Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1146 AM EDT Sun Oct 01 2023

Day 1
Valid 12Z Sun Oct 01 2023 - 12Z Mon Oct 02 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
EAST COAST OF THE FLORIDA PENINSULA...

...16Z Update...

...Florida...

Only a few minor tweaks were made to the ongoing Slight Risk area
across the east coast of Florida this afternoon. A very slow
moving front is finally allowing the associated convection near
Cape Canaveral to begin to drift southward...a trend that is
expected to continue through the day. Meanwhile, convection just
off the coast of Miami should begin to redevelop over the mainland
as diurnal heating locally increases surface-based instability.
Once again due to lack of forcing, any storms that form will be
nearly stationary, resulting in localized high rainfall totals.
Much of the east coast of FL has been at or above normal for
rainfall over the last couple weeks, so soils are generally at or
near saturation, especially when adding the urban factor. Thus,
the Slight Risk remains in place. The Slight was nudged northward
to account for ongoing heavy rainfall near Cape Canaveral, and was
also stretched southward to include more of Miami and its southern
suburbs as well. The southward expansion is due to the
aforementioned expectation for afternoon and evening convection to
develop over urbanized areas.

There remains considerable uncertainty as to how far inland/west
the rainfall will extend from the coast, but there is good
agreement that it will not be as far west as the west coast, so
the Marginal Risk was trimmed out of the Ft. Myers/Naples area, as
any heavy rain in most of the guidance is likely to form southeast
of there over the Everglades, and thus not pose a flash flood
threat.

...Eastern NM/Portions of the TX and OK Panhandles...

No significant changes were made other than a small northeastward
extension of the Marginal Risk area to include more of the TX/OK
Panhandles based on the latest guidance trends. Otherwise the
afternoon through overnight showers and storms are still expected
in the Marginal Risk area, though only isolated flash flooding is
expected as a result.

Wegman

...Previous Discussion...

...Florida...

We've upgraded the Florida Marginal to a Slight Risk. There's been
a consistent signal for heavy rain to occur over central/southern
Florida's Atlantic coast for a couple days now, but the signal has
increased significantly over the last few runs. PWATs will be well
over 2 inches with some shallow instability. The presence of a
surface front will allow for thunderstorms to continuously develop
and dissipate through the afternoon. HREF 24 hour exceedance
probabilities of 5" are quite high as well.

...New Mexico/west Texas...

The deep diffluent pattern over the Great Plains will continue to
generate convection in the Southern Plains on Sunday. Instability
around 1000J/Kg and PWATs of over 1" could support some instances
of flash flooding this afternoon and evening. The global models
appear to be well clustered over the Southern High Plains as well,
however the GFS appear to suggest that the heaviest rainfall will
be scattered across the marginal risk area, so an upgrade is not
necessary at this time. Ensemble mean exceedance probabilities of
1" and 2" in 24 hours including the 00z GEFS are well clustered
over the northeastern New Mexico and the Texas panhandle.


Kebede


Day 2
Valid 12Z Mon Oct 02 2023 - 12Z Tue Oct 03 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN HIGH PLAINS OF NEW MEXICO AND TEXAS...

...Southern High Plains of New Mexico and Texas...

Confidence has increased enough to warrant an upgrade of the New
Mexico/west Texas Marginal to a Slight Risk. The Southern High
Plains will be under the influence of deep diffluent flow from a
potent upper trough propagating through the West on Monday. This
will be coupled with the arrival of a strong upper jet aloft and
favorable low level jet dynamics. Cape values in the 1000-1500J/Kg
range and moisture anomalies well over 2 standard deviations
(1-1.5in") should support thunderstorm activity (some severe) with
excessive rainfall potential.

...Florida...

Albeit weaker than today's threat, Monday's heavy rainfall threat
will shift to southern Florida as the surface front, which will
continue to be the focus for storms, shifts southward. Instability
will weaken on Monday so any excessive rainfall threats will
likely be few and far between. The main threat from potential
heavy rainfall will be over urban areas.


Kebede

Day 3
Valid 12Z Tue Oct 03 2023 - 12Z Wed Oct 04 2023

,...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS
OF THE GREAT PLAINS...


A rapidly developing mid-latitude cyclone, supported by a deep
upper-level trough will produce thunderstorms (some severe) and
moderate to heavy rainfall across the Great Plains on Tuesday.
Confidence has diminished a bit since the last issuance regarding
excessive rainfall over parts of the Northern Plains, therefore
the Slight Risk has been removed. The broad Marginal area remains
due to the potential for excessive rainfall, particularly over
parts of the Central Plains where favorable instability within
potential severe storms could enhance rainfall rates. PWATs will
be well over 1 inch across the Plains on Tuesday with impressive
low level winds.


Kebede


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt