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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1952Z Apr 09, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
351 PM EDT Tue Apr 9 2024

Day 1
Valid 16Z Tue Apr 09 2024 - 12Z Wed Apr 10 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EAST TEXAS, THROUGH THE SOUTHERN MISSISSIPPI VALLEY...

...16Z Update...

The general consensus this morning is for a greater westward
expanse of the heavy rain threat as convection blossoms in-of east
TX through northern LA along our stationary front currently
positioned south of the I-20 corridor from the TX Hill Country to
50-75 miles south of the interstate line in LA. Significant
thunderstorm development has occurred over the past hour with rates
hovering between 1-2"/hr with a few overachieving cells reaching
upwards of 2.5"/hr given the latest hourly obs out of central TX.
Environment is rich with moisture as of the latest 12z soundings
out of SHV indicating PWATs up around 1.7" which is a solid 2.5-3
deviations above normal climatologically. Large scale ascent ahead
of a deep upper low over the boot heel of NM will continue through
the period with multiple shortwaves ejecting out of the base of the
trough, interacting with the downstream environment and subsequent
boundary juxtaposed over the South. Latest 12z HREF blended mean is
much more robust compared to the last run with widespread 3-5"
totals now encompassing east-central TX along and east of the I-35
corridor near Waco and points east-northeast. As a result, HREF EAS
probabilities have soared for all three intervals (1"/2"/3") with
the 3"/24-hr contour now indicating up to 70-80% for that area near
Waco to the northeast. 30-60% stretches all the way out to
Shreveport with a large 20-30% area for at least 3" now expanding
south into central LA which is aggressive considering that area is
not anticipated to see the heavy rain threat until a little later
in the period. Neighborhood probabilities are more bullish given
the definition, but even those probabilities are very high
(80-90+") for at least 5" within that target zone between I-35 out
to Palestine, TX where 3+" fell last night and early this morning.

Rainfall rates are sufficient for significant flood potential,
especially for areas that have been hit overnight through the
morning as areal FFG indices for 1/3/6 hr(s) have all taken a
nosedive compared to where they were yesterday. HREF probabilities
for 2"/hr or greater are between 20-40% this afternoon across
portions of east TX and northern LA but ramp up further this
evening thanks to an expected complex of thunderstorms initiating
over south-central TX, growing upscale as it approaches the TX/LA
border and continues its propagation to the east. This is due to a
stronger mid-level vorticity maxima ejecting out of MX and helping
organize the areal thunderstorm development within a very favorable
environment.

Given all the above trends and forecast probabilities, have
expanded the Moderate Risk area further west into portions of
central TX and points to the east-northeast. This threat will be on
the higher end of the MDT threshold and some discussion of a High
risk was entertained. This will be monitored over the course of the
period, but have maintained the MDT for now. Further west over the
TX Panhandle, severe thunderstorms due to the favorable ascent
pattern downstream of the upper low has allowed for locally heavy
rainfall across the northern Caprock. This convection is scattered
in nature for a heavy rain threat as we are missing a primary focal
point to pin for training potential that is customary for
significant rainfall setups. Still, any smaller towns and low-water
crossings across the area between Lubbock and Amarillo will have
the threat for localized flooding concerns, thus maintained the
SLGT risk in place.

Kleebauer

...Previous Discussion...

BLUF: Compared to yesterday afternoon's Day 2 ERO, there's a slight
expansion of the Marginal Risk area across OK, northwest AR, and
southern MO. Otherwise, the Slight Risk area was largely unchanged,
while the Moderate Risk was shifted a tad south into the southern
Piney Woods of eastern TX, though northern LA and into the
ArklaMiss.

The trends in the guidance, particularly the high-res CAMs, has
resulted in a southward shift in the heavier QPF footprints,
largely owing to the deep-layer CAPE trends (greater farther south) and
thus the impact on short-term rainfall rates (highest farther
south). Deep-layer forcing ahead of the amplifying southern stream
trough pivoting into the southern Plains by Wednesday morning will
cover quite a bit of real estate, with the broad diffluent flow
aloft and coupled left exit/right entrance region forcing from the
southern stream/northern stream jet streaks respectively. For this
region the Marginal and even Slight Risk areas remain quite large.
However, within the Moderate Risk area will be an overlap with a
more favorable thermodynamic profile as 850 mb moisture
flux/transport anomalies get closer to +3 standard deviations above
normal, PWs to 1.75+ inches, and MUCAPEs closer to 1000 J/Kg.
Within the Moderate Risk area, the latest (00Z) HREF indicates the
highest probabilities...between 25-40+ percent...of 24hr QPF
exceeding 5 inches, with 15-25+ percent probs of exceeding 8
inches.

Hurley


Day 2
Valid 12Z Wed Apr 10 2024 - 12Z Thu Apr 11 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER THE MID-
SOUTH INTO PORTIONS OF THE FLORIDA PANHANDLE...

...20Z Update...

