Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1147 AM 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...

...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...

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: 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