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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 221655

Area Forecast Discussion
National Weather Service San Diego CA
950 AM PDT Sun Sep 22 2019

The last full day of summer will be a warm one with high clouds at 
times. Monday will be a little cooler as a trough of low pressure 
moves in from the north. The trough will stall over northwest Mexico 
on Tuesday and result in gusty offshore winds in the mountains and 
inland valleys. Moisture wrapping around the stalled upper low may 
bring a few showers Wednesday, mainly to the mountains and deserts. 
It will turn cooler toward the end of the week as a large trough 
settles over the West.   



This weeks weather will be somewhat of a roller coaster ride, 
starting out warm and dry today, then transitioning to a Santa 
Ana pattern Tuesday and Wednesday before turning much cooler Friday 
and Saturday.  

The last full day of summer will be a warm one, accompanied by a 
a viel of high cirrus clouds associated with a weak trough drifting 
across SoCal. Monday will be a little cooler, especially west of the 
mountains, as a deep trough racing down the California interior 
increases our onshore flow. This trough evolves into a closed low 
over the Lower Colorado River Valley Monday night before drifting 
into NW Mexico Tuesday. This feature will force the deep layer 
tropical moisture into Arizona where heavy rain is likely in Monday 
and Monday night. In SoCal, the greatest impacts from this system 
will likely be the gusty offshore winds in the mountains and inland 
valleys on the backside of the low Tuesday. Downslope flow will 
bring warming to the inland valleys and elevated fire weather 
concerns. Wrap around moisture may reach the mountains and deserts 
and generate scattered showers or an isolated thunderstorm Wednesday 
and Wednesday night. 

On Thursday the upper low slowly fills and drifts east as a longwave 
trough begins settling over the western states. Most of its 
shortwave energy will stay to the north, but lower heights and 
stronger onshore pressure gradients will bring cooler weather and a 
deeper marine layer Friday and Saturday, perhaps deep enough for 
scattered light showers along and west of the mountains. 


221530Z...Coast...Mostly clear with unrestricted vis through this 
evening. More widespread clouds to develop after 05Z Monday with 
bases 1000-1500 ft MSL, spreading up to 15 miles inland overnight. 

Valleys/Mountains/Deserts...SCT-BKN high clouds AOA 20000 ft MSL and 
unrestricted vis through Monday morning. 

No hazardous marine weather is expected through Thursday.


A dry and warm day is expected today with weak offshore flow 10 to 15 
MPH with locally higher gusts through the passes, turning onshore 
in the afternoon. Low RH with minimums of around 15% this afternoon 
will create a period of elevated fire weather conditions.

Cooler with higher RH on Monday as onshore flow strengthens ahead of 
a low pressure system dropping south through the deserts. This 
feature will stall over northwest Mexico and set up a period of 
gusty northeast winds 20 to 30 MPH with local gusts 40 MPH across 
inland areas on Tuesday. Gusty northeast winds in the San Bernardino 
Mts and into the Inland Empire will create several hours of near 
critical fire weather conditions due to heat and minimum RH of near 
10%. Temperatures of 90 to near 100 degrees are possible in the 
Inland Empire if the winds mix to the surface. Late in the day, this 
weather is expected to moderate as the sea breeze advances inland. 
Weaker offshore flow may persist through Wednesday morning, but it 
will be cooler and RH higher.

A large, cool, early season trough will develop over the West after 
Wednesday with unseasonably high RH and possible showers along and 
west of the mountains. Westerly winds will be strong and gusty at 
times over the mountains and deserts.