916
FOUS30 KWBC 302337
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
737 PM EDT Tue Apr 30 2024

Day 1
Valid 01Z Wed May 01 2024 - 12Z Wed May 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST KANSAS & NORTHEAST OKLAHOMA...

...Kansas and Oklahoma...
The primary change made in this area was to shift the inherited
Slight Risk area south to include more of northeast Oklahoma with
this update. The 18z HREF, along with 18z GFS 1000-500 hPa
thickness diffluence field and broad MU CAPE gradient, strongly
suggest that any convection initially near the cold front/dry line
intersection near the KS/OK border should drop roughly southeast
this evening into tonight, driven forward by outflow boundaries,
before drifting southward. There remains considerable uncertainty,
as is typical, with exactly where the line of storms forms and how
quickly it moves to the south as the guidance can be slow with
progression. This region has been wet recently, with large swaths
showing 300%+ of their average weekly precipitation, so soils
should be sensitive. Should the western flank of the convective
line show backbuilding character, Moderate Risk impacts would be
possible.


...South Florida...
A Marginal risk remains for portions of south Florida. While conditions
have been drier than normal across much of the Florida Peninsula
lately, there is an area of populated Palm Beach County that
received 3"+ this morning, which could have made ground conditions,
outside of paved areas, more sensitive. The mesoscale guidance
suggests that increasing effective bulk shear associated with the
tail end of an upper trough in the eastern Gulf of Mexico should
allow convection to survive when CIN would normally set in for such
PWs (1.75") -- past 04-05z. However, eastward propagation offshore
into the Gulf Stream may force activity off the coast around that
time anyway. Believe heavy rainfall in southern Dade County
should have ended by 01z, so left them out of the Marginal. But,
that activity has shown the convective potential -- hourly rain
totals to 3" with local amounts to 5", which would remain possible
within the lingering Marginal Risk area. This accounts for any
convection moving over or through the large bubble of MU CAPE
across the Central Peninsula towards Palm Beach County that manages
to train, merge, or develop a random mesocyclone.


Roth/Wegman/Kleebauer


Day 2
Valid 12Z Wed May 01 2024 - 12Z Thu May 02 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS AND SOUTHERN PLAINS...

...2030Z Update...

The main change from the inherited ERO for Wednesday and Wednesday
night was to split the inherited Slight risk into 2 separate
Slights, one in the central Plains following the Missouri River
between Nebraska and Iowa, and a second for southwest Oklahoma and
much of northern and eastern Texas.

With the benefit of CAMs guidance, all of the models that go out to
the end of the period 12Z Thu indicate that the overnight
convection Wednesday night will primarily be in the form of two
simultaneous but otherwise completely separate squall lines that
will both move eastward, one across Nebraska into Iowa, and a
second across north Texas. The guidance is in good agreement that
there may be an isolated shower or storm in between, but otherwise
the area in between is largely dry. Thus, due to a local minimum in
the guidance over Oklahoma/Kansas with good agreement in the CAMs
as to the same, the inherited Slight was split into 2 separate
smaller Slights. Despite very wet antecedent conditions expected in
KS/OK, especially after tonight/Day 1, the very isolated nature of
any additional convection Wednesday suggests the threat is
marginal.

For both Slights, antecedent conditions will also play a big role
in the development of any new flash flooding. The squall lines
responsible for the rain in both areas are likely to be moving at
a fairly rapid clip. In Texas, cell mergers with newly forming
showers and storms ahead of the line will locally produce added
rainfall, whereas such a scenario is not expected for the northern
Slight. Since fast moving squall lines have a very difficult time
on their own with causing new flash flooding, the antecedent wet
conditions in these areas will have to make up the difference.

Wegman

...Previous Discussion...

...Midwest/Central Plains...

A broad, closed upper level cyclone will persist over eastern MT
into the adjacent northern plains and southern Canadian Provinces
by Wednesday with a potent, mid-level shortwave rounding the base
of the mean trough and exiting into the northern high plains by the
second half of the period. A vigorous upper diffluent pattern will
transpire by Wednesday afternoon and evening with a corridor of
convection initiating towards nightfall. A stout LLJ will enhance
areal shear in-of the central and northern plains within the Quad
state area between NE/KS/IA/MO. Any convective development will
congeal and grow upscale into a large cluster of heavy
thunderstorms within the above area leading to widespread heavy
rains capable of flash flooding as rates likely reach towards that
1-2"/hr marker with locally higher possible. The one factor that
may limit the higher end potential will be the progressive nature
of the convection as pattern yields faster storm motions limiting
the prospects for training. This does not mean there will not be
some areas that can overachieve and maximize the potential based on
the prime mid-level energy involved, along with a strong meridional
push of some very moist air approaching 2-2.5 deviations above
normal for PWATs based on the latest GEFS and NAEFS output. The
threat remains solidly within the SLGT with a chance at any upgrade
if future CAMs indicate a better potential for totals to exceed 3"
in a larger footprint than what is currently depicted.


...Southern Plains...

The location on a secondary rainfall maximum across TX is becoming more
clear as a quick moving shortwave embedded within the subtropical jet
ejects out of MX and moves to the northeast by Wednesday evening. Guidance
suggests that somewhere across eastern TX would be the primary target
for convective development with the heaviest rainfall located within
the tongue of higher theta-E`s positioned east of the line from ABI
to DRT. A cluster of thunderstorms will develop over west TX with a
steady propagation to the east during the afternoon and early
evening hours. By nightfall, a prominent LLJ aligned across
central and east TX will provide necessary maintenance to any
convective clusters that will migrate over the state after peak
diurnal instability. Surface dew points running into the upper-60s
and low-70s relay the moisture rich environment that will provide
the foundation for significant rainfall potential as the mid-level
vorticity maxima continues to move over the region providing the
additional large scale forcing that will keep the convection
rolling through the evening. Current deterministic output is
consistent with local totals breaching 4" across portions of
central and north TX with the latest ensemble bias corrected QPF
sufficiently in the range of 2-3" over an area extending from
southwest OK down through the DFW metroplex and adjacent surrounds.

