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< Day 2 Outlook
WPC Day 3 Excessive Rainfall Outlook Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 2007 UTC Sun Mar 26, 2023
Valid: 12 UTC Mar 28, 2023 - 12 UTC Mar 29, 2023
Forecast Discussion
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1000 PM EDT Sun Mar 26 2023
Day 3
Valid 12Z Tue Mar 28 2023 - 12Z Wed Mar 29 2023
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
CENTRAL CALIFORNIA COAST AND SAN FRANCISCO BAY AREA...
...California...
Only minor changes were made to the previous risk areas, with the
main expansion being to stretch the Marginal Risk to include Los
Angeles county. An upper level polar cutoff low begins the day
Tuesday morning centered off the Oregon coast, and tracks
southeastward to be centered over the northern California coast by
Wednesday morning. A 100 kt westerly jet streak wrapping around
the low will direct a plume of Pacific moisture and precipitation
into California as it tracks south. PWATs will approach
0.75-1.00", which is about +1.5-2 sigma above the climatological
normal and IVT peaks at about 500-600 kg/m*s along central
California. This will bring with it a brief increase in snow
levels as high as 7,000 ft along coastal ranges during the day
Tuesday. The attendant surface low will follow a similar track as
the upper level cutoff low through this period, making it a
vertically stacked low, that will result in largely unidirectional
flow throughout the column south of the low. Since the vertically
stacked low has polar origins, it will have plenty of cold air and
a lack of plentiful Pacific moisture with it. Thus, this rainfall
event is not expected to be as intense as the atmospheric rivers
impacting the state in recent weeks. Nevertheless, the residual
impacts from the atmospheric rivers remain across coastal central
California, and the injection of up to 3 inches of rain into the
coastal mountains are still expected to cause or worsen flooding
impacts as that rainwater flows down the mountains and swells
already high rivers. Thus, the Slight Risk was maintained due to
steady forecasted rainfall through the Bay area and especially the
mountains north of there, while also spanning south to include the
westernmost Transverse Ranges of Santa Barbara County. Further
inland, slightly lower snow levels, much higher mountains, and
more recovery time from the last round of atmospheric rivers were
factors that led to the continuance of the Marginal Risk, as much
of the heaviest precipitation is likely to fall as snow. The
highly favorable antecedent conditions for flash flooding remain a
significant factor in the Slight Risk area.
...Central Gulf Coast States...
The Marginal Risk area along the Gulf Coast was confined to the
south and along the central Gulf Coast compared to the recent
issuance in order to match recent guidance and the likelihood that
intense rainfall rates will remain closer to the coastal regions.
By this time, the lingering frontal boundary plaguing the Deep
South will shift farther to the south and limit the available
moisture/instability for areas to the north. Still, convection
ongoing from Monday night moving into eastern LA and southern MS
is likely to tap into 1.5" PWATs and have rainfall rates
potentially exceeding 1"/hr. Urban regions and locations where
rain has fallen over the last few days will be most at risk to
localized flooding concerns.
Snell/Wegman
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt
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