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 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
346 PM EDT Sun Oct 01 2023
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...
...20Z Update...
...Southern High Plains...
No major changes were made to the ERO risk areas for Day 2/Monday.
The Slight Risk remains in place for eastern NM and portions of
the TX Panhandle as a deep trough ejects out of the Rockies, which
when added to increasing Gulf moisture ahead of the trough will
result in widespread shower and thunderstorm development in the
Slight Risk area. Training will be the primary concern as the
storms largely struggle to move east, instead tracking NNE to NE.
Expect heavy rainfall rates as high as 2 inches per hour with the
strongest storms. NASA Sport imagery shows the area is at to a bit
below normal as far as soil moisture conditions go, so it will
likely take multiple rounds of storms to result in widely
scattered flash flooding except where any bigger towns or flood
sensitive areas are hit with the heavier rains.
...Florida...
The front over the center of the Peninsula now will make its way
to the southern tip of the state by Monday/Day 2. Predominantly
easterly flow will allow for a renewed round of convection in the
Miami-Ft. Lauderdale area, and as with the past few days will be
slow moving and moisture-laden. Thus, the threat for flash
flooding remains in the urban areas predominantly. Out over the
Everglades it's much more difficult for heavy rains to result in
flash flooding, so the Marginal Risk area was trimmed from the
west to highlight the greatest threat being in the urban areas.
For the Keys, any excessive rainfall induced flooding would likely
be compounded by tidal flooding.
Wegman
...Previous Discussion...
...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 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
346 PM EDT Sun Oct 01 2023
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 AND THE EAST COAST OF FLORIDA...
...20Z Update...
Once again no major changes were made to the inherited forecast.
Over the Plains, the Marginal Risk area looks good for the
uncertainty as to where the strongest storms will develop. The
overall area was nudged a row of counties east, and trimmed on the
western side, particularly across western SD, where the center of
the low is expected to track, which will end the rainfall threat
more quickly. More of ND may be able to be trimmed with future
updates due to lack of instability on the northern side of the
low, but the confidence wasn't quite there to make that change.
...Florida...
An easterly wave is expected to move into the Space Coast Tuesday
and especially Tuesday night. This area has been hard-hit with
rainfall the past few days, and so while the rainfall forecast at
this point is meager along the coast, there remains some chance
that slow-moving/training storms could cause localized flash
flooding, especially right along the coast. This is a
low-confidence Marginal, and any bigger changes in frontal
position could result in bigger changes to the Marginal Risk area.
Wegman
...Previous Discussion...
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 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
234 PM EDT Sun Oct 01 2023
The south-central U.S. will be an area of focus for widespread
heavy rain especially on Wednesday, as the right entrance region
of the jet will provide ample lift, and plenty of instability is
in place just ahead of the cold front. A Slight Risk remains in
place there in the Day 4/Wednesday Excessive Rainfall Outlook, as
despite the drought conditions, high rain rates that could
overcome the dryness are likely. By Thursday, the cold front will
push southeast and clear out rain potential on its backside, but
could still provide forcing for convection along and ahead of it
into southern and eastern parts of Texas and into the Lower
Mississippi Valley. Model guidance still varies with placement of
any heavy amounts, but ensemble and clustering signals are growing
to favor issuance of a small Slight Risk area over Texas for the
Day 5/Thursday ERO to cover the threat. Some lingering rain may be
possible in South Texas late week.
Farther north, the cold front should track through the
Midwest/Upper Great Lakes and the Ohio/Tennessee Valleys Wednesday
into Friday, the into the East heading into the weekend, producing
showers and thunderstorms along and ahead of it. This rain should
overall be lighter than farther south, but may be enhanced in the
Northeast especially in favored terrain, as the front could begin
to tap Atlantic moisture within southerly flow to increase
rainfall, especially considering preference for an amplified
supporting upper troughing by late week.
The amplified trough and the cold front will lead to cooler
temperatures than average while the upper ridges to the east and
west will support above normal temperatures. The Interior West
should see cooler temperatures than average especially through
midweek, with with lows 5-10F below normal and highs around 10-15F
below average, but warming to near normal for the latter part of
the week. Meanwhile warmer than average temperatures by around
15-20F above normal are forecast midweek for the Midwest into the
Great Lakes and the Interior Northeast, with highs in the upper
70s to low 80s. Some daily records for highs/warm lows may be
possible Wednesday. As the cold front and upper trough march east
into the latter half of the week, temperatures will cool to below
average across the central and then east-central U.S., limiting
the above normal temperatures to the Northeast. A trailing front
may reinforce the cooling trend by Friday-Sunday, and there could
be concerns for the first frost or freeze of the season to occur
in the northern High Plains. Approach/arrival of the eastern
Pacific upper ridge should raise temperatures along the West Coast
to above average, by as much as 5-15F, and some record warm lows
may be possible.
Schichtel/Tate
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
234 PM EDT Sun Oct 01 2023
The south-central U.S. will be an area of focus for widespread
heavy rain especially on Wednesday, as the right entrance region
of the jet will provide ample lift, and plenty of instability is
in place just ahead of the cold front. A Slight Risk remains in
place there in the Day 4/Wednesday Excessive Rainfall Outlook, as
despite the drought conditions, high rain rates that could
overcome the dryness are likely. By Thursday, the cold front will
push southeast and clear out rain potential on its backside, but
could still provide forcing for convection along and ahead of it
into southern and eastern parts of Texas and into the Lower
Mississippi Valley. Model guidance still varies with placement of
any heavy amounts, but ensemble and clustering signals are growing
to favor issuance of a small Slight Risk area over Texas for the
Day 5/Thursday ERO to cover the threat. Some lingering rain may be
possible in South Texas late week.
Farther north, the cold front should track through the
Midwest/Upper Great Lakes and the Ohio/Tennessee Valleys Wednesday
into Friday, the into the East heading into the weekend, producing
showers and thunderstorms along and ahead of it. This rain should
overall be lighter than farther south, but may be enhanced in the
Northeast especially in favored terrain, as the front could begin
to tap Atlantic moisture within southerly flow to increase
rainfall, especially considering preference for an amplified
supporting upper troughing by late week.
The amplified trough and the cold front will lead to cooler
temperatures than average while the upper ridges to the east and
west will support above normal temperatures. The Interior West
should see cooler temperatures than average especially through
midweek, with with lows 5-10F below normal and highs around 10-15F
below average, but warming to near normal for the latter part of
the week. Meanwhile warmer than average temperatures by around
15-20F above normal are forecast midweek for the Midwest into the
Great Lakes and the Interior Northeast, with highs in the upper
70s to low 80s. Some daily records for highs/warm lows may be
possible Wednesday. As the cold front and upper trough march east
into the latter half of the week, temperatures will cool to below
average across the central and then east-central U.S., limiting
the above normal temperatures to the Northeast. A trailing front
may reinforce the cooling trend by Friday-Sunday, and there could
be concerns for the first frost or freeze of the season to occur
in the northern High Plains. Approach/arrival of the eastern
Pacific upper ridge should raise temperatures along the West Coast
to above average, by as much as 5-15F, and some record warm lows
may be possible.
Schichtel/Tate
