Quantitative Precipitation Forecast
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FOUS30 KWBC 210849
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
449 AM EDT Thu Mar 21 2024

Day 1

Valid 12Z 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...

No major changes to the previous thinking. 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.

...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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