FOUS30 KWBC 190738
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
338 AM EDT Mon Aug 19 2024

Day 1
Valid 12Z Mon Aug 19 2024 - 12Z Tue Aug 20 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHEAST U.S...

...Northeast...

Broad meridional flow across the Eastern CONUS will continue with a
dual surface low structure progged to move northeast through the
period with a cold front swinging eastward out of the adjacent Ohio
Valley up through Quebec, clearing the eastern seaboard later in
the evening. The deep cyclonic flow ahead of the trough will
continue advecting warm, moist unstable air poleward with
increasing theta-E's and accompanying instability along and ahead
of the cold front situated across Southern New England up through
Interior Northern New England by the late-morning and afternoon
time frame. The combination of increased mid-level forcing from the
trough and surface convergence along the cold front will aid in
developing a line of convection in-of the above area with a primary
focus north of I-80 with the best chance for heavy rainfall likely
north of the LI-NYC-Hudson Valley corridor. This puts areas that
have seen significant rainfall within the crosshairs of another
locally enhanced rainfall prospect before the cold front moves past
and drier air sweeps through in wake of the front.

In terms of potential, the upper threshold will be less than what
occurred today due to a shorter time frame of impact due to the
frontal progression, as well as a more narrow sector of favorable
instability that will play a significant role in the heavy rain
footprint, and overall flash flood prospects. 00z HREF neighborhood
probabilities paint some of that story with a focal point of 20-40%
probabilities for >3" located within the confines of Southern CT up
through Interior New England, including parts of the complex
terrain within MA/VT/NH. The alignment of these higher
probabilities coincide with the better MUCAPE field forecast during
the morning and afternoon hours before the cold front passes the
area. A favorable footprint of at least 1"/hr rates off the
ensemble (HREF) also give credence to the potential with an area 
of 50-90% encompassing the above region with low-end probabilities
for at least 2"/hr showing up across Northern VT and NH. These
areas have been impacted a few times in recent memory with the area
across CT very fresh from today's barrage that brought locally 8+"
to parts of the state. Area FFG's across New England are relatively
low with soil moisture averages falling just above the 50th
percentile, or near normal. CT is not so lucky with the top soil
layer completely inundated from this past event leading to very
sensitive grounds for flooding in this area of interest.

Considering the above factors, a moderately favorable environment 
for heavy rainfall, and with a strong agreement from the UFVS First
Guess Fields denoting a recommended high-end SLGT risk, a Slight
Risk was added across much of CT, West-Central MA, up through much
of VT and NH. A MRGL risk was maintained further south for isolated
flash flood prospects from scattered convection migrating eastward
during the frontal passage that could lend a few chances to those
within the urban corridor from Hampton Roads up through the NYC
metro. 

...Southwest U.S...

The region will continue to lie within the western flanks of mid-
level ridging situated over the Southern Plains into the
Continental Divide. Relatively moist airmass with differential 
heating during the afternoon will yield scattered thunderstorms 
across the Desert Southwest with locally enhanced rainfall 
plausible within the terrain areas of AZ up through the Inter 
Mountain West. A few shortwaves embedded in the flow will aid in 
some better organized convective clusters capable of providing 1+" 
rainfall amounts in a short time, enough to cause some issues if 
they fall within complex terrain, remnant burn scars, or slot 
canyons located across the region. The threat remains on the lower 
end of MRGL, but still within the threshold leading to a general
maintenance of the risk from previous forecast. 

...High Plains...

Multiple stronger shortwaves will eject eastward along the northern
periphery of the ridge, encountering a formidably unstable airmass
as they enter into the High Plains of the Northern and West-Central
U.S this afternoon and evening. Consensus has grown on a few areas
within the High Plains of MT into SD, as well as the CO/WY Front
Range for a cluster of thunderstorms capable of locally heavy
rainfall developing later this afternoon, carrying through the
evening as they ridge along the theta-E gradient in place across
the center of the CONUS. HREF neighborhood probabilities were
favorable for at least 1-2" of rainfall (60-90%) in either of these
areas with some lower-end probabilities for upwards of 3" in any
cells that propagate through the High Plains. The setup should be
on the progressive side overall, however some backbuilding
potential behind any MCS development over the Northern Plains could
produce a more locally significant rainfall footprint where if/when
it develops. The zones of potential include the Southeast WY and
Northeast CO corridor along with Southeast MT into Western SD as
the secondary focal points in the setup. ML output was targeting
this area as the latest runs with an axis of higher QPF positioned
within the same zones as above. First Guess Fields are also
pin-pointing a broader MRGL risk extended into the above zones with
even a some signals for a potentially higher risk (SLGT) embedded.
With the top soil layers trending drier than normal, the prospects
for a more widespread significant rainfall are not anticipated,
thus an additional MRGL expansion into the High Plains was enough
to cover for the potential. 

Kleebauer


Day 2
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...

Mid-level ridge across the Southern U.S is still on track to
maintain a stronghold on the overall pattern for much of the
Central and Western U.S by Tuesday. Ridge axis will expand and
strengthen according to the model consensus leading to unfavorable
conditions for convective development over under the ridge itself.
Arizona will continue to lie right on that western fringe of the
ridge placement with enough lingering instability and moisture
promote a threat of isolated thunderstorms capable of producing
locally heavy rainfall. The threat is certainly on the low-end of
the potential, but non-zero, especially along the International
border between Santa Cruz and Pima counties in AZ. The previous
MRGL was maintained with no change in the alignment. 

A small note to add that lingering convection across the Northern
and Central Plains stemming from activity on D1 will have the
ability to maintain enough intensity to promote an isolated threat
for flash flooding the very beginning of the period before
diminishing. The global model consensus is all over the place on
exactly where the heaviest rain could occur, so there was not
enough agreement to institute a MRGL risk. There is a chance for a
small addition in later updates if there is more agreement within
the model suite, but there's still time to hash out those details
in the next succession of updates. CAMs guidance will hopefully
shed more light on the necessity, if any for an addition.

Kleebauer


Day 3
Valid 12Z Wed Aug 21 2024 - 12Z Thu Aug 22 2024

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

A more robust moisture advection pattern will ensue across the
Southwest U.S with some Pacific moisture being pulled northward
thanks to the combination of the Southern ridge shifting eastward
and becoming more elongated north-south, along with a shortwave
trough quickly moving inland through CA leading to a pathway for 
elevated moisture and embedded shortwaves pivoting out of Sonora 
and nearby Baja to move northward. Ensemble QPF is much greater
compared to the recent D2 time frame thanks to the poleward
advancement of anomalous moisture (+2 to +3 deviation PWATs based
off the latest NAEFS) and subsequent instability increase. The
ascent pattern across the Great Basin is the driver for a better
signal further north of AZ thanks to diffluent flow out ahead of
the shortwave. A widespread signal of 0.5-1" of rainfall at this
lead is indicative to the potential for more widespread convective
impacts across the Southwest and parts of the Inter Mountain West.
As of now, the MRGL in place will suffice, but a targeted upgrade
is plausible considering the expected evolution of the pattern
heading into mid-week. 

Kleebauer
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