Quantitative Precipitation Forecast
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000
FOUS30 KWBC 250051
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
851 PM EDT Sun Mar 24 2024

Day 1
Valid 01Z Mon Mar 25 2024 - 12Z Mon Mar 25 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM CENTRAL
TEXAS INTO SOUTHEASTERN OKLAHOMA WESTERN ARKANSAS...

...01Z Update...
Ongoing convection ahead of a dryline/Pacific cold front centered
over western Texas is expected to continue, with additional
development along the southwestern flank as strengthening low level
flow supports increasing moisture advection into an area of
sufficient ascent and instability. Most of the CAMs, including
recent runs of the HRRR, appear to be too slow to develop
convection, but all generally show the same signal for southwest-
to-northeast training convection developing this evening and
continuing into the overnight. Recent runs of the RAP show PWs
increasing to around 1.25 inches ahead of the slowly advancing
surface boundary, while mid-level energy and left-exit region upper
jet forcing help support large-scale ascent.

With some adjustments made for current radar trends, the Marginal
Risk for the 01Z Update reflects the 18Z HREF, which shows high
neighborhood probabilities for an inch or more along an axis
stretching from the central Texas Hill Country northeastward across
North Texas into southeastern Oklahoma and western Arkansas. The
HREF indicates the heaviest amounts are mostly likely to fall
across southeastern Oklahoma, where it is showing high neighborhood
probabilities for amounts exceeding 2 inches.

Pereira

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...

...20Z Update...

No significant changes to the placement and orientation of the SLGT
risk across the Lower Mississippi Valley as the overall pattern
evolution on guidance maintains decent continuity from the previous
forecast cycle. There was an expansion of the MRGL up into the
central Midwest, mainly covering northern IL into extreme southern
WI. This was in conjunction to an increasing signal of convection
developing in-of of the warm sector across northeast MO, advancing
rapidly northeast with the aid of a strong mid-level steering flow
and deeper moist advection thanks to a roaring 55-60kt LLJ
extending from the Gulf coast up through the central Midwest.
Elevated PWATs on the order of 2 deviations above normal within the
environment will be plentiful for locally heavy rainfall falling
over a region with fairly low FFG indices thanks to recent
precipitation and some urbanization characteristics extending from
Rockford over to Chicago. Per request, as well from the Chicago
WFO, have expanded the MRGL risk area further north.

The Lower to Mid-Mississippi Valley will still be the primary
beneficiary of the heaviest rainfall potential over the CONUS.
Advancing cold front into a plentiful, buoyant environment will
generate periods of heavy rainfall with two main areas of focus;
the Ozarks on Monday morning and early afternoon, then across the
Mississippi Delta into western AL by nightfall thanks to convection
re-firing after the nocturnal LLJ redevelops over the
aforementioned area. PWATs between 2-2.5 deviations above normal
and sufficient speed shear will help organize cells into a line and
slowly trudge eastward through Monday evening. Rainfall rates on
the order of 1-2"/hr will be common within the heaviest precip
which will offer some localized flooding concerns within poor
drainage areas and within any urbanization constructs. Convection
will advance eastward into AL by the end of the period with
diminishing convective risks thanks to environment stabilizing
away from the primary upper trough. This is the reason for the
sharp delineation from SLGT to MRGL to nothing for places further
east.

Kleebauer


...Previous Discussion...


...Lower Mississippi Valley...

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 SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE FLORIDA PANHANDLE INTO SOUTHEAST AL AND SOUTHWEST GEORGIA...

...20Z Update...

In coordination with the Tallahassee WFO, have upgraded a small
portion of the FL Panhandle into the southern portions of AL/GA
given the latest trends in convective rainfall anticipated Tuesday
evening along the cold front. Recent rainfall across the FL
Panhandle has allowed for some decent saturation over areas
historically with higher FFG indices. Despite that, there is some
concern for locally heavy rainfall within a slow-moving regime to
develop Tuesday evening that could drop locally 2-4" totals in-of
areas that have been hit recently with heavy rain. Highest theta-E
signatures designate a primed area of instability over parts of the
Gulf Coast into the very southern portions of AL/GA. Some
deterministic are being more aggressive with the setup compared to
others, so the door is open for local flood prospects if cells do
form and exhibit some training characteristics with the steering
flow aligned parallel to the slow-trudging boundary that will enter
the area by Tuesday evening. As per the request of the WFO, have
added that targeted SLGT risk to cover for the area that has the
best opportunity to see some heavier rainfall the back half of the
D2 period.

Kleebauer


...Previous Discussion...


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: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt


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