FOUS30 KWBC 031328
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
928 AM EDT Tue Sep 3 2024

Day 1
Valid 16Z Tue Sep 03 2024 - 12Z Wed Sep 04 2024

....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL AND SOUTH TEXAS...

...13Z Outlook Update...

...Texas...
Slight risk areas were combined and expanded to include more of 
south Texas (near the Del Rio area). This morning, slow-moving 
convection has materialized and produced areas of 1-2.5 inch/hr 
rain rates just east of the Rio Grande. These storms are in a 
sufficiently moist/unstable enviornment with weak flow aloft for 
continued heavy rainfall. Low FFG thresholds (near zero in spots)
were also noted. The flash flood threat should persist for most of
the D1 forecast period under this regime. Reference MPD #0968 for 
additional mesoscale details.

...Southeastern Mississippi eastward to northern Florida...
Latest CAMs indicate a robust axis of convection materializing amid
weak flow aloft from the Pine Belt of Mississippi eastward to the
Jacksonville Metro area. HREF probabilities of 24-hr, 3 inch+  
precipitation totals maximize near Jacksonville, where models 
depict a persistent fetch of deep convection originating along a 
frontal boundary over the Gulf Stream. Multiple rounds of 
precipitation are expected today through the early overnight 
hours especially across northeastern Florida. Localized flash 
flood potential is expected where rainfall can occur over 
sensitive/urbanized locales. The Marginal risk has been expanded 
northwestward and eastward to include areas of isolated flash flood
potential.

The remainder of the forecast is unchanged. Refer to the previous
discussion below for more details.

Cook

...Previous Discussion...

...Texas...

The mid-level vort is expected to exit the region although
precipitable water values will remain high. Flooding issues will be
possible in the area with a widespread 1 to 1.5 inches forecast
with locally higher amounts. A Slight Risk remains in effect for
central portions of the state although it was expanded y 1 tier of
counties to the south to reflect the latest trends in the guidance.
A slow moving tropical low will continue to bring periods of heavy
rain in the form of slow- moving showers and thunderstorms from
the Gulf into the adjacent Texas Coast. Most areas will not see
steady rain the entire time, but very localized areas of training
storms resulting in flash flooding. The heaviest rains in the
state are expected to be along the Gulf Coast from Galveston south
and west through Corpus Christi. Locally heavy rains to 2 inches
per hour will be possible.

...Mississippi and Alabama Gulf Coast...

The Marginal Risk was maintained for the potential of slow moving
and training storms to the east of the low along a stalling front
into coastal Mississippi and Alabama. The abundance of atmospheric
moisture and instability, as well as, the slow-moving nature of
the synoptic systems increases confidence that an isolated flash
flood or two could develop along this stretch of coast.

...Idaho/Montana...

There is potential for isolated to scattered heavy rainfall as an
advancing shortwave encounters an anomalously moist airmass. PW
values rising to near 1 inch will support storms capable of heavy
rain that may produce isolated flash flooding should the storms
impact particularly flood sensitive or more urban areas.

Campbell


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

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE MISSISSIPPI
DELTA REGION AND ADJACENT GULF COAST...

...Gulf Coast and Lower Mississippi Valley...

The surface front will continue to be draped west to east across
the South, albeit further south than previous forecast cycles. This
frontal boundary will provide focus for showers and thunderstorms
to spread across the entire Gulf Coast from south Texas all the
way to the Florida Peninsula on Wednesday. Most of the guidance
shows 1 to 3 inches will concentrate over this part of the region
thus increasing the risk for excessive rainfall and local flooding
potential.

There is the potential for a few locations for local
maximums to exceed 6 inches near northern Louisiana/Arkansas.
Onshore flow across southeast Louisiana and southern Mississippi
could yield 1 to 4 inches as well. The focus for the heaviest rains
will be in the central Gulf Coast from Louisiana east through the
Florida Panhandle. Locally heavy rains are likely, especially from
New Orleans east through Pensacola, where a higher-end Slight is in
place. With increasing amounts of rain forecast in and around New
Orleans, continued increases in rainfall there may require an
eventual targeted Moderate Risk should trends continue.

Given the trends, the back edge of the Marginal Risk was trimmed
eastward toward the Texas coast in addition to reducing the
northern boundary by 1 tier of counties across Alabama and
Mississippi.


Campbell/Wegman


Day 3
Valid 12Z Thu Sep 05 2024 - 12Z Fri Sep 06 2024

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

...Gulf Coast and Lower Mississippi Valley...

The area of precipitation mentioned for the Day 2 period will
shift eastward across the central Gulf Coast with the heaviest QPF
still focusing across portions of southeast Louisiana, southern
Alabama, southern Mississippi and clipping western portions of the
Florida panhandle. Again, there was a notable southern shift in
placement within a majority of the guidance from previous
cycles.coastlineThe inherited Marginal Risk areas was removed
from southern Tennessee and northern portions of Alabama,
Mississippi and northwest Georgia while trimmed eastward across
central Louisiana. The Slight Risk spans along the Gulf Coast from
west-central Louisiana to eastern Choctawhatchee Bay Florida.

Campbell
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