Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
902 PM EDT Thu Sep 19 2024
Day 1
Valid 01Z Fri Sep 20 2024 - 12Z Fri Sep 20 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN COAST RANGES OF CALIFORNIA AND UPPER MIDWEST...
...01Z Update...
...Central California...
A few slow moving cells will persist over the central CA Coastal
Ranges into this evening under an upper low. An additional inch or
two is possible in the heavier cells. The Marginal was shrunk a bit
based on radar depiction and recent HRRR QPF. More info can be
found in MPD 1035.
...Upper Midwest...
Most activity is fast-moving hail-producing cells, but some
congealing/merging and repeating activity should continue this
evening over southeast MN, southwest WI, and northeast IA. Thus the
Marginal Risk was expanded south a bit, mainly over the Driftless
area in the ARX CWA. More stratiform precip over central WI
warranted removal of some areas of north-central WI. Recent HRRRs
are in decent agreement with reality with potential for a quick
few inches of rain. More info can be found in MPD 1034.
Jackson
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
902 PM EDT Thu Sep 19 2024
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 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
902 PM EDT Thu Sep 19 2024
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 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Thu Sep 19 2024
The southern stream upper low over the central High Plains early
Sunday and pivoting northeast will provide ample dynamical support
for thunderstorms with heavy rain on the cusp of the instability
gradient. On Sunday, models generally show support for 2 to locally
4 inches of rain across parts of the central Plains into the Mid-
Mississippi Valley. Continue to depict a broad Marginal Risk for
those areas, and there may be a need for higher risk levels within
the Marginal in the future if models align more agreeably on a
heavy rain axis. Antecedent conditions are generally dry, but heavy
rain rates of 1 to 2 inches per hour could pose flash flooding
issues regardless. Chances for moderate to heavy rain should shift
east into the Great Lakes and Ohio Valley on Monday as the
shortwave moves east. Rainfall totals look to decrease as the
shortwave weakens, but with some upper level support and
instability, a Marginal Risk still seems warranted for Day
5/Monday, though perhaps lower end than on Day 4/Sunday. Rain is
forecast to focus along the eventually stalling front in the east-
central U.S. Tuesday-Thursday. Rain chances also are likely to get
renewed in the south-central U.S. by Tuesday with another front and
energy aloft.
Farther west, most areas should stay dry under mean ridging. The
Washington coastal ranges/Cascades could be an exception at times.
Then in South Florida, showers and storms may focus near a frontal
boundary lingering in a moist environment. These do not look to be
too heavy next week until around midweek, when there may be some
surge of tropical moisture northward ahead of any potential
tropical development farther south in the Caribbean that may slowly
make its way north into the Gulf during next week. There is still
considerable uncertainty in the track and timing of any possible
tropical system, so continue to check updated forecasts.
Cooler than normal temperatures particularly in terms of highs
will peak across the central Plains on Sunday, with highs generally
only reaching the low 60s underneath the upper trough. Meanwhile
farther east, above normal temperatures by 5-15 degrees are
forecast for the south-central Mississippi Valley northeast to the
Ohio Valley and Great Lakes region ahead of a front. The above
average temperatures will decrease in scope into Monday, focusing
mainly over the Ohio Valley and then gradually moderating there
too. The Eastern Seaboard could see below average highs into early
next week. On the other hand, building ridging in the West will
allow for some warmer than average temperatures by 5-15 degrees,
bringing temperatures above 100F to the Desert Southwest once
again.
Kebede/Tate
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Thu Sep 19 2024
The southern stream upper low over the central High Plains early
Sunday and pivoting northeast will provide ample dynamical support
for thunderstorms with heavy rain on the cusp of the instability
gradient. On Sunday, models generally show support for 2 to locally
4 inches of rain across parts of the central Plains into the Mid-
Mississippi Valley. Continue to depict a broad Marginal Risk for
those areas, and there may be a need for higher risk levels within
the Marginal in the future if models align more agreeably on a
heavy rain axis. Antecedent conditions are generally dry, but heavy
rain rates of 1 to 2 inches per hour could pose flash flooding
issues regardless. Chances for moderate to heavy rain should shift
east into the Great Lakes and Ohio Valley on Monday as the
shortwave moves east. Rainfall totals look to decrease as the
shortwave weakens, but with some upper level support and
instability, a Marginal Risk still seems warranted for Day
5/Monday, though perhaps lower end than on Day 4/Sunday. Rain is
forecast to focus along the eventually stalling front in the east-
central U.S. Tuesday-Thursday. Rain chances also are likely to get
renewed in the south-central U.S. by Tuesday with another front and
energy aloft.
Farther west, most areas should stay dry under mean ridging. The
Washington coastal ranges/Cascades could be an exception at times.
Then in South Florida, showers and storms may focus near a frontal
boundary lingering in a moist environment. These do not look to be
too heavy next week until around midweek, when there may be some
surge of tropical moisture northward ahead of any potential
tropical development farther south in the Caribbean that may slowly
make its way north into the Gulf during next week. There is still
considerable uncertainty in the track and timing of any possible
tropical system, so continue to check updated forecasts.
Cooler than normal temperatures particularly in terms of highs
will peak across the central Plains on Sunday, with highs generally
only reaching the low 60s underneath the upper trough. Meanwhile
farther east, above normal temperatures by 5-15 degrees are
forecast for the south-central Mississippi Valley northeast to the
Ohio Valley and Great Lakes region ahead of a front. The above
average temperatures will decrease in scope into Monday, focusing
mainly over the Ohio Valley and then gradually moderating there
too. The Eastern Seaboard could see below average highs into early
next week. On the other hand, building ridging in the West will
allow for some warmer than average temperatures by 5-15 degrees,
bringing temperatures above 100F to the Desert Southwest once
again.
Kebede/Tate
