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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2241Z May 23, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
640 PM EDT Thu May 23 2024

Day 1
Valid 2230Z Fri May 24 2024 - 12Z Fri May 24 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE ARKLATEX
TO ARKLAMISS AS WELL AS FOR PORTIONS OF THE TENNESSEE VALLEY AND
SOUTHERN APPALACHIANS...

...22Z Update...
Updated the Slight and Moderate Risk areas across the Southern
Plains-ArkLaTex areas to Lower MS Valley and TN-Lower Ohio Valleys,
as well as with the Marginal Risk over the Lower MO Valley, based
on the latest observational and mesoanalysis trends along with the
latest HRRR and HREF guidance. There are active Mesoscale
Precipitation Discussions or MPDs in these outlook areas; for
further information, please go to
www.wpc.ncep.noaa.gov/metwatch/metwatch_mpd.php.

...16Z Update...
The inherited Slight risk area was divided into 2 separate smaller
ones with this update based on the latest guidance. The theme from
the ensemble CAMs was to highlight the area around southern
Arkansas with a separate area on the west side of the southern
Appalachians into Kentucky, eastern Tennessee and extreme
southwestern Virginia. Areas east of the mountains have somewhat
higher FFGs and more tolerance for the possibility of localized
training convection, so the Slight was cut off at the Great
Smokies, which fits well with where current convection is adding
moisture to the environment across southeast Kentucky and northeast
Tennessee. Additional convection expected this afternoon with
daytime heating and a period of a few hours of clearing between
current areas of convection should allow for this afternoon's
storms to have the potential to produce locally heavy rainfall and
resultant flash flooding.

The squall line crossing southeast Arkansas and into northwest
Mississippi is also adding moisture to the environment and locally
lowering FFGs as it crosses the area. The line alone is unlikely to
produce flash flooding given its fast movement perpendicular to the
axis of the line...however it will set the stage for additional
convection likely to develop this afternoon, which in areas with
training could cause widely scattered instances of flash flooding.

Elsewhere only minor changes to the Marginals, including a small
southwestward expansion in central TX for the currently-weakening
convection, trimming out the Sand Hills and dry slot of the
northern Plains low, and including the northernmost counties in
Georgia into the Marginal for possible southward development of
this afternoon's convection.

Wegman

...Previous Discussion...

...ArkLaTex into the Mid-South...
A stationary front is expected to stretched from northeast Texas/
southeast Oklahoma into Arkansas, while slowly weakening. Precipitable
water values expected to remain 1.5 to 2 standard deviations above the
mean along the boundary. Additional shortwave energy moving in from
the west across the Southern Plains will support additional
convection in this high moisture axis and there is potential for
additional heavy rains from northeast Texas/southeast Oklahoma,
east across the Lower Arkansas Valley into the Lower Mississippi
Valley. The stationary front weakens and southerly low level flow
pushes the high PW values farther to the north. Hourly rain totals
to 2.5" and local amounts to 5" are possible. While some model
spread continues, convergence seen in the 00z Canadian Regional,
00z UKMET, and 00z ECMWF in the Slight Risk area led to some
increased confidence, despite the weaker look in the 00z HREF
probabilities and more bifurcated 00z GFS solution in this region.
The Slight Risk remains and has shown some expansion equally due
to the heavy rainfall which occurred Wednesday and because of
future heavy rainfall concerns Thursday and Thursday night.


...Northern Plains/Upper Midwest...
The marginal risk area across eastern portions of the Northern to
Central Plains into the Upper Mississippi Valley remains.
Additional convection is likely ahead of the next strong area of
height falls/cooling aloft rotating from the Northern Rockies into
the region. This area has seen heavy rainfall amounts as of late,
keeping stream flows high.  The strong upper trof, and associated
strengthening low level southerly flow and increasing PW values,
there will be potential for localized heavy rainfall amounts that
may fall across areas that received heavy totals over the past 24
hours.

Roth


Day 2
Valid 12Z Fri May 24 2024 - 12Z Sat May 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM CENTRAL
TEXAS TO THE MID-ATLANTIC AS WELL AS PORTIONS OF THE UPPER MIDWEST...

...2030Z Update...

No significant changes were made to the Marginal risk areas across
portions of the south and the upper Midwest. From Texas to
Virginia, storms are once again expected to develop along the
stalled out front. However, with lesser upper level support and
moisture flow parallel to the front out of the southwest rather
than perpendicular, supporting flow into the storms will be reduced
Friday as compared with today. Thus, the threat level remains
Marginal across this area. It's possible that portions of the front
may need a local Slight by Friday, with the most likely area in the
same portions of the current Slight in and around southern
Arkansas. However, for now the signal for heavy rain in the
guidance is diffuse enough that confidence is not there for a
Slight. This could change if afternoon convective development
across southern Arkansas overperforms tonight.

The Marginal in the Upper Midwest remains largely unchanged with
the cold frontal rainfall expected across Wisconsin and Illinois.
Wisconsin up to Minnesota has been very wet in recent days, while
urbanization in and around Chicago, the Quad Cities, and points in
between locally lower FFGs in this area. Some training storms may
pose an isolated flash flooding threat into Friday evening.

Wegman

...Previous Discussion...

...Upper Mississippi Valley into the Upper Great Lakes...
The next strong area of height falls moving into the northern High Plains
will continue to push eastward through the Northern Plains and into the
Upper Mississippi Valley. The overall favorable large scale conditions
ahead of these height falls from Thursday will continue into Friday
along the associated front where strong frontal convergence in an axis
of PW values 1 to 1.5 standard deviations above the mean will support
potential for moderate to locally heavy rainfall totals. This front
does look progressive, which may limit very heavy amounts. The marginal
risk was drawn to where there may be overlap of locally heavy rains
Friday through Friday night with heavy rains that have fallen as of
late. Hourly rain totals to 2" and local amounts to 4" are expected
where cells merge or manage to train.


...ArkLaTex, Tennessee Valley, & southern Appalachians...
A broad marginal risk area continues and was expanded in areal
coverage from continuity based on the most recent QPF guidance and
to some degree heavy rainfall that occurred near the ArkLaTex on
Wednesday. Precipitable water values across these regions will
remain above average, with additional shortwaves in from the west
supporting scattered convection and locally heavy rainfall amounts.
Spread remains within the guidance, which keeps the threat level
Marginal. The ingredients in place support hourly rain totals to
2.5" with local amounts to 5". Should the guidance show more
convergence in future runs, a Slight Risk could be prudent as
portions of the area saw heavy rainfall Wednesday and should see
heavy rains again on Thursday.

Roth


Day 3
Valid 12Z Sat May 25 2024 - 12Z Sun May 26 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL PLAINS...

...2030Z Update...

Few overall changes were needed as the area covering much of
eastern Kansas and Missouri remains the area of focus for
convection on Saturday. However, as convection in this region will
be largely dependent on the behavior of convection from previous
days, guidance agreement is poor.

Since there is a large amount of spread in this area, and with
recent heavy rainfall, the Slight was expanded to cover more of the
potential scenarios for flash flooding in this area. Most of the
guidance took the unusual step of trending north, which is the
direction much of the expansion of the Slight went, but since
convective behavior tends to trend south, very little change was
made to the southern end of the Slight. It's likely that with
better agreement closer to the event that the Slight will be able
to be shrunk to follow those trends.

Wegman

...Previous Discussion...

The Marginal Risk in the area continues and was expanded in areal
coverage to account for the most recent QPF guidance, with the
Slight Risk a new addition. 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.

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