Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 192110
AFDSGX

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

.SYNOPSIS...
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.

&&

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

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
Basin.

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.

&&

.AVIATION...
191927Z...Through 20/0000 UTC, primarily P6SM vis and FEW-SCT clouds
AOA 20000 ft msl for all areas, with any local SCT-BKN clouds 1500-
2500 ft msl likely clearing out by 2200 UTC. 20/0000-1500 UTC, OVC-
BKN stratus will develop over the coast, valleys, coastal slopes and
into the passes, with bases 2000-3500 ft msl, tops around 4000-5000
ft msl, mountain obscuration of coastal slopes, and drizzle/isol
light showers. Elsewhere, P6SM vis and FEW-SCT clouds AOA 20000 ft
msl will continue.

After 20/0000 UTC, west winds 15-25 kt with gusts 35-40 kt over
mountain ridges, desert slopes and into adjacent desert areas will
create mod-stg uddfs/llws over and E of the mountains. Patchy BLDU
is also possible in the desert areas. KPSP and KTRM will see
increasing NW winds after 2200 UTC. Forecast confidence is high.

&&

.MARINE...
A trough moving in over the West will strengthen the northwest winds
across the coastal waters this evening through Friday, with wind
gusts of 25 kt likely, accompanied by short-period combined seas of
5-9 ft. Highest seas and strongest winds can be expected near and
south of San Clemente Island. Conditions will slowly improve on
Saturday, with light winds and calmer seas expected early next week.

&&

.BEACHES...
A 2-3 ft/14-15 second south southwest swell from 200 degrees will
continue to impact beaches north of Solana Beach through today. This
will produce elevated surf of 3-5 ft with sets to 6 ft, along with
strong rip and longshore currents. The south southwest swell and
surf will diminish tonight, and be replaced by a short-period
elevated northwest swell in addition to a long-period steep angled
northwest swell from 295 degrees late Friday through Saturday. This
will likely bring another increase in the surf up to 3-6 ft along
with an increased rip current threat. Although surf lowers somewhat
on Sunday, rip currents will still be a concern. The Beach Hazards
Statement has been extended through Sunday afternoon.

&&

.FIRE WEATHER...
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
Tuesday.

&&

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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
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
     Valleys.

     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.

&&

$$

PUBLIC/FIRE WEATHER...Maxwell
AVIATION/MARINE/BEACHES...Harrison



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