Southwestern California Discussion

FXUS66 KSGX 181039

Area Forecast Discussion
National Weather Service San Diego CA
239 AM PST Thu Jan 18 2018

Expect another fair and mild winter day today, ahead of a fast-
moving trough to kick off the weekend. This system will have
limited moisture, but will produce gusty winds in the mountains
and deserts, and drop temperatures sufficiently for some light
snowfall in the mountains later Friday into early Saturday. It
will be noticeably cooler all areas through Sunday, then a bit
warmer into the first half of next week as afternoon high
temperatures slowly return to near average values.



Another batch of high clouds was drifting south over the region at 2
AM PST. Patchy fog was visible at times over the coastal waters just
offshore of San Diego County. Weak offshore sfc pressure gradients
were holding from the deserts supporting easterly winds over the
mts/foothills of San Diego County with local gusts to 25 MPH. The
sfc dewpoints were a bit lower along the coast this morning, so
drainage winds, coupled with the offshore gradient and high clouds
should keep any fog threat to a minimum.

It looks like another fantastic day is on tap across SoCal, thanks
to a high pressure ridge aloft. Afternoon temperatures will again
be running some 8 to 14 degrees F above average just inland from
the coast. The ridge will flatten over CA on Friday as a trough
slides east across the Pacific. The trough has a strong jet moving
through it and a modest amount of moisture ahead of it. The
combination of the right-rear quadrant lift associated with the jet
and deep cyclonic flow and vorticity look sufficient for
accumulating snowfall over the the higher mountains Friday night
into Saturday morning. Accompanying the snow will be gusty westerly
winds. A Winter Weather Advisory has been issued. Weekend travelers
should check the latest forecast and road conditions before heading
out. For details please see LAXWSWSGX.

The colder air associated with the trough will linger through the
weekend, followed by modest warming early next week as the trough
drifts east and heights/thickness recover. The 00Z GFS/ECMWF runs
bring another trough/inside slider across CA later next week which
will likely rev up winds and the marine layer again, but at this
time, any precip looks spotty unless it can manage to dig farther to
the west than currently forecast.


181000Z...Coast...Patchy fog impacting coastal airports is remotely
possible this morning through 15Z. If and where it occurs, bases
would be 200-400 feet msl with vis below 1sm. Also areas of vis 3-
5sm. Low clouds will develop and move ashore after 03Z this evening.
Low bases around 500 feet will rise and spread into some valleys
overnight. Across the region, VFR conditions with variable thin high
clouds today and tonight.


Isolated patches of fog with visibility 1nm or less this morning. A
storm arriving Friday will bring hazardous winds and seas late
Friday through Saturday. Refer to the Gale Watch for details.


Northwest swell and surf will decrease today, but increase again
tonight through Friday. High surf will peak Friday night into
Saturday morning before diminishing late Saturday. Consult the High
Surf Advisory for details.


Skywarn activation will not be needed through early next week.


CA...Winter Weather Advisory from 5 PM Friday to noon PST Saturday
     for Riverside County Mountains-San Diego County Mountains.

     Winter Weather Advisory from 2 PM Friday to noon PST Saturday
     for San Bernardino County Mountains.

     High Surf Advisory until 8 PM PST Saturday for Orange County
     Coastal Areas-San Diego County Coastal Areas.

PZ...Gale Watch from Friday afternoon through Saturday evening 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



NWS SGX Office Area Forecast Discussion