Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 220942

Area Forecast Discussion
National Weather Service San Diego CA
242 AM PDT Thu Mar 22 2018

A weakening low pressure trough will interact with deep tropical
moisture today to produce light to moderate rain that will sweep
across the region, starting across Orange and San Bernardino
Counties early in the day, then spreading south. Heaviest rainfall
will be on the coastal slopes of the mountains. Rain will
gradually decrease tonight and Friday morning. A colder trough
will move across California Sunday for cooler weather and light
showers into Monday. The trough will move very slowly east and
offshore flow will develop for some foothill winds. Higher
pressure aloft will bring warmer weather Wednesday.



The main moisture plume and steady rain band still lurks to our
northwest along the central coast early this morning. Ahead of
that we have received some light to locally moderate intensity
showers through mainly the L.A. Basin. Winds have increased mainly
along the ridgetops of the San Gabriel and San Bernardino
Mountains. Top gusts there have exceeded 40 mph. These winds will
increase this morning. The rain will gradually push through
Southern California today from northwest to southeast. The
moisture plume falls apart and energy weakens considerably as it
progresses. As a result, total rainfall from this storm will
decrease sharply from northwest to southeast. See hydrology
discussion below for the rain details. The snow levels will be
quite high, above 8000 feet for most of this storm. The rain and
the strong mountain winds will gradually decrease tonight and
Friday morning. Residual low level moisture will keep skies from
clearing much Friday, but become partly cloudy. A large low
pressure trough continues over California this weekend, and
guidance features a shortwave trough rotating through Southern
California Sunday. That appears to bring showers and colder air
Saturday night through Monday morning. Amounts won`t be much with
this feature as it moves into Arizona Monday. It will feel colder
Sunday and Monday. Offshore flow will get going Monday and Tuesday
with the northerly flow over our region behind the trough. We
could get some offshore winds in the foothills. The trough will
very slowly move out of our region Tuesday and Wednesday and some
legit high pressure aloft appears to build to bring fair and much
warmer weather.


Rainfall amounts will be greatest over northern areas and especially
along the coastal-facing mountain slopes. Forecast additional
storm totals:

Orange County and Inland Empire: 1 to 2 inches
San Bernardino and San Gabriel Mountains: 2 to 5 inches
Riverside and San Diego County Mountains: 0.75 to 2 inches
San Diego County Coast and Valleys: 0.40 to 1 inch
Upper Desert: 0.25 to 0.5 inch
Lower Desert: 0.10 to 0.25 inch

Shorter duration bursts of heavy rain as high as 1/2 inch per
hour are possible today over northern areas, where a Flash Flood
Watch will be in effect. Overall rainfall will be light to
moderate, so rainfall rates should not exceed infiltration in many
areas, resulting in a beneficial rainfall.


230900Z...SCT060 BKN150 through 18Z with intermittent -RA/-DZ near
KSNA, KONT and KSBD. From 18Z Thu through 00Z Fri...BKN-OVC040-080
TOPS 150 lowering to OVC025-040 with RA spreading south into San
Diego County. Vis locally 3 miles in the heavier rain bands.
Mountains becoming obscured in clouds and fog at times. After 00Z
Friday...SCT-BKN040-060 with -RA transitioning to -SHRA before

Gusty southwest and west winds in the deserts and along the desert
slopes through 06Z Friday with periods of moderate LLWS and/or
up/downdrafts. Significant LLWS is not anticipated at KPSP or KTRM
at this time.


Areas of rain and breezy southwest winds (peak wind gust near 20 kt)
through Friday morning. Winds shift northwest Friday afternoon from
15-20 kt with gusts to 25 knots at times Saturday and Sunday. Choppy
short period seas will likely accompany the winds Saturday and
Sunday. Quieter conditions for early next week.


Skywarn activation is requested for Orange and San Bernardino


CA...Flash Flood Watch from 6 AM PDT this morning through late
     tonight 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.

     Wind Advisory from 8 AM this morning to 7 PM PDT this evening
     for Apple and Lucerne Valleys-San Bernardino County




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