845
FOUS30 KWBC 022350
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
750 PM EDT Thu May 2 2024

Day 1
Valid 01Z Fri May 03 2024 - 12Z Fri May 03 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST TEXAS & FAR SOUTHWEST LOUISIANA...

...Texas and Louisiana...
The outflow boundary from earlier convection is shifting back
northeast towards Houston, with MU/ML CAPE trends up a bit as the
instability hole caused by the earlier complex recedes northward.
Further northward return as this boundary is possible overnight as
it remains convectively inactive. A couple convective clusters
forming along the dry line across northwest TX are moving east
early this evening and the convection with the higher risk of
heavier rainfall is expected to drop east- southeast as right
movers along the 1000-500 hPa thickness gradient and instability
gradient into southeast TX/southwest LA, with rainfall
rates/totals increasing as it gets into the area around sunrise
just as MU CAPE rises back into the 2000-3000 J/kg range.
Precipitable water values should rise back into the 1.5-1.75"
range. Effective bulk shear of 40-60 kts lies across the area,
which is more than sufficient to allow for convective organization,
including the mesocyclones which have already developed.
Considering rainfall totals and very low flash flood guidance
values in the previous complex`s wake, the Moderate Risk area was
maintained in or near Southeast TX. A Slight Risk surrounds this
area, covering both where heavy rain has fallen as of late and
where heavy rain from the expected complex is expected to fall.
Hourly rain totals to 2.5" with additional amounts to 5" are
anticipated over saturated soils/filling rivers and bayous.


...Mississippi Valley/Midwest...
Radar trends and mesoscale guidance shows clusters of showers and
thunderstorms in and near portions of OK this evening into tonight
with less organized, in some cases dying, activity most elsewhere
within the broader Marginal Risk area. A general 1.5-3" of rain,
with locally higher amounts possible, which would fall within
several hours, hence the continuation of the Slight Risk in that
area. The northeast portion of the Slight Risk was removed as the
risk has lowered across much of MO. Flash flood guidance values are
currently running between that 1-2"/hr marker for exceedance, and
soils have some degree of saturation, so it`s within the zone of
opportunity to cause some flash flooding potential within the rates
alone.

Roth


Day 2
Valid 12Z Fri May 03 2024 - 12Z Sat May 04 2024

...THERE IS SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
TEXAS AND LOUISIANA COAST AND THERE IS A MARGINAL RISK OF
EXCESSIVE RAINFALL FOR MUCH OF TEXAS, THE LOWER MISSISSIPPI VALLEY,
TENNESSEE VALLEY, CENTRAL PLAINS, AND THE PACIFIC NORTHWEST...

...Southern Plains and Southeast...

21z update... A copious amount of rain (at least 8 to 12 inches)
has fallen over parts of southeast Texas Thursday morning with more
heavy rain to pass through for the remainder of the day 1 period
and into the day 2 period. This very water-logged part of the
region will be very sensitive to any additional precipitation. A
Slight Risk was raised for this period to cover the most sensitive
locations with the expected additional rain over the region, which
spans from southeast Texas, across southern Louisiana and clips the
extreme southwest edge of Mississippi.

Campbell

A trio of areas extending from the southern plains through the
Southeast US will draw attention for the period. The primary area
of focus will reside in the Lower Mississippi Valley where some
guidance is now suggesting the development of convection upstream
within the axis of an approach cold front, developing upscale and
maneuvering through the ArklaTex into LA by the beginning of the
forecast period. Locally heavy rains would be a certainty if this
evolution occurred and will need to be monitored closely with
regards to timing and magnitude

...Northeast...

Showers are expected to move through parts of the Appalachian and
western portions of the Northeast as a warm front lifts through the
region. Hi-res guidance suggests there will be several hours where
the thunderstorms will be be capable of producing hourly rainfall
rates up to 2/2.25 inches/hour, particularly across western New
York, western Pennsylvania and northern West Virginia. 3-hr FFG
values across this part of the country are as low as 1.5 inches so
there will be an increased threat for excessive rainfall and local
flooding concerns. A Marginal Risk area was raised from western New
York to central West Virginia.

Campbell

...Central Plains...

A fairly robust mid-level perturbation will eject east-southeast
out of WY/CO with convection initiating over the Front Range,
eventually congealing into a MCS as it migrates eastward through
southern NE into KS. Models are in agreement of at least some
modest QPF with 1-2" scattered across the aforementioned areas and
QPF maxima running upwards of 3" at this time. Current QPF maxima
is within the vicinity of southern NE into northern KS, but the
expectation is for multiple pockets of higher QPF embedded within
the complex as it moves to the southeast. As of now, the system
looks to remain progressive, but will monitor closely as we
approach the event in question to see if we can hone in on any
specific area for potential upgrades, or maintain continuity.

...Pacific Northwest...

21Z update... QPF trended a bit further south along the northern
California coast so in turn the Marginal Risk area was expanded a
smidgen southward to reflect this trend.

Campbell

A relatively weak IVT pulse will enter coastal OR and northwest CA
by the second half of the period with locally heavy rainfall
expected through Saturday morning. IVT index is running at an "AR
1" which signals a weaker atmospheric river event, but one that can
still produce modest rainfall totals to the coastal plain. Ensemble
trends have come up a bit from previous forecasts with QPF running
between 1.5-2.5 over the span of 12 hrs (00-12z Saturday), with a
bit more as we head into D3. As a result, kept continuity with the
MRGL risk from previous forecast package.

Kleebauer


Day 3
Valid 12Z Sat May 04 2024 - 12Z Sun May 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR WEST AND NORTH
TEXAS INTO SOUTHWEST OKLAHOMA...

...Southern Plains...

21Z update... Very minor adjustments were made to the Marginal
and Slight risk areas to reflect the latest guidance and WPC QPF
trends. The best concentration for higher QPF and elevated flooding
risk continues to be north-central Texas and southwest Oklahoma.

Campbell

A cold front will move south through the course of Saturday with
sights on north TX by the afternoon hours as it progresses out of
the plains. A dryline will be focused over west TX by the morning
and will begin mixing eastward through peak diurnal instability.
Aloft, a shortwave will eject northeast out of MX which will
provide areal ascent within the confines of where the dryline and
cold front will intersect. Guidance is in agreement on a rapid
development of convection in-of the western rolling plains,
northeast Permian Basin up to the Red River with heavy rainfall
likely along and east of the dryline as dew points in the upper-60s
and low-70s will exist on the moist side of the boundary. The
convergence of the front and dryline will induce a large cluster of
heavy thunderstorms capable of significant rainfall and rates
upwards of 1.5-2"/hr based on local climo and expected PWAT
anomalies of 2-2.5 deviations above normal. Current rainfall totals
are already pushing 2.5-3" on the ensemble bias corrected QPF with
likely higher totals once we factor in CAMs later into the
equation. A SLGT risk was maintained and expanded to encompass the
1"+ contours within the ensemble bias corrected forecast and WPC
Guidance which is factoring in higher localized totals within area
convection. Would not be surprised to see this be labeled a higher
end SLGT, or an even higher risk given the early signals and the
forecasted synoptic evolution.


...Central and Southern Appalachians

21Z update... The overall Marginal Risk area was shifted east and
reshaped to a more southwest-northeast orientation that spans from
western North Carolina to eastern Pennsylvania. The bulk of the convection
closer to Appalachians is expected to fall during the day 2 period
while convection further east is expected for the day 3 period.
There is some spread in the timing and location of the QPF in the
latest guidance, but general take is up to 1.5/2 inches expected
for this period which could result in local areas of excessive
rainfall and flooding.

Campbell

A cold front will approach from the west with slow progression
through the Ohio and Tennessee Valleys before aligning parallel to
the Appalachian front by mid-day Saturday. A broad ridge to the
east will provide a convergent signature within a band of elevated
PWATs along and east of the terrain in the Mid-Atlantic. Modest
instability and upper forcing along with the surface convergence
pattern will lead to scattered convection over portions of the
western Carolinas up through western Pennsylvania through most of
the period, especially the afternoon and evening Saturday. Locally
heavy rainfall will be plausible in the setup with weak steering
flow aloft allowing for meandering and/or training cells within the
confines of the complex terrain in the interior. FFG indices are
relatively low, but not significant so, so the threat currently is
fairly minor, but within the threshold for a MRGL risk that was
inherited from prior forecast packages. Another instance of
continuity with a better analysis in future forecast cycles with
the help of CAMs and associated ensemble(s).


...Northern Sierra Nevada Foothills...

A fairly robust IVT pulse will slide down the west coast with
southwesterly flow aimed orthogonally to the northern and central
Sierra`s by Saturday. Higher elevations will be greeted with more
snow, but the adjacent foothills will remain warm enough to benefit
from all rainfall. Rates will be borderline overall, but the
prospects for over 1" of precip has grown in the past succession
of runs with the ensemble bias corrected guidance now upwards of 2"
now forecast within the elevations below 8000` MSL, falling mainly
in a span of 8-12 hours. The threat is on the lower end of MRGL,
but does have merit given the signal at leads. Maintained
continuity from previous forecast and will assess whether it needs
adjusting, or removal pending future trends in guidance.

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