164
FOUS30 KWBC 290228 AAA
QPFERD

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


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

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
SOUTHEASTERN TEXAS INTO 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. An impressive combination
of mid/upper forcing, low level convergence, instability and
moisture should be present to support a flash flood risk across
this region.

The 12Z model consensus indicates about 1000-1500 J/kg MLCAPE
ahead of what is expected to be a lingering nocturnal MCS from
eastern TX into LA. Precipitable water values are forecast to be
approaching 2 inches just ahead of the forecast leading edge of the
MCS with southerly 850 mb winds of 30 to 40+ kt just ahead,
advecting in moisture from the south. The region will also lie
within the diffluent and divergent right-entrance region of 100 to
130 kt upper level jet max. Timing and placement differences
remain within the 12Z components of the 12Z HREF concerning
placement of the expected MCS at 12Z Monday, but the best
probabilities for rainfall rates in the 2-3 in/hr range exist from
north-central LA into southwestern LA and southeastern TX where
backbuilding/training are most probable. The southwestern flank of
the MCS is expected to move offshore into the Gulf of Mexico
during the day as the leading edge continues to advance eastward
across the Lower MS Valley. The forecast evolution of the MCS has
it weakening and/or moving offshore by 00Z which should end or at
least significant decrease the flash flood threat for the region.

Farther north, a broad Marginal encompasses locations in the middle
MS and lower OH Valley, eastward into the TN Valley. These
locations will be within an anomalous moisture axis (standardized
PWAT anomalies of +1 to +2) and at least weak instability, located
ahead of a cold front approaching the lower OH Valley during the
day. Convective coverage will expand with daytime heating and while
most storms should stay progressive enough to limit flash flood
concerns, a similar orientation/magnitude of the mean steering and
850 mb flow suggests potential for localized training which may
pose some flash flood concerns despite dry antecedent ground
conditions.

Otto

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

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE SOUTHERN PLAINS INTO THE UPPER MISSISSIPPI VALLEY...AND OVER
THE UPPER TENNESSEE/OHIO VALLEYS INTO THE APPALACHIANS...

...Northeast...
A lower end Marginal Risk was maintained for central PA into
Upstate NY and portions of VT where localized 1 to 3 inch rainfall
totals may occur. Guidance indicates the presence of a NW to SE
oriented stationary front across this region on Tuesday, which
will likely help enhance lower level convergence, along with the
approach of a cold front from the west. Lift ahead of an
approaching mid to upper-level shortwave and associated right
entrance region of a jet max in southern Ontario/Quebec will aid in
convective development. MLCAPE of ~500 to 1000+ J/kg is probable
via the 12Z NAM (GFS tends to bias low with instability) over
portions of PA and NY, although a bit less certain over VT. PWs are
forecast to be quite high for late April/early May, with values
from the 12Z NAM/GFS/ECMWF just over 1.25" (approaching
climatological 99th percentile). Given the ingredients in place, it
seems plausible that at least an isolated flash flood risk could
evolve.

...Upper Ohio/Tennessee Valleys into the adjacent Appalachians...
Some degree of overnight showers and thunderstorms is expected to
continue into Tuesday morning from KY into TN, south and east of a
cold front approaching from the west. It`s not clear if storms
lingering into Tuesday morning from the overnight will pose a flash
flood concern but some redevelopment of convection is likely
during the afternoon hours. Moisture anomalies are forecast to
decrease during the day as an upper trough passes by overhead, but
a small window for localized flash flooding appears possible prior
to winds shifting around to the northwest near/after 00Z.


...Midwest to Upper Mississippi 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 a negatively
tilted shortwave moves across the north-central U.S. The attendant
cold front will be progressive, 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.

...Southern to Central Plains...
Further south the front is expected to become stationary over KS
with a dryline extending southward from western OK into
northwestern TX. Locations near the front could result in a
training/backbuilding convective risk, although large scale forcing
appears weak due to a lack of height falls across the region.
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.

Even farther south into OK and TX, while there could be a weak
perturbation aloft that helps to initiate convection, daytime
heating and erosion of CIN might be the main driver of convective
initiation along a dryline. 0-6 km bulk shear is forecast by the
12Z GFS to be somewhat lackluster but seasonably moist airmass and
recent heavy rain may be enough to support localized areas of flash
flooding, especially if there is overlap with remaining
hydrologically sensitive locations.

Otto


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