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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

                            
959 
FXUS66 KSGX 230937
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
237 AM PDT Sat Sep 23 2017

.SYNOPSIS...
A low pressure trough over the West will keep it comfortably cool 
again today. The trough will weaken next week, allowing surface high 
pressure to build over Great Basin, which will turn the winds 
offshore over southern California. This will set the stage for 
sunny, warmer days, and clear, cool nights through the end of the 
month. The drying northeast winds will become gusty at times along 
the coastal foothills, especially Monday and Tuesday, increasing 
the threat of wildfires.  


&&

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

Once again overnight low clouds were mostly confined to the 
foothill and lower coastal slopes, with only scattered clouds 
farther west. Weak onshore sfc pressure gradients continued KSAN to 
the lower deserts, while they were weak offshore from NV. At 2 
AM PDT winds were mostly light. 

The deep, longwave trough over the West has reached its peak this 
morning and will begin lifting out through the Great Basin and on 
through the High Plains Mon/Tue. This will allow surface high 
pressure to build south across the CA deserts and turn the winds 
over SoCal offshore. The winds should peak Mon/Tue mornings before a 
secondary trough develops back across the Great Basin and settles 
as a cut-off low over the Lower Colorado River Valley again on Wed. 
This system will have little net sensible weather effect over SoCal, 
but it will serve to limit the extent, strength, and duration of the 
offshore event by weakening the sfc gradient.

Even though the offshore winds will be modulated, they will still 
mark a noticeable weather change here, punctuated by warmer days with 
much lower humidity, and cooler nights with few if any clouds until 
the marine layer can become reestablished again, perhaps late next 
week. Indications are that it will remain on the warm side and dry 
for early October with high pressure over the SW.

&&

.AVIATION...
230830Z...Coast/Valleys/Coastal Slopes...SCT-BKN clouds with bases 
2500-5000 ft MSL locally obscuring higher terrain through 17Z. 
Clouds becoming FEW-SCT after 17Z with mostly clear skies and 
unrestricted vis prevailing through Sunday morning.

Mountains/Deserts...Clear with unrestricted visibility through 
Sunday morning. 

&&

.MARINE...
No hazardous marine weather is expected through Wednesday.

&&

.FIRE WEATHER...
Offshore flow will develop on Sunday, and become gusty below the 
coastal slopes, canyons and passes of San Bernardino, Riverside, and 
Orange Counties Mon/Tue. The RH will drop considerably, on Sunday 
with minimum daytime values in the 10-15% range through Thursday. In 
the favored, offshore wind-prone areas, we are still expecting to 
see local winds of 25 to 35 mph with isolated gusts topping 40 mph, 
especially Mon/Tue mornings. Several hours of near critical fire 
weather conditions are possible in these foothills areas, mainly 
north of San Diego County Mon/Tue due to the low RH and winds 
exceeding 25 mph. However, with the limited coverage and duration of 
critical conditions, no fire weather product issuance is anticipated 
at this time.

&&

.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...SS