Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
311 PM EDT Sat Sep 30 2023

Day 1
Valid 16Z Sat Sep 30 2023 - 12Z Sun Oct 01 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE GREAT BASIN, NEW MEXICO AND FAR WEST TEXAS, AND FLORIDA...

...16Z Update...

The only change of significance this morning was to remove the
Marginal Risk area across WI and the U.P. Ongoing rainfall across
the U.P. has been weakening, while the entire area's soils are
much drier than normal, per NASA Sport soil moisture imagery. Much
of the rainfall that has already fallen in this area has been
beneficial, owing to those dry soils. Later this evening into
tonight, renewed shower activity is likely to develop, however the
guidance has been decreasing on total rainfall amounts, with most
of the CAMs suggesting just showers, with very little convection
expected.

Elsewhere the 3 Marginal Risk areas in FL, NM, and NV were largely
unchanged. In FL, a stalled out front that locally resulted in 4
to 6 inches of rain in the Daytona Beach area has been drifting
northward and weakening with daytime heating. Expect widespread
convective coverage as usual again this afternoon, but no one spot
is expected to see large amounts of rain as compared with the
surrounding areas. The Marginal Risk remains in effect. Light to
moderate rainfall is ongoing across NV, but is unlikely to result
in any more than isolated flash flooding due to lower rainfall
rates. Showers and thunderstorms are still expected to develop
this afternoon and evening across NM, where the rainfall event has
yet to begin.

Wegman

...Previous Discussion...

...Great Basin/Nevada...

A deep upper-trough will transport a plume of moisture (1-2 PWAT
stndv) into the West today. A cold front associated with a
deepening low pressure system will act as a focus for convection
over the Great Basin, in particular. There's been some
consolidation and better agreement between the hi-res and global
guidance with respect to the location of the axis of heaviest
precipitation. Some parts of central Nevada, southeastern Oregon
and southwestern Idaho could see rainfall rates around 0.25"/hr
with isolated higher amounts. Ample forcing over elevated terrain
could lead to more efficient/enhanced rain rates. Between 1-3" of
rainfall is expected through tonight.

...New Mexico/west Texas...

Convection supported by a deep upper trough to the West and
leading shortwave energy will support another localized flash
flood threat over parts of eastern New Mexico and into west Texas.
Decent instability (1000J/kg+), modest moisture flux and a
favorable low level jet may produce bursts of heavy rainfall over
our marginal risk area. Urban areas and dry washes are most
vulnerable of flash flooding.

...Upper Midwest...

An embedded shortwave within an omega ridge will continue driving
convection through this afternoon. The heaviest rainfall may
already be occurring over parts of eastern Minnesota and
northwestern Wisconsin. That being said, impressive forcing,
instability (1000J/Kg+) and moisture (2-3 PWAT stndv) could still
generate excessive rainfall leading to isolated instances of flash
flooding.

...Florida...

A slow moving shortwave trough will allow for the continuation of
scattered to isolated thunderstorms across the central/southern
peninsula today. Some cells may produce localized heavy rainfall
within the marginal risk area. Antecedent conditions will also
contribute to the sensitivity to any isolated heavy rainfall
threats.


Kebede


Day 2
Valid 12Z Sun Oct 01 2023 - 12Z Mon Oct 02 2023

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

...19Z Update...

No significant changes were made to the inherited ERO areas for
Day 2/Sunday. A slow moving front over FL will continue to support
slow/nearly stationary cells developing along the Atlantic coast
and drifting west/into the I-95 corridor again on Sunday from the
Space Coast south to the Everglades. Portions of the Space Coast
around Daytona Beach saw multiple inches of rain and localized
flash flooding this morning, so any additional rainfall,
particularly on the order of multiple inches, would likely result
in additional flash flooding. As there's little agreement on where
any higher totals would be, for now leaving at a Marginal until
such time as there's better agreement on where heavier rainfall
totals may fall...always a challenge in FL.

Day 2/Sun remains the quietest of the 3 short-term days as far as
rainfall totals into NM/TX. Any upgrades will be highly contingent
on over-performance this afternoon in this area, as a lack of
rainfall overall should preclude any more than isolated flash
flooding.

Wegman

...Previous Discussion...

...Southern High Plains of New Mexico/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
o flash flooding Sunday afternoon and evening. The global models
appear to be well clustered over the Southern High Plains as well,
however the past couple of GFS runs appear to suggest that the
heaviest rainfall will occur on the southern edge of the marginal
risk area so an upgrade is not necessary at this time. Ensemble
mean exceedance probabilities of 1" in 24 hours including the 00z
GEFS are well clustered over the northeastern New Mexico and the
Texas panhandle.

...Florida...

Shortwave energy present across the state of Florida will support
isolated flash flooding concerns on Sunday. Scattered to isolated
thunderstorms will continue to develop along a quasi-stationary
front draped across the southern peninsula. There will be plenty
of instability in place so there's always the risk of some cells
over performing current forecasts with efficient rainfall rates
that exceed flash flood guidance.


Kebede

Day 3
Valid 12Z Mon Oct 02 2023 - 12Z Tue Oct 03 2023

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

...19Z Update...

For Day 3/Mon there is a much better signal for heavy rain in all
of the guidance along the NM/TX border. The Marginal Risk in this
area was maintained, as overall rainfall totals have come down a
little bit, and the entirety of the NM/TX border making up the
eastern end of NM is in play for locally heavier rainfall totals.
Thus, the area is in a higher-end Marginal, with a decent chance
that come better agreement and CAMs guidance, that a portion of
this area may need to be upgraded to a Slight. For now however,
have opted to leave as a Marginal due to the relatively diffuse
signal in this area.

...Southeast Florida...

The third straight day of rainfall along the Atlantic Coast,
particularly from West Palm Beach southward through Miami,
necessitated introducing a Marginal for the I-95 corridor. The
same very slow moving front should continue to make southward
progress, but by afternoon the abundance of moisture and
instability should allow for renewed convective development in
this area.

...Northeast WY through ND...

A Marginal Risk area was considered here along a stalled front,
but antecedent dry conditions and poor overall signal left this
area for now risk-free. However, the area will continue to be
monitored.

Wegman

...Previous Discussion...

...Southern High Plains...

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 with excessive rainfall potential. The latest guidance
has certainly trended wetter and is well clustered over the
Southern High Plains region for the location of heaviest rainfall
potential. The ensemble mean excellence probabilities are in good
agreement for 1 and 2" within the drawn marginal risk area.
Mitigating factors for an upgrade at this time include relatively
dry soils and high flash flood guidance. Potential severe
thunderstorms could overcome the mitigating factors by producing
higher rain rates, but there is still some uncertainty around the
severe weather threat at this time.


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