FOUS30 KWBC 282030
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 PM EDT Sun Apr 28 2024

Day 1
Valid 16Z 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...

...16Z update...

15Z radar imagery and surface observations showed a broad area of
stratiform rain with weakening embedded convection from central
AR into east-central TX. There is good agreement in the 12Z hires
model suite for redevelopment/reintensification of thunderstorms
between 18-00Z this evening. It is still a bit unclear if remnant
outflow will help to initiate storms and/or the dryline farther
west in central TX. Either way, a broad region of thunderstorms
aligned from NNE to SSW from eastern OK/western AR into eastern TX
appears likely. Within this area of thunderstorms will likely be
portions that align with the steering flow supporting training.
Current thinking is there could be a bi-modal distribution of
heavy rain, first across portions of eastern TX into western LA
and far southern AR where inflow/instability should be maximized.
This is in line with the 12Z HRRR, ARW, ARW2 and FV3. A second
area farther north is possible from far eastern OK into southern
MO/northern AR where synoptic scale lift will be greatest, with a
relative minimum in precipitation over central AR. Confidence for
a Moderate is still not there but perhaps a late update to
Moderate is possible as the convective situation becomes clearer
later this afternoon.

Otto

...previous discussion follows...


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
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