945
FOUS30 KWBC 280828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Sun Apr 28 2024

Day 1
Valid 12Z Sun Apr 28 2024 - 12Z Mon Apr 29 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS EASTERN PORTIONS
OF THE SOUTHERN PLAINS INTO THE LOWER AND MIDDLE MISSISSIPPI
VALLEY...

Convection will be ongoing at 12z this morning across portions of
eastern OK and TX, into western AR/MO. Still looking like activity
will be progressing off to the east at a greater clip by this time,
and the overall convective intensity should also be on a gradual
downward trend. Thus, while some lingering flash flood risk into
this morning is likely over these areas, the risk should be on a
decreasing trend.

By this afternoon 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


Day 2
Valid 12Z Mon Apr 29 2024 - 12Z Tue Apr 30 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE LOWER MISSISSIPPI VALLEY..

The convective and heavy rain threat shifts eastward into the lower
Mississippi Valley on Monday. Thunderstorms with heavy rain are
likely to be ongoing at the start of the period Monday morning
across portions of far southeast TX into portions of LA and
MS...and the main flash flood threat looks to be focused in roughly
the first 12 hours of the outlook period. Models continue to show
an impressive combination of mid/upper forcing, low level
convergence, instability and moisture to support a flash flood risk
across this region.

Most indications are that convection will be progressing off to
the southeast Monday morning given a gradual eastward push of the
convergence axis and cold pool prorogation. This is expected to
limit the extent and magnitude of any flash flood risk. However,
depending on the exact orientation of the convection by this time,
we could see some backbuilding/training on the southwest flank of
the MCS. This is most likely over LA, and this could drive a more
organized flash flood risk. Even without any more pronounced
training, the expected organized MCS will still be capable of
hourly rainfall as high as 2-3"/hr given the expected instability
and moisture in place. This should be enough to drive an isolated
to scattered flash flood risk, especially over urban areas. It
should be noted that there is some uncertainty on the timing of
the MCS, with the 00z and 06z HRRR suggesting the organized
convection is almost offshore by 12z. While this is a possibility
(and would require shrinking the Slight risk), this is more of an
outlier scenario at the moment....with most models supporting the
MCS more over central LA at 12z.

Additional convection is likely from MS into portions of the TN and
OH Valley. This activity will have the support of a compact
shortwave pushing east across the region. The progressive nature
of this feature, and more limited instability to work with, both
suggest any flash flood risk should stay localized in nature across
these areas. The Slight risk was cut back over these areas.

Chenard


Day 3
Valid 12Z Tue Apr 30 2024 - 12Z Wed May 01 2024

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE PLAINS INTO THE MISSISSIPPI VALLEY, AND OVER PORTIONS OF THE
NORTHEAST...

...Northeast...
A Marginal risk was maintained across portions of northeast PA
northward into NY and VT. Guidance indicates the presence of a
stationary front across this region on Tuesday, which will likely
help enhance lower level convergence. This combined with an
approaching mid level shortwave and weak surface low should be
enough to kick off convective development. Instability in excess
of 1000 j/kg is probable over portions of PA and NY, although a bit
less certain over VT. PWs are forecast to be quite high for late
April, with values from the ECMWF around 1.25" (approaching
climatological 99th percentile). Given the ingredients in place,
it seems plausible that at least an isolated flash flood risk could
evolve.

...Plains and MS Valley...
A broad Marginal risk extends from north central TX north into the
Upper Midwest. The northern portion of this risk over IA and MN/WI
will see the strongest forcing as a low pressure and ample
mid/upper level lift move across. This will be a progressive
system, which suggests convection will be moving quick enough to
prevent a higher flash flood threat. However do see enough
instability and moisture to support briefly heavy rates, which
could cause some localized urban flooding and/or isolated flash
flood issues where ground conditions are more sensitive from recent
rainfall.

Further south the front is expected to become stationary over KS.
This could result in a training/backbuilding convective risk,
although large scale forcing does look pretty weak. Nonetheless,
the presence of the stalling front and increasing low level flow
into this boundary does suggest at least some flash flood risk
could evolve.

Forcing is weaker further south into OK and TX. However there will
be a well defined dryline in the vicinity, and there are
indications in the models that at least isolated convection could
develop along this feature. Model QPFs would not support a flash
flood risk, however global guidance often under forecasts dryline
QPF anyways. Not thinking we will see great convective coverage,
but given some of this area will be saturated from recent heavy
rainfall, thought a Marginal risk was warranted.

Chenard


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