134
FOUS30 KWBC 191915
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
315 PM EDT Thu Sep 19 2024

Day 1
Valid 16Z Thu Sep 19 2024 - 12Z Fri Sep 20 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN COAST RANGES OF CALIFORNIA, UPPER MIDWEST, AND THE
URBAN CORRIDOR OF SOUTHEAST FLORIDA...

...16Z Update...

No changes were made to the inherited Marginal Risk areas in
California, Wisconsin, and Florida with this update.

In California, the Southern Coast Ranges continue to be the focus
for rainfall capable of flash flooding as light to moderate
rainfall rates fall over the many burn scars in the area, which
could result in localized flash flooding. A deep, almost cutoff
positively-tilted upper low will continue drifting southeast down
the coast. Where forcing is maximized along the Southern Coast
Ranges, locally heavy rain may occur. The forcing would be
maximized by a combination of upslope/terrain influences and the
upper level forcing.

In Wisconsin, the Marginal remains intact largely due to some
modest agreement in the CAMs that 2 rounds of strong storms may
move across the region back-to-back, which may cause isolated flash
flooding in flood-prone and urban areas.

In Florida, the stalled out front and PWATs above 2 inches will
continue to support slow-moving convection capable of 2+ inch per
hour rates. At high tide, poor drainage off the urban areas will
briefly and locally increase the flooding threat.

A couple areas were considered for Marginals...the Blue Ridge to
the Great Smokies of western Virginia and North Carolina will have
a non-zero flash flooding threat due to terrain influences and
lee-side low development, as well as for the coast of southeast
North Carolina where the sea breeze may interact with predominantly
offshore flow to result in locally heavy rainfall on saturated
soils from the recent passage of Potential Tropical Cyclone 8.
Ultimately both these areas were passed over for a Marginal largely
due to enough unfavorable meteorology (meager moisture primarily),
and some storm motion allowing any potentially heavy rain cores to
move away from their respective formation regions.

Wegman

...Previous Discussion...

...California...

A fairly stout closed upper low over the Pacific will be the
driving force behind increased forcing and region moisture
advection into the southern half of California this period. Recent
wildfires have brought about a plethora of burn scars within the
terrain of the Southern Coast Ranges that have become increasingly
susceptible to flash flooding prospects due to hydrophobic soils
allowing for consistent run off capabilities. The pattern evolution
supports a strong mid and upper forcing regime to be co-located
within the Central and Southern CA coasts, as well as the immediate
terrain to the east of the coastal plain. Latest 00z HREF mean QPF
distribution pinpoints a regional maxima within the confines of
the Southern Coast Range, intersecting an area with multiple burn
scar locations that will likely exhibit flash flood effects thanks
to the current antecedent soil environment. There`s a consensus
within both the mean QPF and probability fields for >1" of rainfall
in-of the aforementioned area with even some prospects for locally
>2" as noted within the neighborhood probabilities for D1. With
coordination from earlier updates by the local WFOs involved
prompting the MRGL risk, and with the signal remaining prolific
enough from the latest guidance, the MRGL risk was maintained to
project continuity.

...Upper Midwest...

An increasingly favorable pattern will invoke a period of
scattered to widespread thunderstorm activity capable of producing
locally enhanced rainfall cores leading to 1-2"/hr rain rates in
the strongest convective cores. Steepening lapse rates, increasing
bulk shear, and modest surface to mid-level buoyancy will create an
environment conducive for not only thunderstorm initiation, but
prolonged updrafts capable of maintaining significant storm
structure within a core of elevated PWATs advected ahead of a mean
trough to the west. Latest HREF EAS signals for >1" are pretty high
(60-80%) across much of Northwest WI, an area where most CAMs are
pinpointing as the primary area of focus for this afternoon`s and
evening`s convective prospects. Neighborhood probs are a bit less
emphatic than what normally constitutes a higher impact potential,
but still fairly significant nonetheless for potential totals
ranging between 2-4" over portions of Eastern MN into Northwest WI.
The antecedent conditions present within the zone of highest
probabilities is borderline unfavorable for flooding with NASA
SPoRT indicating soil moisture percentiles closer to 40-50% over
the area. Considering the favorable environment and SPC outlining
the area with a Slight Risk for severe weather, believe the setup
is good enough for a targeted MRGL within the core of the best
probabilities based off the latest hi-res ensemble.

...Southeast Florida...

