811
FOUS30 KWBC 091557
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Thu May 9 2024

Day 1
Valid 16Z Thu May 09 2024 - 12Z Fri May 10 2024

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

...16Z Update...

The expected evolution synoptically has not changed from the
previous forecast with the primary adjustments in the SLGT risk
made to account for the precip that has already occurred, as well
as a further west expansion over TX for the trends in short term
guidance. The area out by I-35 is becoming more likely to see
scattered heavy convection later this afternoon with the primary
impacts aligning with the urban corridors from the DFW metro down
through Austin and surrounding Hill country. Initiation will
transpire along a tight theta-E gradient present within the
confines of a stationary front bisecting the northern portion of
the state. There`s a significant difference in available
instability to the north and south of the boundary with a solid
3500-4500 J/kg of MLCAPE along and south of the stationary front
with a precipitous drop-off as you move north into the Red River.
Textbook surface convergence signal is present when assessing the
latest mesoanalysis with the 12z sounding out of DFW presenting a
"low-hanging fruit" for a convective temp (82F) that will easily be
eclipsed later today. Already seeing a rouge cell spur up over
north-central TX as the leading edge of mid-level vorticity creeps
north out of the sub-tropical jet with the primary shortwave
expected later this afternoon.

Downstream over the Southeast U.S, our first round of convection is
ongoing thanks to the propagation of a MCS that spawned yesterday
evening across TN with heavy rain well documented within the
complex path. This is the ground work for some areas within the
corridor from central AL into GA that will see the best prospects
for flash flooding due to priming of the soils from the 1.5-3" of
rain with the first system. The aforementioned shortwave ejecting
out of the sub-tropical jet will be the focus of a secondary MCS
development this evening with a path continuing to the east along
the remnant outflow boundary present from the current convection in
the Deep South. The flow will run parallel to the boundary leading
to increasing chances for training and redevelopment, even in wake
of the future MCS as it makes progress through LA-MS-AL. Latest
HREF probabilities have remained fairly consistent to the previous
iteration of the ensemble with a bullseye for highest total QPF
this period residing over eastern AL into west-central GA due to
the back-to-back nature of impact from the morning and evening
MCS`s. The neighborhood probability field for at least 3" of
precip from the primary MCS this evening leaves a train of 50-70%
readings from LA all the way into GA by the end of the period. This
correlates well with the rate exceedance probabilities for 2"/hr
and 3"/hr that have spiked from previous runs, both running between
40-70% and 15-25%, respectively over the same area mentioned
above. The elevated probabilities were more than enough reason to
maintain the previous SLGT risk with expansion a bit further to the
north/south based on the forecasted blended mean output from the
HREF, and the bias corrected QPF output from this morning. The risk
is on the higher end of the Slight Risk threshold over a large
portion of the Deep South with the best chance residing in central
LA/MS/AL/GA.

Further north over the Midwest to the Mid-Atlantic, low-end risk
for flash flooding continues with anticipated convective
development for later this afternoon and evening thanks to the
prevalent upper forcing and modest instability within the boundary
layer. The best chance for flooding will likely reside over the
central Appalachians where 1" HREF EAS probabilities are still
hovering within the 10-15% range with area FFG`s remaining fairly
low due to all the recent precip and terrain. The MRGL risk will
likely fall off for the Midwest in later updates as the primary
chances reside over the next 6-8 hrs before dropping off after
sunset.

Kleebauer


...Previous Discussion...

...East Texas-Lower MS Valley-Southeast...
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