This is quickly gaining steam for a higher impact event,
especially when you compound the idea that the northern portion of
the I-35 corridor in TX becomes a focal point. There is some spread
on the exact positioning of the heaviest precip expected, but ML
outputs have been persistent in the area over north TX for the past
succession of runs. Should deterministic guidance converge further
with similar amounts, or if ensemble support becomes more
aggressive with totals and probabilities for exceeding higher end
QPF markers, a Moderate Risk cannot be ruled out in later
issuance`s. Moderate Risk impacts are certainly possible wherever
the heavy rainfall materializes with the highest prospects within
the major urban centers along I-35.

Kleebauer


Day 3
Valid 12Z Thu May 02 2024 - 12Z Fri May 03 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHEAST TEXAS AND NORTHWEST LOUISIANA...

...2030Z Update...

...Texas/Louisiana...

12Z guidance remains in good agreement for a local maximum of
rainfall as high as 5 inches, with locally higher amounts in the
most persistent storms into portions of northeast Texas and
northwest Louisiana. While there are understandable differences in
the guidance as to the exact placement of the rain and certainly
how much...agreement remains good both in the current cycle of
guidance as a trends from the previous that this area will be hit
with storms producing multiple inches of rain, likely during the
daylight hours Thursday. This area has been hard hit in recent
days, resulting in saturated soils and very high river levels, with
a few draining south towards the Gulf still in major flood stage.
As such, additional multiple inches of rainfall should almost all
convert to runoff, likely resulting in scattered instances of flash
flooding, several of which will be locally
significant/considerable.

As mentioned, while there are some differences in the placement of
the heaviest rainfall, the unusually high level of agreement on the
broader area and magnitude of the impending rainfall event when
added to the ongoing flooding in the area increased the confidence
that the area of greatest overlap in northeast TX and northwest LA
of Moderate Risk level impacts. As always when dealing with
convection, once the event moves into the CAMs time frame it`s
likely there will be changes in the placement and magnitude of the
heaviest rainfall. The area will continue to be monitored.

...Elsewhere...

Very few changes were needed for the Slight up into the Midwest, as
there remains much greater uncertainty as to the convective
evolution Thursday in this region. Much of this area is at or above
average for soil moisture given recent (and ongoing) heavy rains,
so reduced FFGs as far north as Wisconsin would support renewed
flash flooding with any persistent storms.

Wegman

...Previous Discussion...

...Summary: Two distinct areas of interest will arise for heavy
rainfall on D3 as a strong mid-level shortwave continues to march
eastward out of TX into the Lower Mississippi Valley by the middle
of the period. Further north, a powerful, negatively tilted trough
will maintain foothold across the northern plains with a strong
vorticity maxima pivoting around the southern periphery of the
closed upper reflection in place across southern Saskatchewan. Each
upper forcing will play a role in locally heavy rainfall impacts
spreading eastward from the continuation of the previous period.
The primary threat is currently front loaded with the best flood
prospects leaning towards Thursday morning and afternoon before the
threat wanes heading into the second half of the forecast period.

...Midwest...

Overnight convection from Wednesday will continue to march eastward
as the congealed cold pools from the initial cell mergers will
advance through the central Midwest with sights on eastern IA into
WI and MN on Thursday morning. The complex will eventually peter
out as it moves northward into a less favorable environment which
will put an end to the flooding threat as we move into the
afternoon. Later in afternoon and evening, a cold front will
propagate to the east out of the central plains with a
redevelopment of convection over eastern KS, IA, and northwest MO.
This will allow for the secondary peak of flash flooding potential
as thunderstorms migrate through the mid and upper Mississippi
Valley, impacting some areas that were just affected in the last 24
hrs. With the lower FFGs expected, the threshold for flooding will
be much lower than climatological norms for some areas within the
expected secondary convective regime. Totals are currently within
the 1-3" range on guidance which is significant enough to warrant
at least the current SLGT risk in place with some chance at further
upgrades pending prior period(s) evolution and precip outcomes.


...Southern Plains and Lower Mississippi Valley...

A fairly significant complex of thunderstorms will likely be
ongoing in the beginning of the period with the cold pool
propagation forecast to impact areas of east TX into Louisiana,
similar areas that were just hit recently with significant rainfall
and flash flooding. The prospects for flooding are now well above
climatological norm as areal FFG indices are solidly 50% of what
they normally are during the time of year (2-3" compared to 4-5").
Forecast QPF is hovering between 2-4" with locally higher with the
ensemble bias corrected forecast showing a wide swath of 2+" and
some deterministic already ramping up over 5" across east TX into
north-central LA. Considering the repeated nature of these heavy
rain threats, the prospects for flooding is growing with a SLGT
risk in place over the Arklatex down into parts of east TX and the
Lower Mississippi Valley. Pending the forecast from the previous
period with CAM guidance assistance, an upgrade to a Moderate Risk
may very well be entertained.

Further north across KS and OK, the cold front to the north will
move south with a push from high pressure nosing in through the
plains in wake of the low pressure moving to the north. An area of
convection will likely form over the central plains and move south
with the boundary leading to more locally heavy rainfall in
association with the area of convection. The threat is less
aggressive compared to what is expected further south, but the
chances are still prevalent. The SLGT risk was maintained from the
previous D4 connecting the two primary areas of interest to the
north and south.

Kleebauer

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