A meandering frontal boundary in-of South FL will become a focal
point for elevated convective coverage, especially just inland of
the urban metro of Miami up to West Palm Beach when sea breeze
initiation becomes tied within the frontal positioning later this
afternoon. The steering pattern is favorable for convective
development to drift back over the urban metro, settling within a
modest PWAT anomaly situated over the area. The core of convection
will likely drop rates between 1-3"/hr with intra-hour rates >3"
more than plausible, a factor that typically causes flash flood
concerns within the urban corridor. HREF blended mean QPF has a
bullseye situated within the Fort Lauderdale/Pompano Beach area
with a small secondary max near Miami. This signal is fairly
classic for a lower category risk, especially when you add the
steering pattern leading to favorable motions back over the cities
along the Southeast FL coast. The previous forecast was maintained
with only a very minor adjustment along the western periphery of
the risk area.

Kleebauer

Day 2
Valid 12Z Fri Sep 20 2024 - 12Z Sat Sep 21 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE URBAN CORRIDOR OF SOUTHEAST FLORIDA, NORTHEAST NEW MEXICO, AND
THE DESERT SOUTHWEST...

...1930Z Update...

No significant changes were made to the ERO risk areas for the Day
2/Friday time period.

...Southwest...

The upper low moving down the coast today will move inland over the
Desert Southwest Friday. Associated moisture will accompany the low
into the usually dry areas of southeast California, southern
Nevada, and northwestern Arizona. The storms in this area will be
associated with the instability maximum at the center of the upper
low where upper level temperatures will be coldest. As is the
nature of flow near the center of lows, the steering will be
chaotic and highly dependent on where the exact center of the upper
low will be at any given point in time Friday afternoon. With any
more persistent showers and storms, expect isolated flash flooding
with any burn scars, dry washes, and other flood- prone areas.
PWATs between 0.5 and 0.75 inches in these areas will peak at about
1.5 sigma above normal for this time of year.

...New Mexico...

Broad upper level diffluence ahead of the positively tilted low
over the Desert Southwest will cause a somewhat narrow 50-100 mile
wide plume of training showers and possibly embedded thunderstorms
to develop over eastern New Mexico and into portions of the far
western Texas and Oklahoma Panhandles. Given the sensitivities of
some of these areas to prolonged rainfall, the Marginal risk
remains in place, especially for any burn scars or flood prone
areas.

...South Florida...

A final day of heavy rainfall potential is expected in South
Florida again on Friday. Once again the primary risk of flash
flooding will be in the urban corridor from West Palm Beach south
through Homestead. Slow-moving and persistent thunderstorms on the
leading edge of an advancing cold front will be the primary drivers
of any heavy rainfall. The timing of high tide with the storms will
also play a big role in any potential isolated flash flooding due
to much poorer drainage into the ocean during high tide. The front
will clear South Florida by Saturday turning the predominant flow
more northeasterly. This will cut down on the PWATs some, while
also greatly limiting, though not eliminating the coverage of
storms by Saturday. Thus, the Marginal flash flooding threat should
end for now with the conclusion of Friday afternoon`s storms.

Wegman

...Previous Discussion...

...Southeast Florida...

A secondary period of impact from active sea breeze convection and
propagation into the urban corridor is forecast to transpire on
Friday afternoon. The signal is not as robust as the previous
period, but any heavy rainfall prior would precede this potential
event with lower FFG signals leading to a bit more sensitivity to
flash flood prospects. The ensemble mean QPF is a touch less
aggressive as stated, so it will remain a period to monitor to see
if the threat trends higher once we move closer to the range of the
CAMs. For now, maintained general continuity as the signal
persists for a flash flood threat in-of the urban corridor
extending from Miami up through West Palm Beach.

...New Mexico...

The prolific closed upper circulation out across the Southwest will
begin pressing eastward with an increased upper forcing pattern
thanks to a broad diffluent axis downstream of the mean
trough/upper low. Most of the latest global deterministic eject a
rapidly moving shortwave out of Chihuahua, traversing over the
Eastern half of NM by the afternoon and evening hours Friday. A
tongue of modest instability based off the mean MUCAPE forecast
from all the deterministic is forecast to bisect that eastern side
of the state with the best focus aligned from the Sacramentos up
through the western side of the Caprock and into the High Plains of
Northeastern NM. Flash flooding threats will likely be tied to
urban areas and some of the remnant burn scars located within the
Sacramento Mountains, however these setups are notoriously tricky,
especially with the propensity for convection to become anchored
along the instability gradient favored in the terrain and adjacent
Caprock. Some of the deterministic output is pretty reasonable for
isolated flash flood prospects, especially when you add the
increasingly favorable upper pattern. A small MRGL risk was added
to encompass the areas above with the threat lying on the lower-end
of the risk threshold.

