896
FOUS30 KWBC 290824
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
424 AM EDT Mon Apr 29 2024

Day 1
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...

Current sat/radar composite shows an organized area of convection
across east TX into northern LA, propagating southeastward on the
flank of a cold pool driven outflow from the storms in question. A
secondary area of convection is moving northeastward through the
Mid-Mississippi Valley into the Midwest with locally heavy rainfall
embedded within the main line. The MCS development over east TX
will continue its forward motion into southern LA and the adjacent
southeast TX coastal areas between Houston and Beaumont with the
greatest impact focused over southwestern LA, including Lake
Charles. Latest hi-res guidance has caught onto the threat with the
warm start deterministic like the HRRR/NAM 3km handling the primary
threat for the period the best thanks to some real-time radar
inclusion in the model guidance. Despite some of the other members
within the HREF CAM grouping being too far north with the current
depiction, there is a better consensus on the timing and expected
impacts over the aforementioned areas.

The latest HREF is further south and west with regards to the
primary QPF signature with the HREF blended mean settling between
2-3.5" of total precip within the first 3 hr window in the period
(12-15z). This is the primary time frame of interest as the cold
pool propagation will continue until the complex moves out over the
Gulf, putting an end to the threat with only some lingering
convection possible for the coastal areas near Houston over into
LA. Heavy rainfall is all but a certainty across southeast of TX
near Houston over into the southern parishes of southern LA. HREF
probabilities for rates exceeding 3"/hr are upwards of 20-40% with
a bullseye near 45% located right along TX/LA border south of I-10.
This area has a higher FFG index within all 1/3/6 hour windows, so
the prospects of significant flooding is reduced with the
anticipated forward motion being progressive enough to limit higher
end potential. Some of the larger towns from Houston over towards
Beaumont and Lake Charles have the most significant threat due to
the impervious surfaces from the larger urbanization footprint
that enhances runoff, so the threat is well within the SLGT risk
threshold that was in place with expansion to the west to account
for the latest trends in radar. Local totals upstream have
eclipsed 5" in several places, and the 00z HREF probs for >5" is up
near 20% within that I-10 corridor to the coast. If this was in
many other areas of the country, this would warrant a greater
consideration for upgrades, but the soils and swamplands
encompassing that part of the country deter the higher end
scenarios with a steady moving complex, thus maintained continuity
with the SLGT. Have also kept the expansion over to the east to
account for New Orleans proper where the eastern flank of the
complex and other thunderstorm development could provide a window
for flash flooding later this morning and afternoon.

Further north, the large scale forcing will lift off to the north
as our storm occludes and weakens over the northern Midwest.
Further south, a trailing shortwave will enter the picture with a
steady eastward progression through the central and southern plains
over into the Tennessee Valley. A slow-moving cold front will
trudge eastward through the afternoon with a modest convergence
signal over the southwestern reaches of the Ohio Valley into the
Mid-Mississippi Valley near southern IL and western KY.
Probabilities for rates to exceeding 1"/hr later this afternoon and
evening are sufficient to acknowledge a threat for flash flooding
as convection fires within the zone of best ascent and low-level
convergence thanks to the diffluent pattern from the approaching
shortwave and the aforementioned cold front. Modest MUCAPE between
1000-1500 J/kg will be located ahead of the cold front with a
decent theta-E ridge in place over much of the Tennessee and Ohio
Valleys. With elevated PWATs bordering 2-2.5 deviations above
normal, the environment is ripe for any convective development to
produce locally heavy rainfall within the corridors of interest.
Deterministic output of 1-2" are common within the current CAMs
suite with the HREF blended mean generally situated between
1.25-2.25" across western TN up through southern IL/IN. A SLGT risk
was entertained, but went with a continuation of the MRGL risk due
to capped hourly rates and higher FFGs situated over the areas in
question.

A tertiary max QPF (1.5-2.5") is located over northern AL where
large scale forcing from our progressive shortwave trough will
induce a round of nocturnal convection over northern MS spreading
east through AL. Progressive nature of the precip and higher FFGs
will also limit the potential to a degree and felt the MRGL risk
was appropriate with the setup, thus maintained continuity from
previous forecast cycle.

