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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1537Z Mar 24, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1136 AM EDT Sun Mar 24 2024

Day 1
Valid 16Z Sun Mar 24 2024 - 12Z Mon Mar 25 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR NORTH-
CENTRAL TEXAS INTO SOUTHEASTERN OKLAHOMA...

...16Z Update...

Maintaining continuity across the MRGL risk area in the southern
plains as guidances remains steadfast on a convective line
developing across north TX through the Red River Valley into
southeast OK. Latest 12z HREF EAS probability for at least 1" of
precip has improved across southeastern OK with probabilities now
exceeding 60% for the region southeast of McAlester, OK. Hi-res
deterministic has convective QPF maxes ranging from 1.5-2.25" with
the blended mean settled on 1.5" north of the Red River. Theta-E
gradient along the leading edge of a cold front spells for ample
instability with shear presence noted thanks to a 55-60kt LLJ
pattern ahead of the mean 850mb trough. This will allow for upscale
growth within a moist sector present over the eastern half of the
southern plains as storms fire and organize, moving steadily
eastward through the overnight hours. Luckily the forward
propagation should be enough to curtail any significant flooding
chances, but a low-end Marginal will suffice given the convective
pattern unfolding. This was plenty to maintain continuity from the
previous forecast.

Limited rates and lack of coverage across the southern CA area has
prompted a removal of the previous MRGL risk in place within the
San Diego CWA. Some locally moderate rain is possible this morning
and afternoon before conditions improve. The heaviest precip is
centered over Mt Laguna which will be the beneficiary of winter
weather thanks to their elevation. This will thwart any flood
concerns, thus ending the need for a MRGL continuation within the
Peninsular Range and San Diego proper.

Kleebauer

...Previous Discussion...


...North-Central Texas into Southeastern Oklahoma...

Developing thunderstorms around sunset today on the western edge of
the Marginal Risk area will quickly develop into a line as it moves
across north-central Texas and Oklahoma tonight. The line will be
close to parallel to the environmental flow, as the parent low over
Kansas turns more northward, thus slowing the eastward progression
of the line. The result will be storms with a higher potential for
training as they track northeastward along the line. The Marginal
is low confidence, but multiple CAMs suggesting a local maximum of
rainfall suggests isolated flash flooding to be enough of a
possibility to warrant a Marginal risk, albeit a low-end one, for
the tonight period.

Wegman

Day 2
Valid 12Z Mon Mar 25 2024 - 12Z Tue Mar 26 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER MISSISSIPPI VALLEY...

...Southern Missouri and Western Arkansas...

An line of thunderstorms will be ongoing from Texas and Oklahoma at
the start of the period Monday morning. Similar to the Day 1
reasoning for TX/OK, the parent low track turning more northward
with time during the day Monday will cause a slowing of the
eastward progression of that line of storms. A robust low level jet
will advect a plume of deeper Gulf moisture into Louisiana and
Arkansas Monday at the same time, which in turn will locally
enhance the storms just as their eastward motion slows. Thus, there
is good agreement on higher rain fall totals in a rather narrow
line from Texarkana north-north east into south-central Missouri.
This will act as the pivot point where the line of the storms will
be close to stationary. Areas to the north into Illinois will have
weaker storms and less moisture due to lack of instability and
distance from the Gulf, whereas storms to the south across northern
Louisiana will have more moisture and instability, but will be much
faster moving. Local rainfall totals to 3 inches in this
"Goldilocks Zone" where the line stalls but storms remain strong
could cause widely scattered flash flooding.

There will likely be an area through central Arkansas with a local
minimum of rain as the upper level forcing with the line stays
further west, and a second strong impulse of upper level energy
tracks eastward further south. For simplicity the Slight includes
this area to not have 2 separate Slight risk areas almost right
next to one another, and to account for inherent uncertainty as to
exactly where this local minimum will ultimately set up.

For Mississippi and far western Alabama, the line of storms will
reintensify Monday evening as the typical strengthening of the
nocturnal low level jet advects environmental moisture
characterized by PWATs locally exceeding 1.75 inches across eastern
Louisiana and southern Mississippi. Leftover instability from the
day will advect into this same area as well. Thus, despite good
agreement on a rapid eastward movement of the line of storms, the
ingredients supporting the storms will all increase for a time as
the line traverses Mississippi and eventually far western Alabama
(though uncertainty increases that far east). Rates to 2 inches per
hour given the abundant atmospheric moisture will be possible, even
as any one area only sees rates that high for up to an hour. Such a
deluge of moisture will quickly overwhelm any local streams and
creeks with a large influx of water, so the Slight remains in
effect.

Once the line gets much into Alabama, they will encounter a
much more stable atmosphere across the state which will have the
benefit of cooling off from the day, increasing the stability
before the storms arrive in the wee hours of Tuesday morning.
Expect rapid weakening of the storms by this point, however given
all the moisture the storms will have to work with, rates will
still be high as the storms become increasingly downdraft dominant.
The Slight was expanded east to account for a greater eastward
translation of the storms in much of the guidance. The faster
eastward forward speed should also work to reduce flash flooding
potential due to less rainfall into Alabama as well.

Wegman

Day 3
Valid 12Z Tue Mar 26 2024 - 12Z Wed Mar 27 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST...

As mentioned in the Day 2/Monday discussion, any lingering
convection ongoing at the start of the period Tuesday morning will
be encountering an increasingly hostile/stable environment for
storms over Alabama. Both rates and storm total rainfall will
quickly diminish Tuesday morning. The inherited Slight risk over
the entire state of Alabama was downgraded in coordination with all
3 Alabama offices with this update. Guidance suggests storms could
refire with afternoon heating over the western Florida Panhandle,
southeast Alabama and possibly western Georgia, but those areas are
historically quite flood resistant with high FFGs and other than
Pensacola are mostly rural. Thus for now, the Marginal appeared to
be the best representative risk category, but a smaller and more
targeted Slight may be needed for this area with future updates if
forecast rainfall notably increases.

Wegman


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