Excessive Rainfall Discussion...UPDATED
NWS Weather Prediction Center College Park MD
1028 PM EDT Sun Apr 28 2024
Day 1
Valid 0221Z Mon Apr 29 2024 - 12Z Mon Apr 29 2024

...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
EAST TEXAS INTO FAR WESTERN LOUISIANA...

...0230z Special Update...

A line of slow-moving thunderstorms will continue to train over
portions of east TX with the primary line situated from
Bryan/College Station, east-northeast to the LA border. Rainfall
rates in excess of 3"/hr have been reported with 2-3 hr totals
approaching 6" in places within the line. A strong convergent
signature is noted on radar with light returns feeding up from the
south against the band of heavy rainfall. Until cold pool
propagation can occur, significant totals in excess of 8-12"+ will
be plausible as indicated via the latest HRRR which has the best
short-term handle on the convection. A targeted Moderate Risk has
been added to the D1 update to reflect the increasingly likelihood
of flash flooding, as well as locally significant impacts likely
within the next 3-6 hours.

Kleebauer

...01Z update...

00Z radar imagery and surface observations showed convection
becoming more numerous and more intense from parts of Southern
Plains into the Lower and Middle Mississippi Valley. High
resolution CAM guidance latched on to that idea...building
confidence in their solutions showing activity persisting into the
overnight night hours before the intensity wanes. The primary
concern for flash flooding appears to be from southern Arkansas
into southeast Texas and nearby portions of Louisiana where
rainfall rates appear to be sufficient to result in excessive
rainfall in the event of training/repeat convection. Farther north
in the mid-Mississippi Valley there is a potential for flash
flooding from a second area of convection due to soils having
become saturated by recent heavy rainfall and therefore more prone
for flash flooding even with lower rainfall rates/amounts
(especially from eastern Oklahoma into southwest Missouri). Not
entirely sold on the latest HRRR idea of bridging the two areas
with rainfall amounts/rates on the order of that produced by
convection to the south...with a corresponding degree of excessive
rainfall threat...but certainly not going to rule it out entirely
while not ruling out the risk of flash flooding there overnight.

Bann

...Previous Excessive Rainfall Discussion...

Expect to see additional convective development along the
front/dryline across TX into eastern OK, with this activity pushing
eastward into AR and southern MO by this evening. This will be
supported by strong and persistent upper level divergence, and
increasing 850mb moisture transport/convergence into the
front/dryline. The 850mb moisture transport axis is a bit more
progressive compared to Saturday, which combined with cold pool
generation from the organized convection, should result in an
eastward propagation of convection. Maybe just enough of an
eastward motion to prevent a more widespread flash flood event.
However, with the orientation of convection likely to match mean
steering flow from the southwest at times...there will still likely
be some brief training of cells. This will support rainfall rates
as high as 1-3"/hr at times, and do expect we will at least see
isolated to scattered flash flooding over a fairly large swath from
eastern TX into southern MO.

HREF EAS probabilities of exceeding 2" get as high as 40-70% over
the area, but 3" EAS probabilities drop to 15-30%. Meanwhile
neighborhood probabilities of exceeding 3" are over 50% over a
large area, but 5" probabilities drop more into the 15-30% range.
With EAS probabilities more indicative of QPF coverage, and
neighborhood probabilities higher end maximum potential...this all
suggests that areal averaged rainfall will tend to stay in the
1-3" range...with localized swaths of 3-5" likely as well. However
5"+ totals are a bit more uncertain...and while I'm sure we will
see a swath or two of over 5" of rain, exactly where remains
unclear. Overall would like to have seen a more concentrated and
higher 5" probabilities to upgrade to a MDT risk. From a hydrology
perspective, soil saturation and streamflows are elevated over
some of this area, and not as much of other locations. Thus not
quite enough sensitivity to justify using this as a reason to do a
categorical upgrade at this time.

Thus, overall still think this event remains as a higher end
Slight risk across eastern TX, western LA, southeast OK, central
and western AR and southern MO. Both the 00z and 06z HRRR runs have
been aggressive with convective training and QPF amounts on the
southwest flank of the area of convection over southeast TX into
far western LA. This is a typical favorable location for
training/backbuilding, so the HRRR evolution is possible. However
the HRRR is further south of the higher HREF probabilities...while
the experimental NSSL MPAS runs are actually further south of the
HRRR, and even indicate the possibility ongoing convection in the
morning persists and propagates fairly quickly through the region.
Thus think there is still a bit too much uncertainty to go with a
MDT risk upgrade...although this appears to be the most likely
location for one...and so will continue to monitor observational
and model trends today.

Across the Midwest into Wisconsin and Michigan, a broad Marginal
Risk remained in place with minor adjustments from continuity. Not
a much instability here, and convection should generally be moving
along at a decent clip. Although like further south, some brief
along track training is possible given steering flow parallel to
the boundary.

Chenard