FOUS30 KWBC 010838
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
438 AM EDT Sun Sep 1 2024

Day 1
Valid 12Z Sun Sep 01 2024 - 12Z Mon Sep 02 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NEW MEXICO/TEXAS, THE NORTHERN GULF COAST, AND THE MID-ATLANTIC...

...Southeastern New Mexico and West Texas...

An approaching 250 mb upper trough will move into far West Texas
today while a surface boundary lingers across the Hill Country.
Precipitable water values are forecast to be around 1.5-1.75" which
is about +1 to +2 sigma with the typical NW to SE gradient.
Afternoon convection should percolate today and could fall over
areas that have seen a few inches of rain on Saturday (with lower
FFG values as a result). Coverage should be somewhat isolated so
the Marginal Risk will suffice.

...Gulf Coast...

Meandering weakness in the mid-lower levels will again help drive
some heavier rainfall along the Upper TX Coast into southern
Louisiana. Maintained the Marginal Risk as the bulk of the guidance
keeps the heaviest rain offshore, though it will be close (again)
just like on Saturday.

...Eastern Tennessee Valley and Mid-Atlantic Region...

A cold front will approach the Appalchians this afternoon as the
lead boundary dissipates, leaving a trough over the coastal plain.
The best forcing for afternoon convection will lie there, generally
over far southeastern Virginia into central/eastern North Carolina.
00Z HREF probs of exceeding FFG values are around 10-20% (via 1-2
inch per hour rates), though there are hints at some higher rates
still. Activity should be mostly progressive, suggesting a
localized threat of flash flooding. Kept the Marginal Risk for this
area.

Fracasso


Day 2
Valid 12Z Tue Sep 03 2024 - 12Z Wed Sep 04 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
HILL COUNTRY OF TEXAS...

...Texas...

Mid-level vort and upper trough will move into West Texas on
Monday atop increased moisture from the southeast. Guidance
continues to show modest QPF of 1-5 inches near and around the Hill
Country, with various foci in the deterministic models. ECMWF-AIFS
was still decidedly north compared to the other models. The CSU
machine learning first guess fields still suggest a broad Slight
Risk over the Hill Country, so at least there is some consistency
over the past 24-36 hours in this guidance vs the models (which is
the point). FFG values are lowest near/south of San Angelo to Del
Rio (due to recent rain) where the HREF probs of exceeding these
values reach ~50%.

...Carolinas...

Cold front will sink farther into the Carolinas from day 1, which
will nudge the excessive rainfall threat south as well. Best
dynamics move offshore with the trough axis so the Marginal Risk
should cover any remaining isolated flooding threat due to
afternoon convection.

Fracasso


Day 3
Valid 12Z Tue Sep 03 2024 - 12Z Wed Sep 04 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR TEXAS...

...Texas...

Mid-level vort will be exiting the region though precipitable
water values will remain high (+1 to +2 sigma initially, then
decreasing late Tuesday). Depending on how much (and where) rain
falls on day 2, there is room to upgrade some of this region to a
Slight Risk pending better model agreement in future runs. For now,
ensemble consensus is just east of the Hill Country but individual
models are quite varied north/south. First guess fields from the
CSU machine learning algorithm do show a Slight Risk outline,
lending credence to a future upgrade.

Fracasso
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