Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 240935
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
234 AM PDT Sun Sep 24 2017

.SYNOPSIS... High pressure over the Great Basin will force weak to
moderate offshore flow over Southern California this week. The dry
northeast winds will lower humidity, and bring warmer days and
continued cool nights. Winds will be gusty at times along the
coastal foothills, and will increase the threat of wildfires,
especially early this week.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

It was a clear and cool night over SoCal. aT 2 AM PDT, many
locations west of the mountains already had temps in the 50s, with
March field at 49F. Surface pressure gradients were trending
offshore, but gradients were only around 4 MBS from SW NV to KSAN,
and remained weakly onshore to the lower deserts. The gradients
should build further offshore this morning, supporting gusty NE
winds, especially below the Cajon Pass and over the Santa Ana Mts.
Some spots along the ridges in San Bernardino County already had N-
NE winds. Peak gust was 24 MPH at Crestline.

Our pleasantly cool weather of the past few days is history now as
the main cold pool associated with the upper-level trough
responsible, lifts NE over the Rockies. Weak ridging aloft will now
support weak sfc high pressure over the Great Basin, and offshore
flow across SoCal. Even so, the longwave trough will remain over the
West, and some additional energy will drop back into it on Tue,
forming a cut-off low over the Lower Colorado River Valley on Wed.
The forecast development and position of the low will fend off the
spread of stronger offshore flow into San Diego County, and keep the
sfc pressure gradient in check farther to the north as well.

Winds will become gusty along the coastal foothills of San
Bernardino, Orange, and to some degree Riverside Counties over the
next few days. This will cause considerable warming and drying of
the air over the coastal basin. The main concern here will be
elevated fire weather conditions. (Please see the fire weather
discussion below). Otherwise, the weather will be dry and seasonally
warm into next weekend, with a trend back to more marine influence
(higher humidity) west of the mts by the end of the week.

&&

.AVIATION...
240835Z...Mostly clear skies and unrestricted vis through Monday
morning. Gusty northeast winds will develop after 10Z through the
passes and along the coastal slopes...with gusts 30-40 knots
possible in the most wind prone locations after 12Z. This could
produce weak to moderate up/downdrafts and llws over and southwest
of the mtns. Winds will gradually weaken during the afternoon.

&&

.MARINE...
No hazardous marine weather is expected through Thursday.

&&

.FIRE WEATHER...
Offshore flow has developed this morning, and will become gusty
below the coastal slopes, canyons and passes of San Bernardino,
Riverside, and Orange Counties Mon/Tue. The RH will drop
considerably today, with minimum daytime values as low as 10% by
Monday. In the favored, offshore wind-prone areas, we are still
expecting to see occasional winds of 25 to 35 mph with isolated
gusts to 40 mph, especially Mon/Tue mornings. Several hours of near
critical fire weather conditions are possible in these foothills
areas, canyons, and passes, mainly north of San Diego County Mon/Tue
due to low RH and winds exceeding 25 mph. However, with the limited
coverage and duration of critical conditions, no fire weather
product issuance is anticipated at this time.

For the second half of the week, mostly light winds are expected,
with a sea breeze each afternoon/evening. RH will show some modest
improvement, but it will remain quite warm and dry through next
weekend.

&&

.SKYWARN...
Skywarn activation will not be needed today.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.
&&

$$

PUBLIC/FIRE WEATHER...JAD
AVIATION/MARINE...SS



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