective of a rare but weak August AR into WA/OR/northern CA.

During this time, ascent increases within the RRQ of a
strengthening upper jet streak, and mesoscale upslope ascent
occurs into the Cascades. This will result in widespread showers
and even scattered thunderstorms in response to MUCAPE reaching
500-1000 J/kg. The HREF neighborhood probabilities for 1"/hr reach
as high as 30-40%, so despite generally rapid storm motions, brief
torrential rainfall is likely, with total rainfall being enhanced 
by repeating rounds of cells. August ARs usually do not result in 
anything more than isolated flood/flash flood instances, but the 
MRGL risk was extended along the Cascades into northern CA, and 
rapid runoff/instances of flash flooding are possible, with 
impactful flash flooding, including debris flows, possible across 
recent burn scars.

...South Florida...
Another day of enhanced coverage of convection is expected across
South Florida and the Keys in the vicinity of the lingering 
surface trough draped across the area. This feature combined with 
fairly anomalous moisture should lead to another day of scattered 
to numerous showers and thunderstorms, some of which will be 
capable of producing intense rain rates (2-3" hourly totals) and 
potentially produce a quick 2-4"+ over some areas. The latest 
guidance points to the greatest focus area to be south of the most 
urbanized corridor but nonetheless, some isolated/localized 
flooding concerns will be possible and the Marginal Risk remains in
place. 

Taylor


Day 2
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...
A cold front will advance eastward into the region as the main
upper trough slowly advances through the Great Lakes. This will
bring another day of scattered to numerous showers and
thunderstorms to much of the region. In the low level southwesterly
flow ahead of the front, with enhanced training likely on S-N 
oriented mean flow parallel to the from from near Washington, D.C. 
through Upstate NY. Here, PWs will be nearly +2 sigma according to 
NAEFS, which will be acted upon by the robust synoptic lift to 
deepen convection. Rainfall rates of 1-2"/hr are likely, which 
through training could produce locally more than 3" of rainfall
based on the latest ensemble probabilities.  

Where this training occurs across urban areas or sensitive soils, 
scattered instances of flash flooding are possible. The recent CSU 
FG fields depict an enhanced risk for flash flooding along I-95 and
into the Tri-State area, leading to higher confidence in the Slight
Risk. 

Further south, the Marginal Risk extends into the Piedmont and
coastal Carolinas where the combination of heavy rainfall over more
sensitive soils due to recent heavy rains may result in isolated
instances of flash flooding. 


...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 Utah where the better instability
and highest PW anomalies are forecast. It's possible a Slight Risk
may be needed in future updates across portions of northern AZ into
southern UT but there is some uncertainty on cloud cover
potentially limiting the available instability and thus the rain
rates may end up lower. 

...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. Given the potential for this to occur over
the more urban areas, the inherited Marginal Risk was minimally
changed to highlight the continued isolated flash flood risk. 

Taylor
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