Moderate risk was expanded back to the west to include eastern LA
through the Lower Mississippi Delta into southern AL and the
adjacent FL Panhandle. Recent trends on guidance are all in
agreement on a significant heavy rain event forming this evening
and propagating eastward along a stalled frontal boundary across
the Southeast US. This will be the focal point for any convective
organization and associated heavy rainfall as depicted by the
latest HREF and hi-res deterministic suite. Probabilities for at
least 3" of rainfall in the period have risen sharply with a large
swath of EAS probabilities now into the 40-60% range for the 3"
marker with neighborhood probs exceeding 70% for at least 5" within
that same corridor outlined by the MDT risk. The combination of
the boundary, backed flow ahead of the strengthening 5H trough/ULL,
and an environment conducive to prime updraft maintenance through
the any convective life-cycle will enhance the risk for long-lived
thunderstorm clusters and linear segments across the mid-South
towards the FL Panhandle near the end of the period.

HREF blended mean has jumped upwards of 4" for a large chunk of the
southern MS/AL area with embedded QPF maxima between 5-6"
characterized by the expected strong thunderstorms within the
evolution of the overall pattern. Areas outside the MDT are still
well at risk for localized flooding concerns due to the high-shear,
moisture laden environment present across the Southeast extending
north by the second half of the forecast period as our trough
ejects to the northeast with flow becoming highly meridional.
Rainfall amounts of at least 1" are now forecast all the way up
into the central Midwest with areas through the Mid-Mississippi
Valley up through IN being the beneficiaries of appreciable
rainfall given the amplified upper pattern and strong low to mid-
level moisture advection occurring east of the Mississippi. Some
concerns linger to the north from previous rainfall that saturated
the local river/stream sources, so the prospects for local flood
concerns has more merit despite the heaviest rain likely over the
Deep South. Regardless, the pattern being highly amplified with a
deep moisture return over all locations east of the Mississippi
will create an elevated flash flood threat with highest potential
over the Southeast US thanks to the combo of primed instability and
surface focus along a stationary front located north of the GoM.

The MDT risk area over the mid-South is considered the higher end
of the threshold and an upgrade to High risk is possible pending
convective evolution overnight into the morning. The best risk for
heightened impacts will lie from central LA through southern MS/AL
with the northern extent likely along the I-59 corridor to about
Birmingham.

Kleebauer

...Previous Discussion...

BLUF: No significant changes made from yesterday's Day 3 ERO.

Once again per coordination with local WFO's, given the continued
progressive nature of the convective pattern from the latest 00Z
guidance (including the 60hr FV3 and RRFS), have maintained a
rather targeted Moderate Risk area over the area (including central
AL that will most likely see significant rainfall during a larger
portion of the forecast period). This area continues to align well
with the highest areal-average QPF per WPC and the NBM (2.5 to 4+
inches). However this remains a low confidence ERO forecast,
especially given the progressive nature of the convection as with the
upper trough and surface front. In addition, some of the extended
CAMs, including the 00Z FV3 and RRFS, are now farther south with
the heavier rainfall footprint during the Day 2 period (12Z Wed-12Z
Thu), i.e. closer to the Gulf Coast. For now, again per
collaboration with the WFOs, have opted to maintain continuity with
only minor adjustments, with the anticipation of better clarity in
the guidance over the next couple of forecast cycles as the event
gets within all of the high-res windows.

Hurley


Day 3
Valid 12Z Thu Apr 11 2024 - 12Z Fri Apr 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE APPALACHIANS TO WEST OF THE I-95 CORRIDOR FROM THE MID ATLANTIC
TO NORTHEAST...

Upper trough across the southern plains will tilt neutral to
negative by Thursday morning and begin ejecting northeastward into
the Ohio Valley area by the end of the period. Highly meridional
flow ahead of the mean trough will allow for a poleward advancement
of deep moisture from the Gulf all the way up through the northern
Midwest and Northeastern US with a cold front extending from the
base of a strengthening SLP moving in tandem with the primary
vorticity associated with the shortwave trough. Scattered heavy
rain instances will be plausible from the Big Bend of FL up through
the Northeastern US with the best flooding risks within the terrain
of the Southern and Central Appalachians and along the urban
corridor from DC to just northwest of NYC where FFG indices are
notoriously low due to urbanization and the recent rainfall.
Further to the northwest, our low pressure will deepen considerably
before occluding once into MI. Axis of deformation will develop
over the Midwest and locally heavy rain on the northwest side of
the surface reflection will take shape bringing totals between 1-2"
from Thursday afternoon through the end of the forecast period.
Lower theta-E values indicate less of a prominent convective risk,
however there will be enough forcing and mid-level instability to
generate some bands of heavy precip for a time before the system
begins scaling back as it becomes vertically stacked.

With cases of flood concerns due to hydrologic responses from the
previous disturbance and the anticipated rainfall, wanted to make
sure the low-end threshold for flash flooding was covered. The MRGL
risk was expanded to include the central and northern Midwest with
a subsequent northern shift in the SLGT risk into the Mid-
Mississippi Valley given the latest trends in the 5H evolution and
convective scheme focusing further north within the tongue of
theta-E advection on the eastern flank of the SLP. The SLGT risk
was maintained from the previous forecast with only minor
adjustments made to correlate with the forecasted rainfall and
convective potential.

Kleebauer

Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt