Quantitative Precipitation Forecast
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FOUS30 KWBC 160036
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
836 PM EDT Fri Mar 15 2024

Day 1

Valid 01Z Sat Mar 16 2024 - 12Z Sat Mar 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH TEXAS...

...01Z Update...
Much of the southern U.S. (from Mississippi eastward) has been
removed from Marginal Risk highlights for this outlook update.
Convection across Georgia has picked up forward speed, has had
marginal rain rates over the past couple hours, and was moving
toward areas of higher FFG.

Farther west, a Marginal was maintained for portions of southern
Louisiana and southeast Texas. A maturing, partially elevated MCS
was making slow eastward progress through the region, resulting in
a few spots of 1+ inch/hr rain rates. Eventually, this MCS will
propagate southeastward toward northwestern Gulf of Mexico waters,
but heavy rainfall could last for at least a couple more hours as
lingering instability (around 250-1000 J/kg MUCAPE) persists along
and downstream of the track of the MCS.

Lastly, robust, deep convection continues to mature along the
higher terrain of northeastern Mexico just west of the Rio Grande.
Models suggest that these storms could cross the Rio Grande and
grow upscale while maintaining slow movement amid strong
instability (3000-4000 J/kg MLCAPE). A few areas could experience
3+ inch/hr rain rates beneath the slowest and most dominant
convection, necessitating a Slight Risk.

See the previous discussion below for more information.

Cook

...16Z Update...

The previous forecast reasoning still holds. The Slight Risk area
in the Mid-South has been removed as the flash flood threat is
decreasing in this area. The line of thunderstorms moving across
the Southeast has been fairly progressive this morning, and
instability drops sharply behind the main convective line,
limiting rainfall rates in the stratiform ran on the backside of
the boundary. Rainfall rates have also begun decreasing in storms
on the leading edge and should continue to trend downwards into
this afternoon and evening, making it difficult for rainfall rates
exceeding flash flood guidance to be realized. However, isolated
stronger storms could still cause localized flash flooding
concerns
in the South and Southeast.

There is a second Slight Risk area in effect for portions of
Texas where the environment will be favorable for convective
development this afternoon. There is quite a bit of uncertainty
and spread among the available guidance as to where the heaviest
precipitation will develop, but the consensus remains that heavy
rainfall is expected. Stronger storms may produce intense rainfall
rates, potentially reaching up to 3-4 inches per hour, which
could cause flash flooding, especially in urban areas.

The broader Marginal Risk remains largely unchanged from Texas to
the Southeast Coast with minor adjustments made in the Southeast
to adjust for latest radar trends.

Dolan


...Previous Discussion...

A cold front begins to accelerate to the southeast across the
Southeast as a broad upper level trough gains real estate across
portions of the Great Lakes and Upper Midwest, with the front
moving slower to the south into the Southern Plains as a cold low
in the Desert Southwest shows minimal movement and keeps the upper
level flow southwest to west- southwesterly which should slow the
surface boundary`s progression. This combination of systems
flattens the ridging across the Southeast and Gulf Coast.
Temperatures at 700 hPa suggest that the heaviest rainfall should
be north of southern TX, where temperatures are <6C, based on GFS
forecasts and ML CAPE should rise to ~3000 J/kg. An axis of
anomalous PW values, 1.5-1.75", will be seen near the front.

Another day of potentially widespread heavy rainfall is expected,
as hourly rain totals to 2.5" and local amounts to 4" remain
possible. There is the typical amount of spread with respect to
the position and maxima within the heavy rainfall axis, but the
overall model consensus suggests potentially heavy rains. Some
heavy rainfall is expected across areas of Mississippi, Alabama
and Georgia that have had above average precip over the past week
or two, resulting in higher stream flows and greater soil
saturation which continues to be the reason for the Slight Risk,
over other considerations. The pair of Slight Risk areas remain to
depict the bimodal distribution of the heavy rainfall pattern in
the southern tier of the country, with the eastern portions
associated with the progressive frontal zone/850 hPa confluence
early on more northerly and the western portions associated with
the slow-moving boundary shifting more southerly. Despite the
slowness of the progression in TX, the guidance appears weaker in
its support for this heavy rainfall. Kept the Slight Risk there as
a precaution since the ingredients still appear to support a
greater than isolated flash flood risk.

The Marginal Risk for areas north of approximately Chattanooga TN
was removed as convective progression through 06z and MU CAPE
trends the last several hours suggested that any threat north of
there would be more or less done by 12z.

Roth


Day 2

Valid 12Z Sat Mar 16 2024 - 12Z Sun Mar 17 2024





...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
SOUTH AND SOUTHEAST TEXAS...

...20Z Update...

The synoptic setup remains favorable for heavy rainfall in
portions of Texas Saturday into Saturday night as a slow-moving
frontal boundary moves across the southern half of the state. Two
upper level shortwaves will move overhead during the period, which
will support a couple rounds of convection that will be
responsible
for heavy rain and could potentially lead to flash flooding.
Latest guidance has been trending slightly wetter and northwards
with the QPF, and the Slight Risk area over portions of Texas was
expanded slightly north and east accordingly. The Marginal Risk
area was also pulled north to include part of North Texas,
including the Dallas/Fort Worth metro area. Although conditions
will be more favorable for strong convection in south-central and
southeast Texas, there may be enough instability to support a
decent round of convection in the afternoon/evening in north
Texas.

Dolan


...Previous Discussion...

Difluence aloft ahead of an upper level low moving slowly across
Arizona teams up with a sagging front in southern TX to continue
the potential for heavy rainfall in the front`s vicinity. The
guidance has become broadly wetter when compared to this time
yesterday, which led to some expansion in the Slight Risk area.
Temperatures at 700 hPa imply that the heavy rain threat should be
mainly north of southernmost TX, based on GFS forecasts. Inflow at
850 hPa from the Gulf should pulse up to 25-30 kts at times,
importing ML CAPE of 2000 J/kg or so. Effective bulk shear should
be sufficient for organized convection, capable of producing
hourly rain totals to 2" and local amounts to 4".

Roth


Day 3

Valid 12Z Sun Mar 17 2024 - 12Z Mon Mar 18 2024





...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF
THE NORTHERN GULF COAST...


The forecast remains on track. A potent upper level shortwave will
move along the stalled frontal boundary over the Gulf Coast,
producing heavy showers and thunderstorms. This feature looks to
be
fairly quick-moving, which should help to limit the risk of flash
flooding, so there is currently only a Marginal Risk area in place
from the Texas Gulf Coast across the northern Gulf Coast to the
Georgia Atlantic Coast. The risk of flash flooding may increase if
stronger convection impacts an urban area or if training storms
develop, which are details that will be better defined in the 1-2
day period when hi-res guidance is available.

Dolan


...Previous Discussion...

Organized convection overriding a front is expected to lead to
heavy rainfall for portions of the northern Gulf coast. Sufficient
850 hPa inflow off of the Gulf of Mexico, up to 40 kts, will lure
precipitable water values of 1.5-1.75" into the region and MU CAPE
up to 2000 J/kg. A slight amount of warming at 700 hPa could lead
to some northward shift in the convection, when compared to
Saturday. The main issue from an excessive rainfall perspective is
the progression noted, as an upper trough sharpens across the Ohio
and Mississippi Valleys and helps act to shift the moisture
plume/best 850 hPa inflow east with time. Hourly rain totals to
2.5" with local amounts to 4" are possible where limited cell
training occurs and/or mesocyclones happen to develop.

Roth


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


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