Quantitative Precipitation Forecast
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FOUS30 KWBC 171542
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1142 AM EDT Sun Mar 17 2024

Day 1

Valid 16Z Sun Mar 17 2024 - 12Z Mon Mar 18 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEASTERN
LOUISIANA...

...16Z Update...

The Slight Risk was trimmed on the west side out of Texas and
portions of southwest Louisiana with this update. An ongoing MCS
across Louisiana remains the focus for any potential flash
flooding. With time, the MCS will be both weakening and moving more
quickly, but in the meantime rates to 1.5 inches per hour still
pose a flash flooding threat over the next few hours, especially in
the included urban areas such as Baton Rouge (for the next hour or
so), Slidell, New Orleans, and Gulfport. Rapid movement will end
the flooding threat by mid-afternoon. A Special update will be
issued at that time.

The Marginal Risk across portions of the upper Texas coast was left
due to isolated slow moving convection ongoing in the area. It was
trimmed from southeast Georgia and northeast Florida as the storms
should be much weaker and less widespread by the time the
convection reaches that area tonight.

Wegman

...Previous Discussion...

Organized convection in the vicinity of a front and a surface low,
which should lie between southeast TX and southwest LA Sunday
morning, is expected to lead to heavy rainfall for portions of the
northern Gulf coast, though confidence in how far inland is not
overly high at this time due to a generally southerly trend in the
guidance QPF when compared to 24 hours ago. 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. However, ML and surface-based CAPE gradients
should be coastal and/or just offshore, which is usually a better
tool with precipitable water values close to 1.75". Difluence
aloft should be fostered by increasing troughing across the Great
Lakes, Midwest, and Mississippi Valley. There remains a signal in
the guidance for cooling temperatures at 700 hPa, which is of
concern as the 1000-500 hPa thickness diffluence appears to be
along the LA coast. A mid-level capping inversion won`t be present
which could allow convection to be outflow-driven more equatorward.
This means that there is some chance that organized convection
could shift slightly offshore the Gulf Coast at some point in its
evolution, leading to decreasing confidence in the heavy rainfall
threat the farther east you go along the Gulf Coast. The guidance
shows a bit of progression to the convective pattern, 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 5" are possible
where limited cell training occurs and/or mesocyclones happen to
develop. Because of the heavy rainfall that has occurred recently,
the Slight Risk accounts for both the previous couple days of
rainfall and the expected rainfall from early Sunday morning
onward. The northern portion has been shifted a little south of
continuity across portions of southeast LA and southern MS, which
was coordinated with the LIX/Slidell LA forecast office.

Roth


Day 2

Valid 12Z Mon Mar 18 2024 - 12Z Tue Mar 19 2024

The probability of rainfall exceeding flash flood guidance is less
than
5 percent.

Roth


Day 3

Valid 12Z Tue Mar 19 2024 - 12Z Wed Mar 20 2024

The probability of rainfall exceeding flash flood guidance is less
than
5 percent.

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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