Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 161006
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
306 AM PDT Mon Oct 16 2017

.SYNOPSIS...
High pressure aloft and weak offshore flow will continue hot and
very dry weather west of the mountains through Tuesday with high
temperatures in the valleys and inland coastal areas in the 90s to
around 100. Cooling and moisture will begin to spread inland with
onshore flow Tuesday night and Wednesday, with greater cooling on
Thursday and Friday as a trough of low pressure moves inland along
the West Coast. This will bring more cloud cover and gusty west
winds in the mountains and deserts. Dry and hot again next weekend
due to strengthening high pressure aloft and offshore flow.

&&

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

Other than patchy high clouds drifting NE across the CWA early this
morning, skies were clear. Winds were generally light, but at 2 AM
PDT there were some areas of NE winds 10 to 15 with gusts around 20
MPH in the offshore wind-prone areas. Surface pressure gradients had
backed off a bit from the NE, but remained around 9 MBS offshore SW
NV to KSAN, and neutral from the east. Temperatures at 2 PM PDT were
anywhere from 3 to 8 degrees F higher than last night.

A high pressure ridge will remain over the SW today, then slowly
deflate through Wednesday as weak disturbances steer moisture around
its western flank and eventually across CA. These disturbances look
too weak to generate any significant precip, however they will carry
increasing instability, clouds, and moisture across central into
SoCal as they top the weakening ridge. This will be monitored as
some elevated convective activity is possible.

A more substantial short wave trough will slide SE across the PacNW
and CA on Fri. This one could carry some welcome rain over northern
sections of the state, but looks dry for the Southland. It will
generate some stronger onshore flow though, and that will build the
marine layer well inland, ending our extended period of very low
relative humidity inland.

The trough moves east over the upcoming weekend, allowing a strong
upper-level ridge to rebuild early next week, accompanied by more
offshore flow and very dry air. Supporting this scenario are both of
the latest operational GFS/ECMWF runs. These similar solutions boost
confidence in a hot start to next week.

Temperatures...look for another very hot day today, with mid-
afternoon temps running close to, or a few degrees F above values
observed on Sunday. Gradual cooling ensues Tue through Thu with max
temps on Thu close to average. Overnight lows remain elevated
through the period. Noticeably cooler by Fri with temps expected to
be a few degrees below average. Gradual warming begins over the
weekend and continues into Tue of next week.

Precip...No significant precipitation is expected through next week.
There is a small chance for convective activity to develop on Wed
based on the latest NAM guidance. This is the result of a large
influx of subtropical moisture aloft and a subsequent lowering of
stability in the mid/upper-levels of the atmosphere. Forcing is very
weak though, and a fairly deep dry layer decreases the chance of
measurable precip below 20 percent. POPS have dropped below this
threshold for Fri as well based on the latest guidance.

&&

.AVIATION...
160900Z...Mostly clear with unrestricted vis today through tonight.

&&

.MARINE...
No hazardous marine weather is expected through Thursday. Winds will
increase Friday with possibly hazardous gusts exceeding 20 kts.

&&

.BEACHES...A swell from 200 degrees will arrive late today and peak
Tuesday and Wednesday. This swell will have a long period which will
create bigger surf, especially on southwest facing beaches. Surf
will begin to subside on Thursday. Consult the latest surf forecast
for details...LAXSRFSGX.

&&

.FIRE WEATHER...

Through Tuesday...winds will be mostly light, but day-time RH will
remain very low with widespread 5-10 percent values away from the
coast and poor overnight recovery. It will also continue to be very
warm to hot across the area. Thus, elevated fire weather conditions
will continue.

Tuesday night through Wednesday...RH will start to increase as the
ridge aloft weakens and onshore flow and the marine layer slowly
rebuilds inland. However, we will also have to watch for the
possibility of convection on Wednesday. Wetting rain chances are
slim, but if these cells do develop in an increasingly unstable
airmass, gusty and erratic surface winds, combined with increasing
onshore flow and possible dry lightning, could pose another wildfire
threat.

Thursday and Friday...Onshore winds will be increasing over
mtn/desert areas and become strong and gusty from the west with
increasing RH driving well inland. Some light rain is possible
across central CA, but wetting rain chances have dropped below
threshold for SoCal.

Saturday through next Tuesday...Strong high pressure aloft and a
return of offshore flow will once again raise wildfire concerns with
very low RH, heat, and locally gusty offshore winds possible.

&&

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

&&

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

PZ...NONE.

&&

$$

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



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