Area Forecast Discussion
Issued by NWS San Diego, CA

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076
FXUS66 KSGX 191652
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
930 AM 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 morning 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. Cooling
may not occur until Wednesday or Thursday of next week.

&&

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

Stratus spread into both coastal and valley locations last night but
has mostly cleared at mid-morning. NKX sounding this morning showed
marine inversion base around 2000 ft MSL with considerable cooling--
mostly 5 to 10 deg C in the 875-975 MB layers and slight cooling up
to 750 MB. Most areas will be about 5 deg F cooler today, except
more than that in the inland valleys and lower coastal mountain
foothills due to the intrusion of the marine air.

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. 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. Will add
mostly 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. In
the forecast this afternoon, will go 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. Some areas may need an excessive heat watch for around
Mon/Tue. 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 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...
191508Z...Patchy clouds over the coast and portions of the valleys,
with bases around 1500-2000 ft msl, tops 1900-2200 ft msl and local
vis down to 1/4 sm in fog in some inland valley locations (KRNM and
KRIV for example), will clear out through 1700 UTC, while P6SM and
FEW-SCT clouds AOA 20000 ft msl continues elsewhere. 1700-20/0000
UTC, primarily P6SM vis and FEW-SCT clouds AOA 20000 ft msl for all
areas. 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. A Beach Hazard Statement remains
in effect through this afternoon. The south southwest swell will
diminish tonight, and be replaced by an elevated short period
northwest swell Friday and Saturday in addition to a long-period
steep angled northwest swell from 295 degrees.

&&

.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 versus 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. Will
have an update on any possible fire weather watch early this
afternoon.

&&

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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement through this 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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