408
FOUS30 KWBC 291551
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1151 AM EDT Mon Apr 29 2024

Day 1
Valid 16Z Mon Apr 29 2024 - 12Z Tue Apr 30 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE MID-SOUTH...

A slow-moving cold front will move east through the afternoon and
evening with a modest convergence signal over the southwestern
reaches of the Ohio Valley into the Lower to Mid Mississippi. The
Mid- South region is between two main upper level disturbances --
on moving along the Red River of the South and the other trekking
across southwest MN per recent water vapor imagery, with some
possibility of an MCV trying to sneak in from the southwest. This
leaves portions of the Lower to Mid Mississippi Valley within a
region of potentially less forward propagation and better upper
level diffluence/divergence. ML CAPE up to 2000 J/kg will be
located ahead of the incoming 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. Effective
bulk shear is sufficient for organized convection, and the flow
will be fairly unidirectional out of the south-southwest to
southwest. Hourly rain totals to 2" and local amounts to 4" are
anticipated, which led to the upgrade to a Slight Risk.

Rainfall elsewhere is expected to have less instability to work
with and have greater forward propagation, so a large Marginal Risk
remains to deal with the possibilities there. Significant
reductions to the risk areas were made across TX and LA due deal
with convective progression. Of note, the 00z/06z HREF appears much
more useful for convective progression closer to the Gulf Coast,
albeit slow, when compared to the 12z HREF..

Roth


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 & CENTRAL/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