FOUS30 KWBC 050828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Thu Sep 5 2024

Day 1
Valid 01Z Thu Sep 05 2024 - 12Z Thu Sep 05 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ALONG THE 
CENTRAL GULF COAST...

...Gulf Coast, Lower Mississippi Valley and Southeast...

In coordination with local forecast offices near the Mississippi
Delta region, a Moderate Risk area was hoisted for this period to 
include portions of southeast Louisiana and southern Mississippi. There
is consensus for heavier rainfall along the central Gulf Coast 
given the bulk of PWs >99th climatological percentile residing 
along the Gulf Coast. The region is also closer to the 
surface-850mb southeasterly flow that will sustain a steadfast 
regime of low level theta-e advection into the nearby stationary 
frontal boundary. Additionally, the latest guidance shows a 
potential for locally higher amounts upwards of 10" per NBM 95th 
percentile to focus in the vicinity of the New Orleans metro and 
points east and south. A Slight Risk spans from the southeast Texas
coast to southern Mississippi.

Deep-layer instability across portions of the Southeast may be 
lacking, however PWs of 2.2-2.4" will make for highly-efficient 
rainfall rates as the quasi- stationary front shifts a bit 
northward by tonight as the upper shortwave trough dips across the
western Gulf Coast. A Marginal Risk spans from extreme southern
South Carolina to central Florida and westward to the central Texas
coast.

...New Mexico and Colorado...

Convection is expected to fire up ahead of an approaching cold
front thanks to PWs topping 0.75" and up to 500 J/kg of MUCAPE.
Soils across this part of the region are sensitive given recent
moisture and burn scars. The Marginal Risk area was maintained for
the localized flash flood potential for the afternoon and evening.

Campbell/Hurley


Day 2
Valid 12Z Thu Sep 05 2024 - 12Z Fri Sep 06 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS 
OF THE CENTRAL AND EASTERN GULF COAST...

...Gulf states and Southeast...

During this period the trough will be digging across the western 
Gulf Coast region and increasing (and persistent) low-level 
frontogenesis along the central-eastern Gulf Coast to the Georgia
Coast. While narrowing with time, the corridor of favorable 
thermodynamics (PWs of 2.2 to 2.5" with tall/skinny CAPE profile) 
and low-level FGEN along the quasi-stationary surface boundary will
favor the more intense short term rainfall rates. This is 
essentially where the Slight Risk was drawn, which also correlates 
well with the highest 24hr QPF exceedance probabilities from the 
NBM. 

While there is some spread on exactly where the highest QPF will
focus there is a general consensus for 1 to 3 inches across the 
region. Local maxes may be closer to with the possibility of very 
isolated maxes of 4 to 7 inches, especially along the coastline.
The NAM, although hedging toward being an outlier solution for 
this period, does show isolated maximums closer to 10 inches. At 
this time, a Slight Risk seems to adequately cover the level of 
threat for excessive rainfall and flooding concerns. There may be 
the need to upgrade with a targeted Moderate Risk along the central
Gulf Coast should guidance continue to trended up.

Campbell/Hurley


Day 3
Valid 12Z Fri Sep 06 2024 - 12Z Sat Sep 07 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE EASTERN GULF COAST AND SOUTHEAST ALONG WITH PORTIONS OF THE NORTHEAST...

...Gulf Coast and Southeast...

Slow moving convection will continue to focus along the draped
frontal system along the Gulf Coast and Southeast U.S. Abundant
Gulf moisture will continue to feed into the region leading to
higher rainfall rates. Multi-day rains will have increased soil
saturation and lowered flash flood guidance by this period thus
there will be a continued threat for excessive rainfall and local
flooding concerns. A Marginal Risk is in effect across northern
Florida and southern Georgia.

...Northeast...

An upper trough/low will pivot across the region during this 
period sending showers and thunderstorms across the Northeast where
multiple recent rains have increased soil sensitivity. A Marginal
Risk area is in effect for eastern New York, northeast
Pennsylvania, northern New Jersey and northwest Vermont.

Campbell
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