Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 140530
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
930 PM PST Wed Dec 13 2017

.SYNOPSIS...
High pressure aloft and offshore flow at the surface will continue
warm and dry weather into the middle of next week, except for a
period of onshore surface winds late Friday into Saturday
afternoon with a possible brief return of the marine layer at the
coast. Stronger Santa Ana winds are likely on Thursday into early
Friday and again Saturday night through Sunday night over the
mountains, foothills and passes. Skies will remain fair overall
with very low relative humidity except near the immediate coast
and locally inland at times with the sea breeze.

&&

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

...Update...

The evening San Diego sounding again is bone dry with PW only at
0.16 inches. A mostly clear sky prevails. Do not anticipate any
stratus or fog impacting land areas overnight. There is some
patchy stratus or fog some 5-10 nm off the southern San Diego
coast, but as an offshore push arrives, think this will stay
offshore and be very spotty. As the offshore push arrives, winds
will increase from the mountain tops to the coastal slopes,
foothills and through and below the mountain passes where gusts
40-50 mph are forecast. The wind though will be delayed for San
Diego County until during the day Thursday.

...Previous Discussion (Issued at 121 PM PST Wed Dec 13 2017)...

Satellite imagery indicated that skies were mostly clear across
the region this afternoon, with wind observations showing some
weak Santa Ana winds continuing (gusts of only 20 mph in the
windiest coastal foothill/pass locations). An upper level low
remained over Baja and Sonora, while an upper level ridge extended
into California and Nevada. The continued offshore flow (Santa
Ana winds) will continue to bring much warmer than normal weather
this afternoon, with highs 5-15 degrees above normal in most
locations. Latest hi-res model guidance actually shows a coastal
eddy spinning up tonight with the possibility of fog occurring
along the coast tonight into Thursday morning. However, confidence
is low as this might be overly aggressive given the very dry air
and offshore pressure gradients that continues across the region.

Currently the offshore pressure gradients from San Diego to Las
Vegas were 2.4 mb, whereas they were 5.7 mb at this time
yesterday. This will change however when a short-wave trough moves
through the Great Basin area tonight and down into Sonora Mexico
tomorrow evening. This will result in strengthening offshore
pressure gradients (peaking around 8-9 mb from San Diego to Las
Vegas Thursday morning) and upper level support in the form of 850
mb winds NE 25-35 kt. Thus, we will get a burst of Santa Ana
winds Thursday morning and afternoon, then gradually weakening
Thursday night into Friday morning. NE to E winds of 20-30 mph
with gusts of 50 mph and locally 65 mph can be expected in the
coastal foothills/slopes and through/below the canyons/passes.
Potential impacts from the winds are described in the Red Flag
Warning.

Winds then turn briefly onshore late Friday through Saturday
afternoon as a trough moves through the West. Latest model runs
show a closed low splitting out from the main trough and moving
near Southern California late Saturday. The 12Z GFS even brings
some precipitation with this low, but am not buying this solution
yet as the 12Z ECMWF keeps the low far off the coast and leaves us
dry. At the very least, expect some decent cooling to take place
Saturday with a possible brief return of the marine layer Saturday
morning. The main trough will move into the Great Basin late
Saturday, once again creating a strong surface high in that area
and bringing a period of moderate Santa Ana winds Sunday through
early Monday. The offshore flow weakens later Monday but may not
transition to onshore until maybe next Wednesday. Expect warmer
than normal and fair weather to continue.

&&

.AVIATION...
140500Z...Mostly clear with unrestricted vis will prevail through
Thu evening. Any very slow stratus or fog that develops tonight will
likely remain well off the coast. Surface east to northeast winds 15-
25 kt with gusts 35-45 kt will develop late tonight mainly in the
foothills and through mountain passes and peak Thursday midday
through afternoon. LLWS and weak/mod up/down drafts are expected
over areas just west of the mountains late tonight through Thu
evening.

&&

.MARINE...
A few patches of fog could occur overnight, mainly beyond about 20
NM from the coast. Otherwise, no hazardous marine weather is
expected through Monday.

&&

.FIRE WEATHER...
Santa Ana winds will strengthen significantly early Thursday
morning, first in the San Bernardino county coastal slopes,
through and below the Cajon Pass, and into the Santa Ana
mountains/foothills around 4 am and then into the Riverside county
coastal slopes/passes by 6 am and finally into the San Diego
county coastal slopes/foothills by 10 am. NE to E winds will peak
at around 20-30 mph with gusts to 50 mph and locally 65 mph in
those areas, with day-time humidity once again lowering to the 5-8
percent range. The winds will become less widespread Thursday
evening and gradually weaken through early Friday morning.
Overnight recovery will be very poor. A Red Flag Warning is in
effect for early Thursday morning through Friday morning for gusty
winds and very low humidity.

Late Friday through Saturday afternoon, there will be a brief
period of onshore winds and slightly higher humidity. Then another
burst of Santa Ana winds is likely Sunday into early Monday, with
humidity falling to 10-15 percent away from the coast. Thus, we
could see another round of critical fire weather conditions
Sunday-Early Monday.

&&

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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Red Flag Warning from 4 AM Thursday to 10 AM PST Friday for
     Orange County Inland Areas-Riverside County Mountains-
     Including The San Jacinto Ranger District Of The San
     Bernardino National Forest-San Bernardino County Mountains-
     Including The Mountain Top And Front Country Ranger
     Districts Of The San Bernardino National Forest-San
     Bernardino and Riverside County Valleys  -The Inland Empire-
     San Diego County Inland Valleys-San Diego County Mountains-
     Including The Palomar And Descanso Ranger Districts of the
     Cleveland National Forest-San Gorgonio Pass Near Banning-
     Santa Ana Mountains-Including The Trabuco Ranger District
     of the Cleveland National Forest.

PZ...NONE.
&&

$$

PUBLIC...Gregoria (Update)/Prev Discussion (Harrison)
AVIATION/MARINE...Maxwell


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