Quantitative Precipitation Forecast
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FOUS30 KWBC 211600
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Thu Mar 21 2024

Day 1

Valid 16Z Thu Mar 21 2024 - 12Z Fri Mar 22 2024



...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE RED RIVER VALLEY TO THE WESTERN GULF COAST...

...16Z Update...
The forecast thinking remains unchanged regarding the isolated
excessive rainfall threat today across the Red River Valley and
Western Gulf Coast. Showers and thunderstorm activity continues to
expand this morning in south Texas in the vicinity of a surface low,
warm front, and inverted trough analyzed between CRP and BEA. The
ongoing convection has reinforced the warm front offshore, which is
expected to prevent more robust surface based instability from
returning inland as reflected in the 12Z CAM guidance. However, can
not rule out isolated runoff issues close to the coastline as the
surface low and warm front migrate east today. Otherwise, a pair of
vorticity maxima and weak surface dryline will lead to thunderstorms
later this afternoon across the Southern Plains. Localized rainfall
totals of 2-4" remain possible over portions of the Southern Plains
this afternoon where storm motions are reduced close to the ejecting
closed low.

Asherman

...Previous Discussion...

A 500 mb southern stream trough is forecast to track eastward from
the southern High Plains this morning to the west-central Gulf Coast
by Friday morning. The eastward advance of this energy will
facilitate development of a wave of low pressure near or just
offshore of southeast TX early this morning and in conjunction with
a northward returning warm front. A sufficient level of instability
and forcing is expected to be in place for the development and
expansion of convection this morning over areas of south Texas and
especially over toward the middle TX coast and offshore. The 00Z
HREF guidance supports the heaviest rains generally focusing
offshore closer to the low center and the attendant frontal zone,
but there may be sufficient rains along the coast or just inland for
an isolated threat for runoff problems and flooding.

Meanwhile, with the core of the ejecting mid-level trough and some
amplification of the 700 mb low center toward the Red River
Valley, there is expected to be multiple clusters/bands of
convection that develop this afternoon and evening which will more
broadly impact central/northern TX and into south-central OK. A
more progressive evolution of the convective threat, and
potentially a QLCS, may then reach across southeast TX and
southwest LA Friday morning. For all of the convective areas across
the southern Plains/western Gulf Coast area, there may be some
spotty 2 to 4 inch rainfall totals. A Marginal Risk area remains
intact at this time.

Orrison


Day 2

Valid 12Z Fri Mar 22 2024 - 12Z Sat Mar 23 2024



...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEAST
FLORIDA...

...Southern Florida...
A well-organized area of convection, and likely a QLCS, is
expected to be traversing the eastern Gulf of Mexico early Friday
morning which is expected to impact much of southwest to southeast
FL. The convection will be driven by a combination of factors
including a very moist and unstable airmass pooling over the
southeast Gulf of Mexico in close proximity to a warm front, and
with strong low- level jet energy/forcing within a very divergent
flow pattern aloft. The 00Z HREF guidance tends to be favor the
potential for the heaviest rains staying southwest of the southern
Florida Peninsula near the aforementioned front and in proximity
the better instability, but this initial round of heavy rainfall
may result in locally 2 to 3 inches of rain alone before it sweeps
through.

A second round of heavy rain is anticipated though ahead an
approaching 500 mb trough tracking eastward along the Gulf Coast.
Strong divergence aloft ahead of the upper trough and a strong
southerly low-level jet of 40 to 50+ kts are forecast to refocus
convection initially out over the central and eastern Gulf of
Mexico which will then advance quickly east and impact south FL.
Between the two rounds of heavy rainfall, as much as 3 to 6 inches
of rain will be possible. The Marginal Risk area was tweaked to
include all of south FL, and with concerns for the more sensitive
urban corridor across southeast FL including areas from Fort
Lauderdale down through Miami and Homestead, a Slight Risk was
introduced here where there may be an urban flash flood threat.

...Southeast into the Southern Mid-Atlantic...
As mentioned before, rainfall will expand eastward across the Gulf
Coast states and eventually into the Southeast/Mid-Atlantic
region region ahead of a potent mid-level trough. Increasing
onshore flow and moisture transport wrapping into the region from
the south and southeast. Precipitable water values over 1 inch for
inland locations and nearing 1.5 inches for the coast can be
expected. 850 mb winds in excess of 50 kt are expected ahead of a
developing surface low to track across the Southeast. Despite the
anomalous moisture for the Southeast (+1 to +2 standardized PW
anomalies), poor mid-level lapses will limit instability to fairly
weak values (up to ~500 J/kg and perhaps near 1000 J/kg near the
coast), which should limit rainfall rates overall. In addition,
the anomalous moisture plume will begin to advance
eastward/northward up the East Coast toward the end of the period.
A Marginal Risk remains in place and was generally tweaked to
account for the 00Z HREF guidance.

Orrison


Day 3

Valid 12Z Sat Mar 23 2024 - 12Z Sun Mar 24 2024



...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE DELMARVA
NORTHWARD INTO COASTAL AREAS OF SOUTHERN NEW ENGLAND...

...Mid-Atlantic into Southern New England...

The latest multi-model consensus continues to support a southern
stream area of low pressure and energy associated with it
advancing northeastward up across the Southeast coastal plain and
generally along the Mid-Atlantic coast on Saturday before then
impacting southern New England by Saturday night. The advance of
northern stream shortwave energy/troughing into the Northeast will
gradually capture/deepen the low center as it advances north-
northeast across the region. Very strong low-level forcing/warm air
advection and moisture transport out ahead of the advancing low
center should favor an axis of heavy rainfall across the Mid-
Atlantic and southern New England coastal plain, with the heaviest
rainfall amounts south and east of I-95. The guidance is quite
impressive with the axis of frontogenesis across this region, which
coupled with the level of low-level moisture convergence/jet
energy ahead of the low may result in a southwest/northeast axis of
rainfall rates that reach as high as 0.50" to 0.75"/hour, and
despite what should be a general lack of instability. Locally as
much as 2 to 4 inches of rain may impact areas of the Delmarva up
toward Long Island and areas of southern New England where these
rates tend to persist and this may result in areas of flooding as a
result. The Slight Risk of excessive rainfall was expanded to
cover these areas.

...Southeast Florida...

Locally organized areas of convection may still be transiting
areas of southeast FL early Saturday morning, as energy and
moisture continues to focus across the region. A cold front will
sweep across the region though early in the day, and this should
allow for the convective threat to then come to an end. Given the
heavy rainfall threat in the prior period, any additional rains
Saturday morning may result in additional runoff concerns. A
Marginal Risk area has been depicted for the southeast FL urban
corridor as a result.

...Northwest California and Far Southwest Oregon...

A Marginal Risk of excessive rainfall is highlighted for areas of
northwest CA and southwest OR as additional Pacific moisture and
energy arrives in association with a larger scale upper-level
trough gradually crossing the West Coast. Additional rainfall
amounts are expected to be as much as 1.5 to 2.5 inches, with
potentially some spotty instances of 0.50"/hour rainfall rates.
Given some of the rainfall expected here before this period, the
antecedent conditions are expected to be rather wet, and the
additional rains may pose some very isolated runoff concerns.

Orrison


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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