Quantitative Precipitation Forecast
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000
FOUS30 KWBC 212053
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
453 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 SOUTH
FLORIDA...

...Southern Florida...


Multiple rounds of organized deep convection currently located over
the central Gulf Coast are forecast to expand and translate eastward
toward South Florida as surface low pressure consolidates along the
Gulf states tomorrow morning. Diffluent flow pattern noted over the
Gulf will continue to intensify as a southern stream jet streak
over Baja California digs into the base of the upper-trough axis,
leading to cyclonegenesis and an associated uptick in low-level
inflow, feeding a very moist and unstable airmass into the
convective complexes along a warm front. Uncertainty remains
regarding the track of the convection along the front tomorrow, with
a noted southward trend to the axis of heaviest rainfall in the
recent HREF guidance compared to last night. Given the uncertainty,
maintained a Slight Risk with a minor expansion westward as
locations that fall within the main axis of heavy rainfall will be
quite wet, with localized rainfall totals upwards of 6-7 inches possible
by Saturday morning.


...Southeast into the Southern Mid-Atlantic...
The deepening low pressure center will drive strong low-level
onshore flow (upwards of 50 kts at 850 mb) and moisture transport
from the Gulf of Mexico and Atlantic into the Southeast and Mid-
Atlantic region, ushering in precipitable water values over 1-1.25
inches to the region. Despite the anomalous moisture for the
Southeast (+1 to +2 sigma 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. A Marginal Risk remains in place and
was expanded slightly westaward into the Appalachians given the
upward trend in rainfall totals corresponding to a more upslope
component to the strong low level flow.

...Northern California into Southwest Oregon...
A Marginal risk was addded as southwesterly onshore flow increases
ahead of an approaching frontal system tomorrow morning. IVT values
may briefly approach 600-700 ahead of the cold front, leading to
periods of moderate to locally heavy rainfall within a band of
anomalous PW values +1 to +2 sigma above climatology. Combined with
post frontal shower activity, localized rainfall totals upwards of
3-4" appear possible along the coastline.

Asherman/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 forecast thinking maintains 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 and divergent jet quadrants 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. A stripe of 2 to 4 inches of rain
appears likely from the Delmarva up toward Long Island and areas of
southern New England where these enhanced rates tend to persist.
Accordingly, the Slight Risk of excessive rainfall was expanded
further northeast along the I-95 corridor given the trend for higher
QPF since overnight.

...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 conditional 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 maintained 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 locally
heavy rainfall expected here before this period on day 2, the
antecedent conditions are expected to be rather wet, and the
additional rains may pose some very isolated runoff concerns.

Asherman/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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