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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 0816Z Mar 29, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Fri Mar 29 2024

Day 1
Valid 12Z Fri Mar 29 2024 - 12Z Sat Mar 30 2024

...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN CALIFORNIA AND DOWNEAST MAINE...

...California...
Overall, the forecast remains much the same, with the latest
guidance showing good run-to-run continuity with an unseasonably
strong system that is forecast to bring widespread precipitation
to the state and the potential for excessive rainfall amounts,
especially along the southern coast.

A vigorous shortwave diving southeast through the eastern Pacific
this morning is expected to develop a closed upper low that will
drop south along the northern California coast later today. This
will be an anomalously deep system, with 500 mb heights dropping
1-3 standard deviations below normal across much of the state by
early Saturday. Amplifying southwesterly flow ahead of the
deepening system will tap a plume of deeper Pacific moisture, that
will spread inland along a well-defined frontal band.

A broad Marginal Risk was maintained across much of central into
southern California, extending from the coast to the Sierra
foothills. A targeted Slight Risk was also kept over the
Transverse Ranges. Guidance continues to show this area as the
focus for some of the stronger low-to-mid level moisture advection
and PWs as the system's front moves into the region later today.
HREF neighborhood probabilities show that accumulations of 3 inches
or more are likely in this area -- raising runoff concerns for
areas below the snow line.

...Eastern Maine....
Upper forcing interacting with ample moisture along a slow-moving
frontal zone will continue to support light to moderate rain, with
precipitation becoming more focused across eastern Maine as a
deepening surface low tracks northeast off of the New England
Coast toward Atlantic Canada. MRMS data indicates amounts of 2
inches or more have fallen across portions of DownEast Maine over
the past 24 hours, while HREF probabilities show another 1-2 inches
likely before precipitation changes over to snow on the backside
of the deepening low. With rain continuing to fall, a Slight Risk
was added in response to the already saturated soil conditions,
which are reflected by 3-hour flash flood guidance values that
have dropped below 0.25 across parts of the area.

Pereira

Day 2
Valid 12Z Sat Mar 30 2024 - 12Z Sun Mar 31 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHERN CALIFORNIA...

...Southern California to southern Nevada and western Arizona...
A heavy to excessive rainfall threat developing Friday into early
Saturday will spread further south as an unseasonably deep low
drops south from the central to the southern California coasts.
Strong onshore flow and moisture advection along the associated
frontal band, will spread precipitation across southern
California, with upslope flow accentuating the threat for heavy to
excessive accumulations along the coast, where a Slight Risk was
maintained along the Transverse and Peninsular ranges below the
snow line. Residual moisture spreading further inland is expected
to fuel a lesser threat for excessive rainfall into the lower
Colorado Basin, including southern Nevada and western Arizona.

Pereira

Day 3
Valid 12Z Sun Mar 31 2024 - 12Z Mon Apr 01 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHERN CALIFORNIA AND SOUTHERN ARIZONA...

...Southern California to southern Arizona...
The upper low impacting the region days 1 and 2 will begin to
transition to an open wave by early Sunday. However, unsettled
weather with some potentially locally heavy rains will continue as
an elongated positively-tilted upper trough remains centered over
the region. Deep southwesterly flow along the leading edge of the
trough will continue to support anomalous PWs (1-2 standard
deviations above normal) across portions of southern Arizona early
in the period. Meanwhile, lingering moisture along with upstream
energy dropping into the base of the trough will support showers
along the coast and an isolated threat for additional flash
flooding concerns.

Pereira


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt