Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1142 AM EDT Thu Apr 18 2024

Day 1
Valid 16Z Thu Apr 18 2024 - 12Z Fri Apr 19 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS, MISSISSIPPI VALLEY, AND WESTERN OHIO VALLEY...

...16Z Update...

Areal coverage of the inherited MRGL risk was expanded to account
for the latest trends in guidance and radar/sat evolution this
morning. Surface low is currently located over southern KS with
expected northeastward motion through the period. A stationary
boundary located over the northern Ozarks and points east will be a
focal point for convection later this afternoon and evening with a
shortwave dipping southeast into the Mid-Mississippi Valley. A
secondary area of focus is unfolding over southern AR into northern
LA as a weak shortwave ejects eastward out of TX and induces some
modest large scale ascent within an increasingly unstable
environment over the Deep South.

Recent radar and IR satellite have indicated a quick growth of
convection across the aforementioned area with radar estimates of
rates approaching 2.5-3"/hr over far northeastern LA. This was a
newer trend within the latest CAMs with persistence into the
afternoon as the convection moves to the northeast in time (See MPD
#0161 for more information). The primary area of interest will
include that AR/LA border near the Mississippi River, eventually
propagating into north-central MS this afternoon and dissipating
prior to evening as the mid-level impulse degrades and we lose the
upper forcing mechanism. This was still enough to warrant a push
south of the MRGL risk with 1 and 3-hr FFG markers running right
around the proposed rates of 2-3"/hr within those cells in
question, backed up by 12z HREF probabilities of 20-30% potential
for at least 2"/hr possible and 40-50% for at least 2"/3-hrs.

Further north, the trend with the convective pattern over MO into
the Mid-Mississippi Valley has been fairly consistent the past
several deterministic runs and is in lock step with ML algorithms
signaling a primary QPF maximum within the confines of eastern MO,
southern IL, into far western KY. This was no different within
this mornings CAMs and aligns perfectly with the current SPC D1
Outlook of a widespread Slight and Enhanced risk co-located within
the northern extent of the MRGL risk area. An upgrade was
entertained, but the progressive nature any convection was a
deterrent for the update. It still bears watching as any storm
that gets rooted at the surface along the stationary front could
induce training potential that isn't being picked up by the latest
CAMs. For now, maintained continuity based on the HREF blended mean
QPF and lower end probabilities for exceeding 2"/hr within the hi-
res ensemble (10-15% maximum).

There are no changes within Texas as surface based convection will
develop over central TX within an area of prime instability.
Coverage is not expected to be widespread and primary interest will
likely be one or two stronger storms that will produce some locally
heavy rainfall over the northern Hill Country to about the I-35
corridor. Probabilities of at least 3" of total rainfall area
generally 10-15% within central TX with a bullseye of 20-30% over
the Hill Country between Brady and Junction.

Kleebauer

Day 2
Valid 12Z Fri Apr 19 2024 - 12Z Sat Apr 20 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE TEXAS BIG BEND AND WEST TEXAS AND FOR PARTS OF THE RED RIVER...

The cold front will continue to press southward through the
Southern Plains and Lower Mississippi Valley before
slowing/stalling. A dryline will also be present from the Oklahoma
Panhandle to the Big Bend area. Convection is expected to be along
and south of the front over the Southern/Central Plains and Lower
Mississippi Valley with the higher QPF near/east of the dryline in
west-central Texas.

The consensus for the area over the Texas Big Bend up through the
Edwards Plateau has grown within this suite of guidance with the
environment primed for higher end convective development from the
Lower Trans Pecos, eastward into the Rio Grande and adjacent
Edwards Plateau. Ensemble probabilities remain on the lower end,
however they are within the 10-20% range for totals exceeding 2"
which would be sufficient for localized flood concerns within the
terrain focused over the area.

The timing differences remain across the Red River and southern
Oklahoma with the front into that area with the convergence
pattern expected towards the back end of the forecast period with a
higher potential for flash flooding. A Marginal Risk area spans
the Big Bend area northeast into the Edwards Plateau and Concho
Valley. Another Marginal Risk area covers portions of southern
Oklahoma, northeast Texas and western Arkansas.

Campbell/Kleebauer


Day 3
Valid 12Z Sat Apr 20 2024 - 12Z Sun Apr 21 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR EASTERN TEXAS,
NORTHWEST Louisiana, SOUTHWEST ARKANSAS AND EXTREME SOUTHERN
OKLAHOMA...

The front will remain stalled across the South meanwhile moisture
will steadily increase. Precipitable water values of 1 to 1.5
inches (upwards of 2.5 sigma) will be in place across much of the
Southern Plains and Lower Mississippi Valley also where there will
be very strong mid-level ascent. This environment will be very
conducive for widespread heavy rainfall. The highest QPF will focus
over central and eastern Texas where areal averages of 2 to 4
inches will be common - a Slight Risk area covers much of this part
of region. Isolated higher amounts will possible, particularly for
locations east/northeast of the Hill Country. A Marginal Risk area
spans from western Texas to eastern Mississippi.

Campbell


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