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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0057Z May 25, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Fri May 24 2024

Day 1
Valid 01Z Sat May 25 2024 - 12Z Sat May 25 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID MISSISSIPPI VALLEY AND FROM THE EASTERN RED RIVER VALLEY
TO PARTS OF THE MID SOUTH...

...0100Z Update...
Hoisted a short-term Slight Risk area over parts of the Mid
Mississippi Valley, from east-central MO into southern IL, to
account for the clusters of pre-frontal convection that has
exhibited more upscale growth this evening in an increasingly
difluent upper level pattern with ample deep-layer instability
(MUCAPEs 1000-2000 J/Kg) and moisture flux (PWs 1.51.75"). The
orientation of these multicellular segments and kinematic/shear
profile will support cell training through late this
evening/towards midnight. For further details, please refer to
Mesoscale Precipitation Discussion or MPD #319 at
www.wpc.ncep.noaa.gov/metwatch/metwatch_mpd.php.

Hurley


...1930Z Update...
Expanded the Slight Risk a little farther west to include more of
northeast TX, including the eastern portions of the DFW Metro. This
is based largely on the latest FFG and 0-10cm RSM trends from the
recent heavy rainfall, while also supported by some of the more
recent CAMs (including the HREF exceedance probabilities) with the
next bout of convection late this afternoon/into the evening hours.

Have also expanded the Marginal Risk areas across parts of the
Midwest-western OH Valley, along with the Southeast to Mid Atlantic.
This too is supported by the observational trends (radar, satellite,
mesoanalysis) while also encompassing SPC's updated outlook areas.
Isolated pockets of 2+ inch/hr rates are anticipated with the
strongest cells across all Marginal Risk areas outside of the
northern portions (i.e. Upper Midwest, where the flash flood risk is
predicated on overall lower FFG, thus runoff response would be more
likely with less-intense rain rates).

Hurley


Day 2
Valid 12Z Sat May 25 2024 - 12Z Sun May 26 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR THE
KANSAS/MISSOURI BORDER...

...2030Z Update...

The biggest change was to expand the Marginal/Slight Risk area
south and west across portions of the Southern Plain given the
convection expected to develop late in the day on Saturday and
Saturday night. Expectation is that atmosphere between the passage
of a warm front earlier in the day and prior to the passage of a
dry line will be unstable enough to support heavy rainfall from any
cells that form...with convective coverage growing by Saturday
evening. A midlevel shortwave trough looks to support convcetion
through favorable shear profiles and cooling mid-level
temperatures. Precipitable water values in excess of 1.75 inches
get advected over eastern Kansas by a 35 to 50 kt low level just by
26/06Z as shown by the NCEP guidance. Given the probabilistic
nature of the Excessive Rainfall outlook...allowed some coverage
of the Marginal risk to extend into QPF-sparse regions given the
difficulties of the models in handling dry-line convection early on
and to account for uncertainty in exactly where convection
develops. Models are more unified in developing QPF farther north
and the potential for excessive rainfall was already well-covered
by the prevously-issued Slight Risk area...only minor changes
needed there.

Bann


...Previous Excessive Rainfall Discussion...

The Marginal Risk remains similar to continuity, with some changes
made to the Slight Risk area. A cyclone emerging into the central
Plains will bring precipitable water values of 1.5"+ across the
region on the heels of sufficiently strong low level
inflow/effective bulk shear. The region lies within a general
instability gradient, with CAPE expected to rise to 4000+ J/kg
within an increasingly capped atmosphere across the southern
Plains. Pockets of moderate to heavy rainfall are forecast within
this region. There is increasing overlap in the QPF pattern near
the KS/MO border within the 00z guidance where three hourly flash
flood guidance is modest, so went ahead with an upgrade to a Slight
Risk for this update. However, given the ingredients, hourly rain
totals to 2.5" with local amounts to 5" are possible where
mesocyclones form and/or cells manage to train.


...Portions of the East...
Near western PA and WV, showers and thunderstorms along a
progressive front could drop locally heavy rainfall within an area
of modest flash flood guidance. Hourly rain totals to 1.5" with
local amounts to 3" are possible where short bouts of cell training
or cell mergers manage to occur. Since the flash flood threat
appears to be isolated, added a Marginal Risk. Down south in the
Carolinas, higher moisture pools near the coast as a front slowly
moves through. With MU CAPE as high as 3000 J/kg and effective bulk
shear expected to be sufficient to organize convection to some
degree, added a Marginal Risk within a regime that can lead to
isolated flash flooding during the warm season.


Roth


Day 3
Valid 12Z Sun May 26 2024 - 12Z Mon May 27 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE MIDWEST AND THE MISSISSIPPI & OHIO VALLEYS...

...2030Z Update...
Few changes were made. The models continue to show a cyclone making
its way east across the Upper Midwest and the Great Lakes with
time. There were some run to run differences in QPF but the
previous outlook still seemed to cover any resulting shifts in
excessive rainfall potential.

Bann

...Previous Excessive Rainfall Discussion...

A cyclone moving across the Midwest and Great Lakes is expected to
draw ample moisture (precipitable water values of 1.50-1.75") into
the region. The area that is expected to see the greatest residency
time with higher moisture and instability is across portions of TN
and KY. However, warm temperatures at 700 hPa appear to be a
limiting factor the farther down its front you go. The best warm
air advection ahead of the low is slated to occur from portions of
IL across OH, where the guidance is more robust in its QPF signal,
with local amounts in the 3-5" range. Considering the progression
of the warm air advection pattern and best moisture across the
region, that degree of heavy rainfall would need to fall quickly;
hourly rainfall potential to 2.5" is possible within the organized
thunderstorms, with embedded mesocyclones, could get them there
given the low- level inflow and effective bulk shear expected. Went
ahead and added a Slight Risk across the region, where flash flood
guidance values are modest.

Roth


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