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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 160319

Area Forecast Discussion
National Weather Service San Diego CA
818 PM PDT Mon Jul 15 2019

A developing trough along the West Coast will bring a cooling 
trend and deeper marine layer this week. Low clouds will be patchy 
tonight, with pockets of locally dense fog over coastal and mesa 
areas. The marine clouds will extend farther into the valleys later 
in the week with fair skies expected in the afternoons. For next 
week, high pressure will strengthen over the Southwest. This 
will bring a warming trend, shrink the marine layer, and possibly 
set up a monsoonal flow from the southeast.



At 8 PM PDT, satellite imagery showed areas of marine stratus 
over the CA Bight and along the O.C. coast, but coverage was much 
less than the last few nights. A large fire and some smoke was also 
visible well south of the U.S./Mexican border. The 00Z Miramar 
sounding had the marine inversion about 1475 FT MSL. This was a 
couple hundred feet lower than last evening. Surface pressure 
gradients were holding weak to moderate onshore with peak westerly 
wind gusts of 25-35 MPH in those onshore wind-prone desert areas. 
Some of the highest daytime temps were logged in the deserts today 
due to a drier atmosphere. Thermal tied a record at 118F and Palm 
Springs wasn't far behind at 117F.

The Heat Advisory and and Warning were allowed to expire. It will 
still be quite hot in the deserts, but not as hot as today and the 
trend is now downward. The sea breeze should be stronger and 
heights will be falling. Those winds are expected to keep most of 
the smoke from the Mexican fire south of the Border. 

Hires models are suggesting the marine layer will be shallow enough 
for some patchy dense fog to form along the higher coastal terrain, 
but there is still uncertainty on timing and extent of the fog by 
morning. A new Zone Forecast was issued to address the patchy dense 
fog and clean up the heat products.

From previous discussion... 

It's amazing what the humidity can do. A drier atmosphere has
allowed temps in the lower desert to soar uninhibited. Thermal at
1 PM was at 116 while PSP was at 111. Thermal's daily record is 
118...easily within reach. With single digit relative humidity the
heat index is 108 and 104, respectively. The heat warning and
advisory are on track to expire safely at sundown. We're watching
the marine layer go more shallow and the cloud deck shrinking in 
coverage and thinning in depth. Sundowner winds (northerly 
offshore winds) from Santa Barbara area seems to be the cause for 
all this cloud deck erosion. All models show this clearing to 
continue, dissolving the cloud deck to almost nothing by this 
evening. As expected, the beaches haven't cleared much, but expect
many to clear by evening. A weak coastal eddy tonight will 
strengthen Tuesday night into Wednesday, which should rebuild the 
stratus cloud deck and provide more widespread and uniform coastal
cloud coverage Wednesday and beyond. The weakening high pressure 
aloft will bring a modest cooling trend Tuesday and Wednesday. 
However, the coast will be warmer with fewer coastal clouds. A 
stagnant summer pattern will continue this entire week with a 
trough over the northwest coast and a ridge to the east over the 
four corners. Dry southwest flow aloft will keep the monsoon 
thunderstorm issue well to our east. The marine layer and its 
cloud deck should rebound enough that coastal clouds and fog 
should get into parts of the valleys during the nights and 
mornings Wednesday and beyond. Daytime temperatures will dip below
average Wednesday as well, and continue that way into the 
weekend. Next week the ridge to the east seems to build 
northwestward, which would produce a modest warming trend, back to
normal or a little above. The ridge axis would migrate north, 
allowing southeast monsoon flow into SoCal. That increasing 
moisture would lead to a growing chance of thunderstorms for 
mountains and deserts at some point next week.


160215Z...Coast...SKC-SCT007 near the coast through 06Z. After 
06Z...BKN-OVC004-008 at the coastal airports with areas of vis 2-4 
miles in fog and occasionally 1 mile at KCRQ through 16Z Tuesday. 
Confidence is low with timing and location of stratus/fog tonight. 
Clearing skies 16-18Z Tuesday. 

Valleys/Mountains/Deserts...Mostly clear with unrestricted vis 
through Tue.


No hazardous marine weather through Saturday. 


A long period south-southwest swell will bring above average surf 
and strong rip currents to the Orange and San Diego beaches through 
Tuesday. Surf will range from 2-4 feet with occasional sets to 5 
feet on the southwest facing beaches of Orange County.  


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