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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 221019

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

Fair and warm weather again today will be transitioning to cooler 
conditions on Monday as an upper-level low pressure system drops 
south over the deserts. The low will intensify over northwest Mexico 
on Tuesday, resulting in some gusty northeast winds over the San 
Bernardino Mountains and Inland Empire. Warmer in the valleys on 
Tuesday due to offshore flow. Some clouds and moisture will wrap 
westward around the low on Wednesday, sparking a small chance for 
showers. Much cooler with more marine clouds for the remainder of 
the week as a large trough settles in over the West with gusty 
onshore flow and a chance of early season showers along and west of 
the mountains. 



Other than a few high clouds, skies were clear at 2 AM PDT. Winds 
were mostly light as well with a mix of directions. Strongest were E-
NE with a peak gusts below 25 MPH. At 2 AM PDT, the sfc pressure 
gradients were offshore around 5 MBS (SW NV to KSAN) and very weak 
onshore to the lower deserts. 

A weak ridge aloft will dominate our weather today with a few high 
clouds and warming. Weak offshore flow this morning will elevate 
temps to around 5F above average west of the mts, while the deserts 
check in near average for the last day of Summer (Autumn officially 
arrives at 12:50 AM Monday morning).

For Monday, an upper low will close off and deepen as it drops south 
over the deserts and over far NW MX Tuesday night. Flow around the 
low will bring gusty NE winds across our mts and coastal foothills 
on Tuesday and eventually wrap in some moisture, increasing clouds 
and a small chance for scattered showers or even an isolated 
thunderstorm Wednesday and Wednesday night. The downsloping winds 
will bring heat to the inland valleys and an increased wildfire risk 
on Tuesday before a cooling trend dominates for the remainder of the 

Thursday through Saturday an early-season cold pool will carve out a 
large longwave trough in the Westerlies. Most of the energy with 
this trough will stay to the north, but it will ramp up the onshore 
flow and build the marine layer over SoCal to a point where some 
scattered, light precipitation could fall along and west of the mts. 
Best chance looks to be Fri night into Sat morning. Absent any 
significant synoptic forcing, any precip that falls will likley be 
light, with some orographic enhancement on the coastal slopes. 
Perhaps one-half inch in favored spots there, and much less at lower 
elevations. Look for daytime temperatures to be well below average 
for this period as well.  


220830Z...Coast...A few low clouds with bases near 1000 ft MSL may 
develop along the coast this morning, but low probability of CIGS at 
the coastal airports. Any low clouds clearing by 16Z. More 
widespread low clouds should develop after 23/05Z with bases 1000-
1500 ft MSL, locally spreading 10-15 mi inland by Monday morning. 
Otherwise SCT-BKN clouds AOA 20000 ft MSL and unrestricted vis 
through Monday morning.

Valleys/Mountains/Deserts...SCT-BKN 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.