Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
826 PM EDT Thu Apr 18 2024

Day 1
Valid 01Z Fri Apr 19 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...

...01Z Update...
Only a few changes needed from the outlook issued earlier today.
Those changes were largely based on trends in short-term radar
imagery and the placement of a well-defined front making its way
from the western Ohio Valley into the Southern Plains. Made
somewhat of an eastward/southward expansion from the Ohio Valley to
the mid-/southern Rio Grande Valley based on early evening
instability fields...but the overall forecast reasoning has changed
little.

Bann


...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 UP THROUGH THE RED RIVER...

...20Z Update...

A small break in the convective potential will ensue on Friday with
a return mainly occurring Friday evening through the end of the
period. A cold front will continue to motion south through TX
Friday before stalling over the south-central portion of the
state later in the afternoon. Out west, a shortwave trough will
propagate eastward off the Pacific with increased large scale
forcing occurring downstream over the Desert Southwest, eventually
into TX by early Saturday morning. The combination of favorable
upper forcing and cooler air advecting behind the cold front will
enable a deepening isentropic environment in-of the southern plains
from the Red River down through north TX. A secondary area of
interest will be over the TX Big Bend where a weak shortwave
impulse moving out of Mexico could spur a round of convection off
the Serranias del Burro with southwestern steering flow taking it
towards the Rio Grande by nightfall.

There's a split in the potential within guidance with some
sneaking the convective cluster over the border with locally heavy
rainfall focused over the Rio Grande and southern Edwards Plateau.
Others maintain a quiet outlook with the convection dissipating
before crossing the border. This was reflected well within the HREF
EAS probabilities remaining low into the 10-15% range with only a
few guidance promoting the potential. Despite the low
probabilities, moisture rich environment and higher end QPF within
the upper quartile of outcomes allows for a maintenance of the
MRGL risk area encompassing the Big Bend into the Edwards Plateau.

Further north, the output from deterministic is very split with a
50/50 distribution on low end QPF development between 00-12z
Saturday and much more favorable QPF for MRGL concerns to arise
over the western rolling plains up through north TX and the Red
River Valley. Considering the GFS/ECMWF ML output for the time
frame being fairly similar and leaning towards the higher end of
the QPF distribution, decided to maintain the MRGL from previous
forecast and link the two areas together with some adjustments on
the edges based on ensemble bias corrected data and HREF blended
mean QPF footprint.

Kleebauer

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

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR CENTRAL AND
EAST TEXAS THROUGH PORTIONS OF THE LOWER MISSISSIPPI VALLEY...

...20Z Update...

The previous forecast remains on track with little deviation in
both deterministic and ensemble based guidance leading to a higher
confidence forecast at this juncture. The previous SLGT risk was
expanded across the southern and western edges to account for some
minor variance in the QPF distribution in the latest 12z ensemble
output. Heaviest precip will be located over central and east TX
with precip totals between 2-4" likely with locally higher
positioning somewhere within the vicinity of the stalled frontal
boundary. 1/3/6 hr FFG intervals are on the higher side for much
of the area encompassed by the SLGT (2"/4"/5" respectively),
however the area within central TX is most prone to flash flooding
due to the composition of the soils and higher propensity for
runoff. There's also been some heavier rain episodes recently that
have primed the top layer of of soils with latest NASA SPoRT soil
moisture composite indicating percentiles between 70-80% within a
large area of east TX. Pending the convective output from future
hi-res deterministic and correlated ensemble data, there's a chance
an area within the SLGT could be upgraded further. For now, will
lean more towards the higher end of SLGT with some time to monitor
trends in the QPF forecasts, as well as assessment of where the
stalled boundary will settle as that is the main feature for the
potential.

Kleebauer

...Previous Discussion...

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