Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 200417 AAA

Area Forecast Discussion
National Weather Service San Diego CA
916 PM PDT Thu Oct 19 2017

A trough of low pressure will move through the West Coast Friday.
This will result in a cooling trend through Friday, with a deep
marine layer bringing stratus with drizzle or light showers late
tonight and Friday from the coastal mountain slopes west to the
coast. A Santa Ana wind will develop this weekend and continue early
next week and bring hot dry weather along with occasional gusty
winds especially through and below passes and canyons. A gradual
cooling trend will likely occur Wednesday and Thursday of next week.



...Evening update...

The high clouds from earlier today are starting to push out of the
region this evening. This should allow widespread coastal stratus to
spread inland into the valleys through Friday morning. Onshore
gradients are increasing and gusty winds have already been occurring
in the high deserts with gusts around 35 mph. The San Diego to
Daggett gradient is up to 6 mb and will increase a bit more on
Friday. Gusts near advisory level will be possible on Friday over
the desert slopes of the mountains and especially into the deserts
where some blowing dust and sand will be possible. Friday will be
another cool day like Thursday, even a bit cooler with onshore flow
maximized and the upper level trough overhead. Some light showers
also remain in the forecast mainly Friday morning into midday with
QPF up to one-tenth of an inch. The only updates to the forecast
database this evening were for coordination of chances for rain
across all of Orange County and spreading the possibility of
dust/sand over all of the desert areas. No other changes at this

...Previous forecaster discussion...

Rather thick high clouds were drifting over most of the region this
afternoon with a few lingering low clouds near the coast.
Temperatures were mostly 5-12 deg F lower this afternoon due to the
cooler profile and deeper marine layer.

Low pressure trough was off the Pac NW coast and will shift east to
just inland from most of the West Coast by Friday morning. Trough
axis moves through So-Cal mid-morning Friday, and with a deep marine
layer and the morning timing with higher low-level relative
humidities, there could be drizzle or light showers. Models have
been consistent with showing some measurable precip, mostly under
1/10 inch, over mainly the valleys and foothills. This models do
have some moisture lingering into Friday afternoon, so precip may
locally linger at least near the foothills accordingly. WRF Bufkit
soundings show the marine air reaching 5000 feet or higher over some
areas from the mountains west to the coast, especially San Diego
County. Westerly winds just above the surface of 10-15 MPH could
orographically enhance the precip on west-facing slopes. Added
mostly 20-30 POPS to the forecast for mainly Friday morning. Areas
of gusty winds will occur Friday on the east mountain slopes and
could reach advisory criteria.

The big issue will be the Santa Ana winds beneath a strong ridge of
high pressure Saturday through Tuesday or Wednesday. This will bring
hot conditions, peaking Monday and Tuesday. The hot conditions could
reach the immediate coast as most of the driving of the offshore
flow will be the surface pressure gradients, with smaller effects
from the mountain wave as cold air advection will be weaker than
with most Santa Ana wind events at least Sunday through Tuesday, so
any inversion which would support a mountain wave would be weak.
There could be an inversion initially Saturday near the mountain
crests which could support mountain waves for locally strong gusty
winds southwest of the mountains. Temperatures will likely reach 100
or higher Monday and Tuesday in many areas west of the mountains,
with a few areas possibly 105-106. In the forecast this afternoon, I
mostly went above MOS guidance as it tends to be too low in this
pattern in October. GFS and ECMWF have been quite consistent with
their temperature profile run-to-run and have 850 MB temps peaking
around 24-25 deg C west of the mountains Mon/Tue, and gusty easterly
winds could keep many areas quite . warm Mon night. Issued an
excessive heat watch for Mon/Tue west of the moutnains. Current wind
guidance suggests gusts in the 40-50 MPH range Saturday mainly below
the San Bernardino County Mountains, slightly weaker Sunday and
Monday, then probably 40-50 MPH again Tuesday but over more
locations as the surface high pressure strengthens over the Great

After a peak in the upper high over California somewhere in the 596-
599 DM range at 500 MB Mon-Tue, the heights lower as a relatively
weak trough approaches from the north. All models have lower heights
Wednesday, but many solutions of the GFS ensemble have modest height
falls. Cooler weather should start Wednesday, but it might be
Thursday until more substantial cooling occurs.



