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

Day 1
Valid 01Z Sun Oct 01 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...

...Great Basin/Nevada...
0100 UTC Update -- Made a few minor tweaks to the Marginal Risk
area, based on the latest observational trends (satellite/radar)
along with trends with the more recent high-res CAMS (especially
HRRR). especially satellite). Robust deep-layer dynamical forcing
is ongoing, with the favorable upper difluent pattern aided by the
90-100 kt upper jet streak migrating through the trough base. Very
strong exit region forcing, which is supporting a compact TROWAL
zone across southern-central NV. Marginal deep-layer instability
(200-400 J/Kg MUCAPEs) and PWs ~ 0.5" will maintain localized
rainfall rates up to 0.50"/hr, especially through 03-05Z, which
are supported by the latest 18Z HREF 40km neighborhood
probabilities. We did remove the Marginal Risk area outside of
areas aoa 8500', since the precipitation will be falling as
(mainly) snow.

...New Mexico/west Texas...
0100 UTC Update -- Excessive rainfall threat is ongoing this
evening, and per the 18Z HREF exceedance probabilities, an
isolated flash flood threat will continue through the overnight
given the favorable synoptic setup east of the upper low (upper
difluence and divergence). Rainfall rates of 1.0 to ~1.5"/hr are
expected along with isolated storm totals over 2 inches. For
further details, please refer to the latest mesoscale
precipitation discussion or MPD #1118.

...Florida...
0100 UTC Update -- Minor changes to the Marginal Risk area based
on the latest observational trends and guidance -- more so to
restrict the coverage along the Gulf coast. The latest HRRRs
continue to show a better (more organized) threat across the
peninsula's east coast, particularly north of West Palm and south
of St. Augustine. While not as impressive compared to the 12Z run,
the 18Z HREF noted probabilities between 60-80+% of 12hr QPF
exceeding 3" through 12Z, with probs between 25-35% of the QPF>5".

Hurley


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 AND PORTIONS
OF SOUTHEAST FLORIDA...

...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