Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 212153

Area Forecast Discussion
National Weather Service San Diego CA
253 PM PDT Wed Mar 21 2018

A low pressure system off the California coast will move slowly
towards the coast spreading a strong atmospheric river of
tropical moisture into southern California. Widespread rain will
develop late tonight from north to south, and become heavy at
times on Thursday. Rain will transition to scattered showers
Thursday evening and decrease through Friday morning. A weak low
pressure system will move along the West Coast maintaining cooler
conditions and a chance of light showers.



Atmospheric river of tropical moisture nosing into the Central
Coast this afternoon and very heavy rain occurred there a few
hours ago with rain rates over 1/2 inch per hour. Out ahead of
the main tropical moisture plume is an area of showers and
embedded light rain associated with a weak s/w affecting SoCal
basically north of an Orange County to San Bernardino County zone.
This area will likely diminish later this afternoon as some drier
air is noted on water vapor satellite. Beyond that this evening
will be mainly dry as the main tropical plume will remain oriented
more towards Pt Conception.

Late tonight as the main shortwave trough shifts east towards
California, the moisture plume will begin to sag south across
SoCal. We expect rain to develop late tonight over the north and
develop south across San Diego County through midday Thursday.
With extreme moisture field and shortwave forcing, rain will
overspread the entire area through Thursday afternoon, heavy at
times. The lower deserts should even see some rain, although not
nearly as heavy, due to the very deep moisture profile.

The rain will transition to showers Thursday night as the
shortwave trough moves through SoCal, then taper off Friday
morning. There could be some residual showers lingering in the
mountains through Friday late in the morning.

Another low pressure trough skims to our north on Sunday bringing
a chance of rain to the area.



Rainfall amounts will be greatest over northern areas and
especially along the coastal-facing mountain slopes where amounts
of 5 to 7 inches, in isolated locations, will be likely. In the
populated valleys and coastal areas, amounts will still be
significant but overall much lower (higher amounts northern valley
and coastal areas)...with 1 to 2 inches in the Northern Inland
Empire and Orange County and 1/2 to 1 inch in San Diego County
coast and valleys. Even with lower overall storm total
precipitation amounts, the potential will still exist for very
heavy but shorter duration bursts of rain as high as 1/2 inch per


212045Z...SHRA mostly in San Bernardino County and far NW Riverside
County, including at/near KONT, KRAL and KSBD, will mostly end by
23Z, with local BKN-OVC bases below 4000 ft MSL, vis 1-4 mi and
mountain obscurations. Otherwise, mostly BKN clouds be 4000-8000
feet MSL tonight and 1500-3500 feet MSL Thursday. Isolated -SHRA and
some mountain obscurations will occur in southwestern San Bernardino
County and Northern Orange County tonight, then more widespread RA
and mountain obscurations will occur Thursday and affect most
coastal and valley airports, with some vis 1-4 miles. However,
in the lower deserts, most clouds will remain above 5000 feet MSL
Thursday with isolated -SHRA.


A storm system will bring periods of rain Thursday through early
Friday. Higher winds and swell are likely to bring hazardous
conditions Friday night through Sunday night. A small craft advisory
will likely be needed then due to wind gusts around 25 knots and
combined seas 8-10 feet with predominate periods of 9 seconds or
less. Seas and winds will lower by Monday.


Skywarn activation is not requested. However weather spotters are
 encouraged to report significant weather conditions.


CA...Flash Flood Watch from 11 PM PDT this evening through late
     Thursday night for Orange County Coastal Areas-Orange
     County Inland Areas-San Bernardino County Mountains-San
     Bernardino and Riverside County Valleys-The Inland Empire-
     Santa Ana Mountains and Foothills.



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