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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 122020

Area Forecast Discussion
National Weather Service San Diego CA
120 PM PDT Thu Sep 12 2019

Upper level high pressure will build over Southern California for
Friday and Saturday and bring very warm and dry conditions into
Sunday. Another unseasonably strong Pacific trough will bring 
cooler weather, marine clouds and mountain winds early next week 
as it moves through northern California and the Great Basin. 



Santa Ana winds with gusts 25 to 35 mph materialized in all 
mountain gaps from Banning Pass to Interstate 8. Peak gusts of 40 
mph at most wind prone western slopes. The sea breeze is now
washing out this offshore flow but very slowly in I-8 and I-10 
corridor where east winds favored. Temps at century park for 
normal hottest inland areas and deserts. Air mass of 23-24C 850 mb
is in place. Clouds cleared to beaches but still in bight. 
Stratus tonight should be similar and mainly San Diego immediate 
coast and enhanced drainage wind in place as the offshore easterly
flow weakens.

Upper level high pressure of 593 DM builds directly over Socal 
brings warming of 25-27C at 850 mb air mass on Friday that lingers
into Saturday. Dry conditions (mixed out air brings dewpoints 
into upper 20s for deserts and 30s for Inland Empire) will allow 
for well mixed air inland...100 to 103 Inland Empire and 108 to 
113F for lower deserts. This will squash the marine layer but 
appears patches will linger on the San Diego coast on Friday with 
some expansion into immediate Orange coast by Saturday morning. 
This should also be fog due to lower inversion height. Hottest day
on coast is Friday and for inland areas its Saturday.

Onshore flow increases on Sunday with beginning of deepening 
marine layer but only slight cooling inland of a couple degrees. 
Good set up for dense fog on the immediate coast and beaches.
Air mass lowers about 2C so still hot inland deserts. Models still
inline with stronger and unseasonable Pacific upper trough 
cutting through northern California by Monday afternoon and into 
Tuesday. This will bring rapid cool down for socal and increased 
mountain and desert slope winds. System takes a similar Great 
Basin track and also weak offshore flow in its wake by midweek. 
This is followed by another surface high over Nevada and Utah so 
weak Santa Ana conditions could return Wednesday and Thursday.


122001Z...Patchy stratus redeveloping along the coast this evening. 
Low confidence on timing for SD County coastal TAF sites. Current 
thinking between 04-07Z with bases near 1000 ft MSL, spreading up to 
10 mi inland by morning for portions of SD County. MOD confidence 
for no cigs at KSNA tonight, though brief cigs after 13Z are 
possible. Areas of vis 3-5 sm 12-16Z in HZ/BR.

Mountains/Deserts...Easterly surface winds gusting up to 25 kt 
expected through the passes and canyons and on the coastal mountain 
slopes through 22Z, then weakening through the afternoon. Areas 
of weak to moderate up/downdrafts and low level wind shear 
possible over/west of the mountains. 


No hazardous marine weather is expected through Monday. 


.FIRE WEATHER...Santa Ana winds in I-10 and I-8 pass this
afternoon continue with gusts 25 mph but will continue to
decrease. Enhanced drainage this evening but weakening offshore 
flow. Hot temperatures and very low RH 7 to 15 percent for far 
inland valleys, mountains and deserts leads to elevated fire 
weather conditions given fuels this time of year through weekend. 
Slight cooling and slight higher RH on Sunday. Mountain passes 
have gusty winds Sunday night into early Tuesday with the incoming
Pacific trough.


Skywarn activation will not be needed today.