000 
fxus66 ksgx 132121 
afdsgx 


Area forecast discussion 
National Weather Service San Diego California 
121 PM PST Wed Dec 13 2017 


Synopsis... 
high pressure aloft and offshore flow at the surface will continue 
warm and dry weather into the middle of next week, except for a 
period of onshore surface winds late Friday into Saturday 
afternoon with a possible brief return of the marine layer at the 
coast. Stronger Santa Ana winds are likely on Thursday and again 
on Sunday over the mountains, foothills and passes. Skies will 
remain fair overall with very low relative humidity except near 
the immediate coast and locally inland at times with the sea 
breeze. 


&& 


Discussion...for extreme southwestern California including Orange... 
San Diego...western Riverside and southwestern San Bernardino 
counties... 


Satellite imagery indicated that skies were mostly clear across 
the region this afternoon, with wind observations showing some 
weak Santa Ana winds continuing (gusts of only 20 mph in the 
windiest coastal foothill/pass locations). An upper level low 
remained over baja and Sonora, while an upper level ridge extended 
into California and Nevada. The continued offshore flow (santa 
Ana winds) will continue to bring much warmer than normal weather 
this afternoon, with highs 5-15 degrees above normal in most 
locations. Latest hi-res model guidance actually shows a coastal 
eddy spinning up tonight with the possibility of fog occurring 
along the coast tonight into Thursday morning. However, confidence 
is low as this might be overly aggressive given the very dry air 
and offshore pressure gradients that continues across the region. 


Currently the offshore pressure gradients from San Diego to Las 
Vegas were 2.4 mb, whereas they were 5.7 mb at this time 
yesterday. This will change however when a short-wave trough moves 
through the Great Basin area tonight and down into Sonora Mexico 
tomorrow evening. This will result in strengthening offshore 
pressure gradients (peaking around 8-9 mb from San Diego to Las 
Vegas Thursday morning) and upper level support in the form of 850 
mb winds NE 25-35 kt. Thus, we will get a burst of Santa Ana 
winds Thursday morning and afternoon, then gradually weakening 
Thursday night into Friday morning. NE to east winds of 20-30 mph 
with gusts of 50 mph and locally 65 mph can be expected in the 
coastal foothills/slopes and through/below the canyons/passes. 
Potential impacts from the winds are described in the red flag 
warning. 


Winds then turn briefly onshore late Friday through Saturday 
afternoon as a trough moves through the west. Latest model runs 
show a closed low splitting out from the main trough and moving 
near Southern California late Saturday. The 12z GFS even brings 
some precipitation with this low, but am not buying this solution 
yet as the 12z European model (ecmwf) keeps the low far off the coast and leaves US 
dry. At the very least, expect some decent cooling to take place 
Saturday with a possible brief return of the marine layer Saturday 
morning. The main trough will move into the Great Basin late 
Saturday, once again creating a strong surface high in that area 
and bringing a period of moderate Santa Ana winds Sunday through 
early Monday. The offshore flow weakens later Monday but may not 
transition to onshore until maybe next Wednesday. Expect warmer 
than normal and fair weather to continue. 


&& 


Aviation... 
132120z...mostly clear with unrestricted visible through tonight. Small 
chance of low clouds and fog developing near the coast. Forecaster 
confidence in fog development is low. Surface east to northeast 
winds 15-25 kt with gusts near 45 kt will develop late tonight 
mainly in the foothills and through mountain passes, and peak 
Thursday morning. Low level wind shear and weak/mod up/down drafts are expected over 
areas just west of the mountains through the period. 


&& 


Marine... 
small chance of fog overnight, however confidence is low any will 
develop. Otherwise, no hazardous marine weather is expected through 
Sunday. 


&& 


Fire weather... 
weak Santa Ana winds will continue today with humidity falling 
into the 5-8 percent range away from the coast. The Santa Ana 
winds will strengthen significantly early Thursday morning, first 
in the San Bernardino County coastal slopes, through and below the 
Cajon Pass, and into the Santa Ana Mountains/foothills around 4 am 
and then into the Riverside County coastal slopes/passes by 6 am 
and finally into the San Diego County coastal slopes/foothills by 
10 am. NE to east winds will peak at around 20-30 mph with gusts to 
50 mph and locally 65 mph in those areas, with day-time humidity 
once again lowering to the 5-8 percent range. The winds will 
become less widespread Thursday evening and gradually weaken 
through early Friday morning. Overnight recovery will be very 
poor. A red flag warning is now in effect for early Thursday 
morning through Friday morning for gusty winds and very low 
humidity. 


Late Friday through Saturday afternoon, there will be a brief 
period of onshore winds and slightly higher humidity. Then another 
burst of Santa Ana winds is likely Sunday into early Monday, with 
humidity falling to 10-15 percent away from the coast. Thus, we 
could see another round of critical fire weather conditions 
Sunday-early Monday. 


&& 


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


&& 


Sgx watches/warnings/advisories... 
California...red flag warning from 4 am Thursday to 10 am PST Friday for 
Orange County inland areas-Riverside County mountains- 
including the San Jacinto Ranger district of the San 
Bernardino National Forest-San Bernardino County mountains- 
including the mountain top and front country Ranger 
districts of the San Bernardino National Forest-San 
Bernardino and Riverside County valleys -the inland Empire- 
San Diego County inland valleys-San Diego County mountains- 
including the Palomar and Descanso Ranger districts of the 
Cleveland National Forest-San Gorgonio Pass near Banning- 
Santa Ana Mountains-including the Trabuco Ranger district 
of the Cleveland National Forest. 


Pz...none. 


&& 



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