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