FOUS30 KWBC 100746
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
346 AM EDT Sat Aug 10 2024

Day 1
Valid 12Z Sat Aug 10 2024 - 12Z Sun Aug 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CAROLINAS INTO SOUTHEASTERN VIRGINIA...

...Carolinas & Virginia...

A stalled out front over the western Carolinas and central Virginia
will act as a focus for storms to develop this afternoon on the
warm/moist southeastern side of the front. The atmosphere will be
characterized with plentiful deep moisture with PWATs over 2 inches
across much of the region. HRRR guidance shows several clusters of
storms forming across southeast Virginia and the central Carolinas
late this afternoon. The storms will then track southeastward
towards the coast into an area where instability could exceed 4,000
J/kg along the coast. This level of instability and moisture will
support potential for storms to produce rainfall rates to 3 inches
per hour. Fortunately the guidance suggests the storms will keep
moving into the evening, but those heavy rainfall rates would have
the potential to cause flash flooding even if antecedent conditions
weren't so favorable.

Debby's remnants tracking across this region over the last few days
has led to very saturated soils all across Virginia and the
Carolinas. With numerous streams, creeks, and rivers already well
into flood stage, additional rainfall from storms Saturday
afternoon will struggle to drain, resulting in additional flooding
and prolonging ongoing flooding. In coordination with all of the
impacted offices, a Slight Risk area was introduced with this
update.

00Z HREF guidance peaked at over 50% chance of exceeding 6 hour 
FFGs in the Slight Risk area, with an over 70% chance of exceeding 
3 inches of rain across much of eastern North Carolina. These
values added to the confidence to upgrade the ERO risk. 

...Rockies into the Plains...

An approaching positively tilted shortwave trough moving into
California will increase lift to its east across the Four Corners 
region. Storms are likely to be widely scattered across this area,
which would support an isolated flash flooding instance or 2,
currently covered with the large Marginal Risk. Additional moisture
in southern Arizona and lift into the Peninsula Ranges of southern
California may cause local flash flooding issues, but the guidance
is inconclusive at best as to where any potential Slight risk area
could be drawn. In coordination with the impacted offices, have
opted to hold at a Marginal for now, but will reevaluate each day's
chances for flooding based on the previous day's convective 
behavior.

MCS development with potential repeating storms in the Plains will
be a concern across Kansas through Oklahoma late Saturday night.
Antecedent conditions are quite dry for soil moisture across this
area, and the CAMs are not in great agreement on the location and
behavior of the storms, despite ample moisture available. Here too
a Slight Risk is close, and may need to be considered with future
updates.

Wegman


Day 2
Valid 12Z Sun Aug 11 2024 - 12Z Mon Aug 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE CAROLINAS
AND FOR PORTIONS OF THE OZARKS AND CENTRAL PLAINS...

...Carolinas...

The stalled out front over the Carolinas will continue to draw deep
tropical moisture northward up the Southeast coast. The front will
have made enough progress to likely keep the rainfall down into
southeastern Virginia, but there will likely be more storms into
South Carolina. Thus, the Slight risk is just a bit south of the
Day 1/Saturday Slight Risk. Nonetheless with ongoing flooding from
Debby, additional rainfall, even if more widely scattered, will
have an outsized potential for impacts from additional flooding.
00Z HREF guidance through 00Z Monday suggests additional elevated
potential for exceeding FFGs in NC, and plentiful moisture and
instability with a stalled out front providing the forcing will
likely help organize the storms, increasing their flooding
potential. 

...Central Plains/Ozarks...

MCS development on the leading edge of a low level jet (LLJ) is
expected from southeast Kansas through northwest Arkansas. Despite
recent dry weather, training convection with PWATs increasing to
around 2 inches will support storms capable of rates to 3 inches
per hour at times. This very heavy rainfall combined with terrain
interactions in the Ozarks will favor widely scattered instances of
flash flooding. In coordination with the impacted offices, a 
Slight Risk upgrade was introduced with this update. Given CAMs
frequent trouble with correctly simulating storms with MCSs, there
are likely to be both temporal and intensity adjustments with the
expected rainfall. Despite this potential, there was good enough
agreement in the guidance to draw in the Slight risk, which for now
remains low-end given the recent dry weather and dry soils. 

...Southwest...

More widespread convection is likely to impact the Four Corners
region Sunday afternoon. The shortwave that will start the day in
northern California will continue eastward across the Intermountain
West. Thus, it appears the best potential for storms capable of
flash flooding will be across central UT and western CO, generally
north of the UT National Parks. Nonetheless with better moisture
further south, AZ and northwestern NM will not be out of the woods,
and isolated flash flooding will remain possible there. Storms are
also likely along the Peninsular Ranges of southern California
again Sunday afternoon, where a small Marginal risk was introduced.

Wegman


Day 3
Valid 12Z Mon Aug 12 2024 - 12Z Tue Aug 13 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE FOUR CORNERS REGION, CENTRAL PLAINS, OZARKS, AND THE CAROLINAS...

Ongoing rainfall from Sunday night may continue into Monday morning
across southern Missouri. With only a few hours of rain in the
morning, the Marginal across Missouri was maintained. However,
should rainfall from Day 2/Sunday overperform, then a future Slight
may be needed. Once again another round of storms Monday afternoon
across the Four Corners region is likely to result in isolated
flash flooding. However, there is greater uncertainty with
coverage by Monday in this region with lesser forcing than on
Sunday.

For the Carolinas, lingering moisture with the same front may cause
yet another round of storms Monday afternoon into the evening. With
greater potential that the bulk of the activity will be offshore by
Monday afternoon, the previously inherited Marginal risk remains
largely unchanged.

Wegman
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