Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 161516
AFDSGX

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

.SYNOPSIS...
High pressure aloft and weak offshore flow will continue the hot dry
weather west of the mountains through Tuesday. A trough of low
pressure will bring cooling Wednesday through Friday along with a
deeper marine layer and gusty west winds in the mountains and
deserts. Dry and hot again early next week with another Santa Ana
weather pattern setting up over the West.

&&

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

7 am temperatures this morning were running several degrees higher
than 24 hours ago in most areas, with the greatest warming at
Ontario (+14F), and Imperial Beach (+14F). Elsewhere, it was 3 to 5
degrees higher at CRQ, SAN, MYF, SNA, FUL, Mt Laguna, Idyllwild, and
Hesperia.

A 500 mb ridge axis was directly over SoCal this morning and it will
drift ever so slowly eastward today. The 500 mb height field and the
boundary layer temperatures don`t change much compared to yesterday,
so expect high temperatures today will be similar to yesterday in
most areas. The forecast was updated to raise high temperatures in
the coastal and valley areas, especially in San Diego County.

On Tuesday the ridge axis drifts east and the flow aloft becomes
more zonal. Nevertheless, tomorrow will be another very warm day
with temperatures 10-15 degrees above average west of the mountains,
and from 5-10 degrees above average in the mountain and desert
zones.

On Wednesday a couple of weak Pacific shortwaves bring mid and upper
level moisture and instability to Southern California. They are weak
systems that lack deep moisture, so the threat for elevated
convection is low. The Precipitable Water increases from 0.66" today
to 1.35" Wednesday with most of the moisture above 700 mb. The
models are outputting small amounts of QPF over the region, so the
hi-rez model trends will be monitored closely today and tomorrow
for the elevated convection potential.

Cooler Wednesday through Friday with a deeper marine layer, stronger
onshore flow, lower heights and gusty afternoon and evening winds in
the mountains and deserts.

*From Previous Discussion*

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.

&&

.AVIATION...
161500Z...Mostly clear skies, unrestricted visibility and light
winds will continue through Tuesday morning.

&&

.MARINE...
No hazardous marine weather is expected through Thursday. Winds will
increase Friday as a front traverses the region, potentially
bringing wind gusts of 20 kt or more to the Southern California
Bight.

&&

.BEACHES...
A south-southwest swell from 200 degrees will build today and peak
Tuesday-Wednesday at 3-4 ft/16-18 seconds. This will produce 3-6 ft
surf with sets to 7 ft north of Del Mar. Highest surf will occur
along southwest facing beaches. Strong rip and longshore currents
will accompany the elevated surf. Swell and surf will begin to lower
on Thursday.

&&

.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...Moede
FIRE WEATHER...JAD
AVIATION/MARINE...Albright



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