633
FOUS30 KWBC 090807
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
407 AM EDT Thu May 9 2024

Day 1
Valid 12Z Thu May 09 2024 - 12Z Fri May 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN TEXAS
THROUGH THE SOUTHERN APPALACHIANS AND SOUTHEAST U.S...

...ArkLaTex-Lower MS Valley-Southeast-Southern Appalachians...
Northern stream upper shortwave pivoting through the Midwest and
Ohio Valley will lead to increased upper level confluence across
the Lower OH Valley-Mid Atlantic regions Thu-Thu night. The ensuing
90-100 kt upper level jet streak will lead to a more focused
(albeit transient) area of divergence aloft/deep-layer ascent
within the right entrance region of this jet streak, along with
strengthening low-level frontogenesis as the cold front becomes
more west-east oriented.

The uptick in dynamical forcing, coinciding with the strong low-
level moisture pooling into the approaching front (850 mb wind and
moisture flux anomalies between 3 and 4 standard deviations above
normal per the GEFS and SREF), and robust deep-layer instability
ahead of the front (mixed-layer CAPEs peaking between 2000-3500+
J/Kg), will lead to more widespread convection within the Slight
Risk area. Scattered stronger cells will produce more intense
rainfall rates, especially along and south of the I-20 Corridor,
where the latest HRRR hourly QPF trends and HREF exceedance
probabilities suggest up to 2-2.5"/hr rainfall rates. In addition,
as the front becomes more west-east oriented, the low-level inflow
is likely to become more parallel to the overall storm motions and
this could favor some repeating rounds or training convection,
again particularly along and south of I-20 in LA-MS-AL-GA. The
environmental ingredients will support isolated 3-5+" totals in a
relatively short period of time.

...Central Appalachians to the Mid-Atlantic...
Low pressure is expected to track from the lower Ohio Valley to
the eastern Great Lakes over the forecast period as shortwave
trough energy aloft and height falls over the region. A strong
southwesterly flow in the low levels will bring higher moisture,
characterized by dewpoints in the upper 50s to lower 60s and PWs
above 1", especially from the Central Appalachians and southern
areas of the Northeast. With the uncapped and warm/moist
environment, there should be scattered to numerous showers and
thunderstorms that develop that will be capable of producing
isolated/localized rain rates in excess of 1-1.5"/hr at times. This
could cause isolated flash flooding, particularly for some of the
areas that have seen recent heavy rainfall and more saturated
ground conditions as well as urban locations.

...Midwest...
Deeper moisture pooled along an inverted trough/TROWAL slowly
moving through the region will be the primary focus for embedded
heavy rain cores that may lead to flash flooding. Tall/skinny CAPE
profiles, an axis of higher/deeper moisture, and warm cloud depths
will promote efficient rain producing showers and thunderstorms
that could total 1-2" in spots and a Marginal Risk was introduced
for portions of this region.

Hurley/Taylor


Day 2
Valid 12Z Fri May 10 2024 - 12Z Sat May 11 2024

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

Only subtle changes were made to the Marginal Risk outlook area
from yesterday`s Day 3 ERO. The advancing frontal boundary across
the Southeast U.S. will be the primary focus for deep convection
Friday into Friday afternoon and will be capable of producing rain
rates in excess of 1-1.5"/hr at times. Showers and thunderstorms
are expected to be ongoing at the start of the period, lingering
from the deep, organized convective line from the previous day.
This activity will be in the presence of a sufficiently high
moisture pool, characterized by dewpoints well into the 60s and low
70s and a PW axis of 1.5-1.75". A subtle shortwave trough noted in
the flow will be the main large scale forcing mechanism to drive
the convection that looks to track through southeast Alabama,
southern Georgia, northern Florida and then eventually off the
Southeast coast. Some localized heavier rain totals and rain rates
will be possible that could pose an isolated flash flood risk.

Hurley/Taylor


Day 3
Valid 12Z Sat May 11 2024 - 12Z Sun May 12 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hurley


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt