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

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