Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 220427

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

A weakening low pressure trough will interact with deep tropical
moisture on Thursday to produce light to moderate rain, 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 transition to scattered showers Thursday
night and decrease on Friday morning. Another cold upper low will
move southeast across northern California and into Nevada this
weekend, maintaining cooler weather with a chance for some light



...Flash Flood Watch pushed back to a start time of 6 AM on

...Wind Advisory issued for the San Bernardino County Mts and high
desert areas on Thursday...

Based on remote rainfall gages and radar, rainfall has largely ended
across the forecast area this evening. This lull may last much of
the night before rainfall redevelops and increases on Thursday
morning. Thus, the start time for the Flash Flood Watch has been
pushed back to Thursday morning. Rainfall so far today has been
under one-half inch and confined to northern portions of the CWA.

We are still awaiting the arrival of the deeper tropical moisture
plume. The 00Z Miramar sounding had a respectable PW of one inch,
with low dew-point depressions throughout much of the column. The
freezing level was just over 12K FT!

The sfc pressure gradient was weak and winds light at 9 PM PDT,
however by Thu morning, developing sfc low pressure over western NV
should ramp up the gradient from the coast to around 10 MBS. At the
same time a strong mid-level jet will arrive to support gusty winds
over the San Bernardino Mts and high desert areas where a Wind
Advisory has been issued for strong S-SW winds/gusts exceeding 50 MPH
at times on Thu.

Satellite imagery over the EastPac shows a stream of cold cloud tops
moving rapidly NE over a building ridge over the SW. This shortwave
ridge will nudge the tropical moisture stream just far enough north
to cut off the precip over SoCal for a time tonight, but eventually
it will thin and swing east over the Bight and across SoCal on Thu.
This is when we have the potential for more widespread and heavier
rainfall. Forcing and instability remain weak, but it won`t take
much lift to squeeze a steady light to moderate rain out of the
moisture choked airmass to come. Greatest rainfall amounts will be
on the coastal mountain slopes where orographic flow can augment
rainfall rates. Please see the Hydrology section below for more

The rain tapers on Thu night and ends Fri morning when partly cloudy
skies return, along with daytime highs in the 60s. Another cold low
will dip south along the PacNW coast and into NoCal next weekend.
This will keep a troughy pattern across the state and bring more
precip. Any precip that makes it this far south looks to be light,
but worthy of mention in the forecast by Sunday. It will be a cool
weekend with highs between 6 and 12 degrees F below average.


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, are 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.

Shorter duration bursts of heavy rain as high as 1/2 inch per hour
are possible on Thursday 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.


220330Z...Generally dry through 09Z Thursday with mixed bases AOA
5000 ft MSL. Areas of -RA developing after 09Z, with bases as low as
3000 ft MSL and visibility of P6SM for all but the coastal slopes,
where FG may reduce visibility to 1 SM or less. Periods of +RA/RA
and bases near 1500 ft MSL possible between 16Z Thursday and 00Z
Friday. These may reduce visibility to 1-3 sm at times. FG will
continue along the coastal slopes with visibility of 1/2 or less. RA
transitioning to -SHRA after 00Z Friday.

Gusty southwest and west winds in the deserts and along the desert
slopes between 15Z Thursday and 06Z Friday. These winds will likely
result in 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. A transition to northwest winds is expected
on Friday. Northwest winds may approach 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 not requested. However weather spotters are
 encouraged to report significant weather conditions.


CA...Flash Flood Watch from 6 AM PDT Thursday 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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