FOUS30 KWBC 150800
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
400 AM EDT Thu Aug 15 2024

Day 1
Valid 12Z Thu Aug 15 2024 - 12Z Fri Aug 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID MISSISSIPPI VALLEY AND LOWER OHIO VALLEY...

...Mid MS Valley to Lower Ohio Valley...
The active weather associated with the large troughing over the
Northern Plains and UPper Midwest this morning will move eastward
today. At the start of the period, convection should be ongoing
across portions of Missouri into Illinois and portions of Iowa, 
tied with the overnight convective line segments. This activity 
should be both relatively progressive and also weakening with the 
loss of the nocturnal low level jet and less favorable influx of 
moisture. However, a few pockets of heavier rainfall and rain rates
will be possible that could total a couple/few inches through mid 
afternoon. 

Further south/southwest, outflow boundaries and the approaching
cold front from the west/northwest will help spark an additional
round of showers and thunderstorms late this afternoon into the
late evening/overnight hours, most likely centered over portions of
southern/central Missouri to southern Illinois, far southwest
Indiana, and portions of western Kentucky. Here, the setup is much
more favorable for 1) robust deep convection, 2) 
training/backbuilding storms with the flow becoming more parallel 
to the storm motions, and 3) an impressive evening/night low level 
jet of 30-35 kts impinging on the boundary in place. 

THe 00Z HREF neighborhood probabilities indicate potential for 
some intense rain rates with the storms, where the 2" hourly total 
probs reach 30 percent for several hours this evening/overnight 
with a slight signal for 3" hourly totals at times. Overall, 
isolated totals of 3-5" are possible with some localized higher 
amounts not out of the question based on the 00Z hi-res guidance 
and the HREF showing a few areas of 25 percent for 5" over the 24 
hour period and a near 15 percent signal for 8" totals. 

For the Day 1 Excessive Rainfall Outlook, still messaging a Slight
Risk for this area for the potential of isolated to scattered 
instances of flash flooding, especially if the heaviest amounts 
fall over portions of southern Missouri where soil moisture is 
already elevated due to recent heavy rainfall events. 

Taylor


Day 2
Valid 12Z Fri Aug 16 2024 - 12Z Sat Aug 17 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE GREAT LAKES TO LOWER OHIO VALLEY...

...Great Lakes to Ohio Valley...
By Friday, the cold front and associated upper level troughing will
shift eastward toward the Great Lakes and Ohio Valley. Expect
another day of scattered to numerous showers and thunderstorms as a
result of the higher moisture ahead of the front along with
favorable instability developing during the peak of the daytime
heating. The set up does favor faster/progressive storm motions but
a few repeating rounds or backbuilding storms will be possible so a
few instances of flash flooding can't be ruled out so the Marginal
Risk remains in place for the area. 

...Mid MS Valley...
Late in the forecast period, the latest guidance is keying on a
subtle shortwave trough embedded in the northwest flow between the
upper ridge over Texas and the large troughing over the Great
Lakes. With the nocturnal low level jet reaching 25 to 30 kts
impinging on the surface boundary expected to be draped across
portions of Kansas and Missouri, there is increased likelihood of
convection developing early Saturday morning. While confidence in
exact placement is a bit higher than normal, the setup and
environmental ingredients suggest potential for some localized 
heavy rainfall and this is showing up in the various deterministic 
and ensemble probabilities this cycle. Isolated/localized flash 
flooding will be possible. 

Taylor


Day 3
Valid 12Z Sat Aug 17 2024 - 12Z Sun Aug 18 2024

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

...Southwest U.S...
The return of deeper monsoonal moisture is expected across the
region, initially late Friday night 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.

...Mid Atlantic to Northeast...
A pronounced upper trough is forecast to move from the Great Lakes
region at the start of the period to the Upper Ohio Valley and
Central Appalachians by 12Z on Sunday. At the surface, a cold front
is expected to pass through with a ribbon of higher moisture
surging ahead of it. The combination of the favorable forcing for
ascent and higher moisture, along with the daytime heating
instability, should be sufficient to bring scattered to numerous
showers and thunderstorms to the region. While the setup should
favor relatively progressive storm motions, some areas could see a
few repeating rounds or backbuilding storms on the
southern/southwest flank of any line segments that may bring
locally higher rainfall totals. Based on the latest deterministic
and ensemble guidance plus ML first guess fields, a broad Marginal
Risk is advertised for the region for localized/isolated flash
flooding. 

...Southeast...
The advancing cold front will become more west to east oriented
across the Southeast during the period. The moisture profiles in
the region will remain elevated (around 1.5 std deviations above
normal) and with robust daytime heating and instability, strong
thunderstorms are forecast to develop along/ahead of the frontal
boundary then quickly move to the east/southeast. A
localized/isolated flash flood threat exists and the Marginal Risk
was extended southward to account for pockets of heavy rainfall
totaling a few inches in places over a relatively short period of
time. 

Taylor
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