FOUS30 KWBC 161235
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
835 AM EDT Fri Aug 16 2024

Day 1 
Valid 12Z Fri Aug 16 2024 - 12Z Sat Aug 17 2024 

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

...Southeast U.S...
Convection across portions of the lower Ohio Valley early this
morning is forecast to reach portions of southern/eastern Kentucky
and Tennessee later today and eventually into northern
Alabama/Georgia. There is lower confidence on how the activity will
hold together or what additional line segments will develop but
with plentiful moisture and sufficient instability in place this
afternoon, several areas of heavier rainfall will be possible.
Based on the latest guidance and 00Z HREF probabilities, pockets 
of 1-2" hourly totals will be possible, most likely across portions
of middle/eastern TN, far southeast KY, and into northern 
AL/northwest GA where the Slight Risk in the Excessive Rainfall 
Outlook exists. Some localized totals of 2-4" will be possible 
within that area and could lead to scattered instances of flash 
flooding. 

...Ohio Valley to Great Lakes...
For today and tonight, a cold front is expected to move eastward
from the Midwest through the Ohio Valley as the main upper level 
shortwave energy digs further into the Great Lakes region. This
combination should provide plenty of support for scattered to
numerous showers and thunderstorms to develop along the advancing
cold front across the Ohio Valley into the Central Appalachians.
The activity should be fairly progressive and quick moving,
limiting the overall duration of any pockets of heavy rainfall at
any one location, but a few spots of merging storm cells or
training could develop that pose a flash flood risk. Further north
into the Great Lakes (Wisconsin/Michigan), storm motions under the
core of the upper trough are likely to be slower and when combined
with the higher PWs and somewhat elevated soil moisture percentiles
(Wisconsin), a few instances of flash flooding will be possible. 

...Southwest U.S...
Increasing monsoonal moisture will begin to lift northward into
portions of the region today, the highest PW anomalies will likely
be tied to far southern Arizona but the 00Z guidance is keying on
enough moisture and sufficient instability developing today
combined with the subtle shortwave lifting through to spark at
least isolated thunderstorms into portions of southern NV and
southern/central New Mexico. Some of the convection will be 
capable of producing intense rain rates and could lead to flash 
flooding over the typical vulnerable locations. 

...South Florida...
A trailing frontal boundary and trough over the region combined
with a very high moisture axis will bring another round of
scattered to numerous thunderstorms to South Florida later today
through the evening hours. The combination of high PWs and
sufficient instability will lead to very intense and efficient rain
producing thunderstorms, capable of producing 2-3" hourly totals at
times. The 00Z guidance is quite bullish for southeast FLorida
coastal areas with potential for some locations to see several
inches today (00Z HREF probs are moderate for 5" and show a slight
signal for 8". The most concerning area is for the highly
urbanized corridor of southeast Florida but also the southwest
coast from near Naples/Fort Myers southward could also see some of
the excessive rainfall from thunderstorms. A Marginal Risk was
introduced this cycle. 

Taylor


Day 2 
Valid 12Z Sat Aug 17 2024 - 12Z Sun Aug 18 2024 

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE EASTERN U.S...SOUTHWEST U.S...SOUTH FLORIDA...AND PACIFIC NORTHWEST...

...Eastern U.S...
The anomalous upper trough and associated cold front will make its
slow progression eastward into the Eastern U.S. Saturday into
Saturday night. Another day of mostly loosely organized convective
segments are expected in the modest axis of higher moisture and
available instability. THe latest guidance points toward areas of
the OHio Valley, Central APpalachians, and Mid Atlantic being the
focus for higher rainfall totals which could total a couple inches.
Based on the environmental ingredients, pockets of 1-2" hourly
totals will be possible where any boundary collisions or storm
mergers allow for a slightly longer duration in rainfall at any one
location but the storm motions overall should be fairly
progressive. However given some of the terrain sensitivities and
urban areas in the risk areas, a few instances of flash flooding
will be possible. 

Further south, the lingering boundary across portions of the
Southeast will again be the focus for another round of mainly
afternoon/evening thunderstorms. Storm motions may be slightly less
and with more robust moisture/instability, some of the rain rates
may be rather intense but short-lived. This may lead to isolated
instances of flash flooding. 

...Southwest U.S...
The return of deeper monsoonal moisture is expected across the 
region, initially late tonight across portions of southern 
Arizona, but by Saturday a plume of higher moisture surges 
north/northeast through the region. This is in response to the 
anomalous upper ridge axis becoming centered over eastern New 
Mexico and western Texas while a strong upper trough approaches the
coast of California. This will set up a favorable flow of this 
higher moisture in between across much of Four Corners region 
during the period. PW anomalies reach +2 to +2.5 standard 
deviations above normal during the period while the latest model 
guidance is suggesting a subtle shortwave trough rippling through 
the flow, which will act to enhance the forcing for ascent across 
the region. Combined this with the expected instability during peak
heating and the set up will become quite conducive for scattered 
daytime to evening showers and thunderstorms. The latest guidance 
is favoring northern Arizona and much of Utah for the greatest QPF 
and potential for higher rain rates, but the threat will exist 
across the larger region. Isolated to widely scattered instances of
flash flooding will be possible, particularly for the more 
vulnerable locations and a Marginal Risk covers the region.

...Pacific Northwest...
There is a increasing signal for heavy downpours across portions 
of the Washington and Oregon Cascades. During this period there 
will be and approaching trough along with a steady influx of PW 
values of 1+ inches. Some of the hires guidance project convection 
to spread/track northward along the terrain with hourly rain rates 
upwards of 0.75 inch/hour. In collaboration with the local forecast
office a Marginal Risk area was raised to highlight the elevated 
threat for excessive rainfall and the potential impacts leading to 
debris flows on the recent burn scar areas from active wildfires. 

...South Florida...
Another round of deep convection capable of producing flash
flooding is expected on Saturday, thanks to the lingering trough
over the region. While the 00Z guidance isn't as robust for Day 2
as it is for Day 1 (and perhaps a bit to the south of the major
urban areas), the repeating rounds / cumulative effect may bring a
few round of flash flooding to the area again on Saturday/Saturday
evening.

Taylor


Day 3 
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...
By Sunday, the timing of the cold front and upper trough is
expected to allow for another round of scattered to numerous
showers and thunderstorms across the Mid Atlantic and portions of
the Northeast. Based on the latest guidance, a focus area of
locally heavy rainfall due to the combination of favorable
dynamics, moisture (PWs near 2 std deviation above normal), and 
potential for training/backbuilding is setting up across portions 
of urban corridor from near Baltimore through southern New York. 
This is where the potential exists for a few rounds of heavier 
rainfall and given the anomalous moisture in place, pockets of 
intense rain rates will be possible over the more urban areas. Some
areas may pick up a quick couple of inches. With this in mind, a 
Slight Risk was introduced for portions of the area where 
scattered instances of flash flooding will be possible.

...Southwest and Intermountain West...
A continuation of the deep monsoonal moisture and daytime to
evening showers and thunderstorms are expected across much of the
Southwest U.S. and into portions of the Intermountain West Sunday
into Sunday evening. Convection will be capable of producing hourly
totals up to 0.75" at times, particularly from northern Arizona
through Utah where the better instability is forecast to setup.
This will bring another day of localized to widely scattered
instances of flash flooding for the more vulnerable locations in 
the region. 

Taylor
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