...Desert Southwest...

Our deep closed upper reflection will migrate slowly to the east
with the circulation likely crossing Southern CA into the interior
Desert Southwest as we move towards the end of the period. As of
now, the current QPF output is not exactly what would constitute a
favorable flash flood scenario. However, considering the expected
pattern evolution and the advection of more moist, buoyant air
northward ahead of the upper low, you have to acknowledge the
prospects of a more scattered pulse convective pattern over an
area that is much more conducive for flash flood potential. Recent
ECMWF AIFS ML output was actually more aggressive than the recent
deterministic in the spatial coverage of precip, and even for the
magnitude of the expected setup. This tends to signal a red flag
for the deterministic not being aggressive enough with such a
strong mid and upper pattern. A low-end MRGL was placed over
portions of Southern CA/NV into Western AZ to outline the favored
area for scattered convective development with modest storm cores
capable of locally heavy rainfall.

Kleebauer

Day 3
Valid 12Z Sat Sep 21 2024 - 12Z Sun Sep 22 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE CENTRAL ROCKIES AND CENTRAL PLAINS...

...1930Z Update...

Only a few minor adjustments to the inherited Marginal across the
central Rockies and Plains were made. The upper level low over the
Southwest will track northeastward towards the Plains on Saturday.
As usual this will help spur on a strengthening LLJ across the
southern Plains, with warm and humid air from the Gulf running into
a cooler air mass over the Plains. Expect the greatest coverage of
heavy rainfall to develop mostly overnight Saturday night, as the
strengthening LLJ interacts with an area of surface high pressure
over the northern Plains. This will force strong frontogenesis in
between, leading to a prolonged period of heavy rain and training
thunderstorms.

Thus, while the meteorology would support at least a Slight Risk,
the tradeoff is that most of this area has been bone dry in terms
of precipitation, and with the drought conditions in place, it will
take a lot of rainfall to overcome the dry conditions and empty
creekbeds to result in flooding. However, given that the surface
low will be forming along the front, and the high to the north
countering the low and the LLJ from the south will strongly favor
training convection, it seems probable that somewhere in the
Marginal Risk area the high bar will be met and exceeded for
flooding. Thus, it becomes where the most likely area of training
convection will set up, which for the moment appears to generally
follow the KS/NE border from northwest KS to southeast NE.
Fortunately much of this area is rural, so urban concerns appear to
be minimized.

It appears likely a Slight risk will be needed across the Plains
but for now there is limited certainty as to how all these various
ingredients will interact to reach that threshold. The Marginal
risk was expanded southwest to include northeast NM and the TX and
OK Panhandles partially due to expected lowered FFGs from the
rainfall on Day 2/Friday. A bit of southeast KS was removed due to
somewhat better certainty that most of the rain will be northwest
of there, and a small bit of the southern Sand Hills was removed
due to very high FFGs.

Wegman

...Previous Discussion...

The broad upper trough, closed low evolution over the West will
translate eastward with an evolving setup capable of widespread
convection across the Rockies through the Central Plains,
especially as the trough axis picks up forward speed and slides
into the Central Rockies during the back half of the forecast
period. A combination of favorable ascent and increased moisture
advection thanks to a developing LLJ pattern east of the Front
Range will lead to widespread thunderstorm activity with several
heavy cores likely to initiate and produce locally enhanced
rainfall. The most favorable locations for flash flooding are
likely to be across the Colorado Front Range and the Central Plains
between Southern NE and Northern KS, a reflection with the current
ensemble fields and bias corrected QPF footprint. There will likely
be an upgrade somewhere within the broad MRGL risk encompassing the
Central Rockies into the Plains, but the finer details are still
another succession of runs away from pinpointing specifics with
regards to rainfall totals and the upper threshold of the
potential. Ensemble bias corrected QPF depicts a widespread 1-2"
across the above area, a testament at range on what could
transpire. This will be a period to monitor closely for upgrades
considering the evolution.

Kleebauer

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