Quantitative Precipitation Forecast
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FOUS30 KWBC 151603 CCB
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1203 PM EDT Fri Mar 15 2024

Day 1

Valid 16Z Fri Mar 15 2024 - 12Z Sat Mar 16 2024



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

...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...

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...

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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