200400Z...BKN-OVC015-025 expanding in coverage through 15Z Friday
west of the mountains with tops 4000-5000 ft. Areas of light rain
and drizzle west of the mountains late tonight through midday
Friday. The foothills and coastal mountain slopes below 5000 ft will
be obscured at times in Clds/-RA/DZ through 20Z Friday. Partial
clearing after 20Z with BKN-OVC020-030 becoming SCT-BKN020-030
through 00Z Saturday. Mostly clear in the deserts and in the
mountains above 5000 feet.

Windy in the mountains and deserts through Friday evening with 15-30
kt winds gusting to 40 kt over the mountains and down the slopes
into the deserts with mod-stg uddfs/llws. Areas of BLDU in and
around PSP and TRM Friday.


Small Craft Advisory in Effect Through 1 AM Saturday. A trough
moving in from the northwest will bring strong northwest winds to
the coastal waters Friday. Wind gusts of 25 kt combined with short-
period combined seas of 5-9 ft will result in hazardous boating for
small craft. Highest seas and strongest winds will be near and south
of San Clemente Island. Sea state and weather conditions will slowly
improve on Saturday. Lighter winds and calmer seas are expected next


A 2-3 ft/14-15 second south to southwest swell from 200 degrees will
diminish tonight. This swell will be replaced by a short-period
northwest swell and a long-period steep angled northwest swell from
295 degrees late Friday through Saturday. This will bring another
round of elevated surf from 3-6 ft and an increased rip current
threat. Although surf lowers somewhat on Sunday, rip currents will
still be a concern. A Beach Hazards Statement is in effect through
Sunday afternoon.


Critical fire weather conditions are likely due to a Santa Ana wind
event Saturday through Tuesday, with biggest concerns Sunday through
Tuesday as temperatures increase and low relative humidity is more
widespread. The concern is more of the duration of the hot dry windy
conditions as 6-hour durations of sustained 25 MPH winds and 35 MPH
gusts may not necessarily be reached each day when RH values are
below 15 percent, but many areas will be close. Current guidance has
strongest winds Saturday with some mountain wave potential and then
again Tuesday with strongest surface pressure gradients. Issued a
fire weather watch starting Saturday for the SBD mountains, Inland
Empire and SNA mountains, then starting Sunday for the rest of
Orange County, valleys and mountains of San Diego County and the
mountains of Riverside County, with all areas continuing through


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


CA...Fire Weather Watch from Sunday morning through Tuesday afternoon
     for Orange County Coastal Areas-Orange County Inland Areas-
     Riverside County Mountains-Including The San Jacinto Ranger
     District Of The San Bernardino National Forest-San Diego
     County Inland Valleys-San Diego County Mountains-Including
     The Palomar And Descanso Ranger Districts of the Cleveland
     National Forest.

     Fire Weather Watch from Saturday morning through Tuesday
     afternoon for 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-Santa Ana Mountains-
     Including The Trabuco Ranger District of the Cleveland
     National Forest.

     Excessive Heat Watch from Monday morning through Tuesday evening
     for Orange County Coastal Areas-Orange County Inland Areas-
     San Bernardino and Riverside County Valleys-The Inland
     Empire-San Diego County Coastal Areas-San Diego County

     Beach Hazards Statement through Sunday evening for Orange County
     Coastal Areas-San Diego County Coastal Areas.

PZ...Small Craft Advisory for winds from 8 PM this evening to 1 AM
     PDT Saturday for Coastal Waters from San Mateo Point to the
     Mexican Border and out to 30 nm-Waters from San Mateo point
     to the Mexican Border Extending 30 to 60 nm out including
     San Clemente Island.



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