Kleebauer


Day 2
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 Marginal Risk was maintained across PA/NY state with some
expansion into VT thanks to marginal convective risk coupling with
some wetter antecedent conditions that arose from the previous day
of rainfall. A west-east situated stationary boundary will remain
draped across Upstate NY through into the adjacent Ontario Province
with a cold front progressing across the Great Lakes and Ohio
Valley into the Northeastern US. Elevated PWAT signatures extending
up through the eastern CONUS will enhance the environment capable
of producing some locally heavy rainfall on Tuesday afternoon and
evening as the aforementioned cold front approaches and provides a
decent convergence pattern across the Northern Mid-Atlantic into
parts of New England. The flow will be progressive in nature, the
max potential is capped with HREF probability fields capping hourly
rates to between 1-2"/hr with 1"/hr still very promising over
western NY once convective initiates (50-70%). This will be enough
to cause some localized flooding within the terrain extending
through the northern Hudson up through the Adirondacks with the
northern extent situated over into VT and the southern edge over
central PA. The combination of low-level convergence and ascent
from an approaching shortwave is best suited over NY state with the
northern and southern periphery approaching the lower end of the
MRGL threshold as the upper ascent pattern will be the primary
driver for each respective location. A max of 2-3" will be forecast
across a few areas in NY, but the general consensus is 0.5-1.5" for
the rest of the areas impacted. This was sufficient for the MRGL
risk to be maintained.

...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 quick moving shortwave will eject out of the southern periphery
of a negatively tilted longwave trough over the northern plains and
southern Canada leading to convective genesis over the northern and
central plains. The synoptic evolution has gained consensus from
the latest deterministic suite which has allowed for some general
agreement amongst the ensembles during the Tuesday evening time
frame. The area that is most likely to see some convective impact
will be the Upper-Mississippi Valley into the northern Midwest
where convective initiation over the adjacent northern plains will
quickly shift eastward with locally heavy rainfall anticipated
within any multi-cell clusters that develop from the west. The
progressive nature of the storm motion will limit the threat to a
lower end of the MRGL risk threshold, but a formidable mid-level
ascent pattern and modest tongue of elevated instability extending
up through IA and southern MN will garner a chance for a few areas
to overachieve and facilitate a MRGL threat of flash flooding. QPF
max of up to 2" is possible with the general consensus leaning
towards 0.75-1.5".

...Southern to Central Plains...

Increasing southerly flow thanks in part to the development of a
surface cyclone over the northern plains will aid in the advection
of elevated theta-E`s and associated instability within the
confines of central and eastern KS on Tuesday afternoon. A cold
front will strengthen along the tail end of the surface low and
progress eastward allowing for a developing surface convergence
pattern in-of the central plains. Large scale forcing from a
progressive shortwave on the tail end of a broad, negatively tilted
longwave trough will act in tandem with the surface to create a
locally dynamic convective regime within the northern periphery of
our theta-E ridge. Best prospects for convection remain over
eastern KS into northern MO with localized totals over 2" plausible
in KS. Hourly rates on the latest HREF are around 1"/hr based on
probabilities, but it was at the end of the run when convection is
just starting to get its act together. Would not be surprised to
see better signals in later runs leading to the MRGL risk area
maintained with an outside chance at a SLGT.

A lower end threat for flash flooding will exist over OK into
northern and west TX as convection tries to fire over the dryline
positioned from the TX Big Bend up through OK. A lot of the
convective threat is conditional in nature, but the moisture and
instability are present and more than capable of some heavier rains
within any convection that does develop. It will come down to
whether a small mid-level perturbation ripples through the flow
and allows for enough mid-level ascent to help ignite the
convection within the confines of the boundary. It`s split on
guidance with a weak signature in the ensemble members/mean. In the
deterministic that is more aggressive, forecast totals over 2.5"
were plausible, so wanted to maintain continuity to account for the
potential.

Kleebauer/Otto

Day 3
Valid 12Z Wed May 01 2024 - 12Z Thu May 02 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL IS FORECAST OVER PORTIONS OF
THE MIDWEST, AS WELL AS THE CENTRAL AND SOUTHERN 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. QPF maximum of over 3"
is likely given the setup which would create the threat approaching
the elevated side of the SLGT risk thanks to wet antecedent
conditions from prior rainfall expected to reduce FFG indices. The
SLGT risk was maintained from prior forecasts and will be assessed
for potential upgrades in later forecast cycles.

Across the southern plains, there`s some question as to exactly
where the secondary precip maximum will occur as a quick moving
shortwave embedded within the sub-tropical jet ejects out of MX and
moves to the northeast by Wednesday evening. Recent deterministic
is coming around the ML model consensus of central and north TX
being 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. Considering the ML
output remaining steadfast from the past succession of runs, the
Red River into the TX Hill Country and points east will likely see
some flare up of thunderstorms with a deep, moist convective
pattern capable of enhanced rainfall rates typical with flooding.
This allowed for a maintenance of the previous SLGT risk in place
with some minor adjustment around the southern edge to account for
the latest ensemble bias corrected QPF and ML output of 24hr QPF
averages.

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