285
FOUS30 KWBC 100818
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM EDT Fri May 10 2024

Day 1
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....

...Summary...
Multiple rounds of convection ahead of a slow moving cold front
will pose a Marginal Risk of excessive rainfall today across parts
of the Southeast. Very minor changes were made to the outlook area
from yesterday`s Day 2 ERO.

...Southeast AL-Southern GA-Far Southern SC-FL Panhandle-North FL...
The advancing frontal boundary across the Southeast U.S. will be
the primary focus for deep convection Friday into Friday afternoon.
Unseasonably robust upper level jet streak (110-130kts at 250 mb)
will traverse the Mid Atlantic region Friday. This would put the
outlook area in the right-entrance region of jet streak for a
little while, allowing for an uptick in divergence aloft/large-
scale ascent along with low-level frontogenesis ahead of the
approaching cold front. Into the early evening hours, pre-frontal,
mixed-layer CAPEs will average between 1500-2500 J/Kg over most
areas per the guidance, which along with the anomalous PW profile
(1.75-2.00"), will support isolated 2-2.5"/hr rainfall rates
underneath the strongest cells. Simulated reflectivity forecasts
per the CAMs do show the potential for some cell training; however,
they all show fairly fast cell motions along with quick forward
propagations of more organized clusters per the Corfidi Vectors. As
a result, while the probabilities of 3" in 6 hours are fairly
decent (spotty 30-40+ probs per the HREF and RRFS), the
probabilities of 5+ inches drops precipitously to 10% or less.
Therefore expect some localized heavier rain totals and rain rates
to pose an isolated flash flood risk today within the Marginal Risk
area.

Hurley


Day 2
Valid 12Z Sat May 11 2024 - 12Z Sun May 12 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE SOUTHERN HIGH PLAINS INTO SOUTH-CENTRAL TEXAS...

...Summary...
Vigorous upper level shortwave trough moving slowly across the
Four Corners region and southern Rockies will lead to a quick
influx of deep-layer forcing and moisture across the southern High
Plains in eastern NM, much of the TX Cap Rock-Rolling Plains, and
into the TX Hill Country/South-Central TX. The Marginal Risk of
excessive rainfall from yesterday`s Day 3 ERO was expanded across
these areas based on the latest guidance trends, where the
potential exists for isolated instances of flash flooding.

...Eastern NM into portions of Western and Central TX...
Ahead of the approaching upper shortwave trough, a gradual uptick
in difluence aloft will ensue across the outlook area.
Strengthening low-level moist S-SE inflow will also take place Sat-
Sat night along and east of the dryline, as 850 mb moisture flux
anomalies peak between 2-3 standard deviations above normal.
Convection forming off the terrain will obtain quite a bit of
forward propagation given the strong deep-layer shear profile,
therefore cell training is not a concern per the simulated
reflectivity guidance form the models. However, available deep-
layer instability (MUCAPEs at least ~1000 J/Kg over the northern
portion of the Marginal area...1500-2500+ J/Kg south) along with
PWs averaging 2 standard deviations above normal will favor maximum
rain rates of 1-1.5"/hr and 1.5-2.0"+/3hr underneath the strongest
cores. Supported by the latest CSU ERO first guess guidance,
runoff from these stronger storms will pose an isolated or
localized risk for flash flooding, even with the antecedent dry
soils over parts of the outlook area.

Hurley


Day 3
Valid 12Z Sun May 12 2024 - 12Z Mon May 13 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
CENTRAL AND EASTERN TEXAS INTO MUCH OF LOUISIANA AND SOUTHWEST
MISSISSIPPI...

...Summary...
Mid-upper level trough traversing the Central-Southern Plains
north of the Red River will lead to a surge of deepening
instability and moisture from the western Gulf of Mexico as a warm
front pushes into eastern TX and southwest LA. Favorable dynamic
and thermodynamic profiles will result in widespread convection
across central-eastern portions of TX into parts of the Lower MS
Valley, where a high-end Slight Risk of excessive rainfall is
anticipated Sunday and Sunday night.

...Central-Southern Plains and Portions of the Lower Mississippi
Valley to the Mid-Upper TX and western LA Gulf Coast...
For now at least, just a modest kinematic response is noted from
the guidance during day 3 ahead of the the approaching shortwave.
850 mb wind anomalies for instance are currently modeled to be
around 1-1.5 standard deviations above normal (while at the same
time 850-700 mb moisture flux anomalies top out between +2-2.5).
The absence of a a more robust subtropical jet streak is the likely
culprit; however, the presence of the 90-100 kt upper jet streak
traversing the Southern Plains and TN Valley will be felt by way of
subtle right-entrance region forcing (focused area of upper
divergence/deep-layer ascent and low-level frontogenesis). This
will enhance the fgen along the leading edge of the surface warm
front, while also retarding it`s east-northeastward motion. Largely
elevated, convection will become more widespread downwind of the
warm front in a favorable deep- layer warm/moist advection pattern
off the western Gulf of Mexico. The lingering low-level
frontogenesis/slow progression of the surface front will allow for
some cell training, especially across parts of central-eastern TX
which had received quite a bit of rain over the past week. Over
this region eastward through central LA and southwest MS, feel the
ERO risk will be more of a "high end" Slight (i.e. at least 25%
risk of rainfall exceeding FFG within 25mi of a point). 00Z models
show pockets of 3-5+ inches of rain during day 3 within this area,
with the past 2 ECMWF and UKMET runs consistently wetter than the
other models.

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