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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 151537

Area Forecast Discussion
National Weather Service San Diego CA
836 AM PDT Sun Sep 15 2019

After another very warm day today, Monday and Tuesday will be 
noticeably cooler and less humid as a trough of low pressure moves 
east across the State. This cooler weather pattern will persist 
through the end of the week, keeping temperatures a little below 
average. Gusty westerly winds will develop this afternoon over 
the mountains and deserts, peeking Monday night. This prevailing 
onshore pattern will rebuild the marine layer and associated low 
clouds and fog inland nights and mornings.



At 8 AM PDT...high clouds had thickened over the region and extended 
well offshore. Before the higher clouds thickened, patches of low 
clouds/fog were visible in satellite imagery along the coast and 
offshore. No coastal observations had any significant cloud or fog 
reports, except near Newport Beach earlier this morning, but a few 
cams did show some fog. The 12Z Miramar sounding had a 9C sfc-based 
inversion and showed cooling of around 2C above 2K FT. It also 
showed the dew point spikes around 10K and 30K FT. Also, 20 KT south 
winds had developed between 6-12K FT.

So far, temps are close to those observed 24 hrs prior, but given 
the extent of the high clouds, expect the diminished sun in some 
areas will stem the temperature rise somewhat this morning and may 
help to shave a few degrees off the max temps today. Still, it will 
be quite warm with daytime highs running some 4-9F above average. No 
forecast changes are in the works this morning.

Relief from the heat is on the way with a cold trough dropping south 
along the British Columbia Coast this morning. The trough will swing 
across CA on Monday with increasing westerly winds, allowing the 
marine layer to build inland. It will be noticeably cooler across 
the entire area with the cooler conditions prevailing all week as 
another trough drops south, reinforcing the onshore pattern over 
SoCal. Some mid and high level clouds are being drawn eastward from 
the Pacific and will decorate the sky into Tue morning before drier 
air arrives and limits most of any clouds to the marine variety.

The ocean sfc temps are currently in the lower 70sF, so the cooling 
will be somewhat muted, but still expect daytime temperatures to be 
running a little below average after today. Slight warming returns 
next weekend as the trough over the West weakens.


151500Z...Coast/Valleys...Remaining VIS restrictions and CIGS will 
lift from the coastal terminals by 17Z this AM, with VFR conditions 
then prevailing through midnight. SCT-BKN high clouds AOA 20,000 FT 
MSL will persist into Monday. Low clouds will redevelop near 08Z 
Monday on the coast, with bases 400-800 FT MSL and tops to 1,500 FT. 
These will spread 10-15 miles inland in San Diego and Orange 
counties. High confidence in VIS 1-3 SM in western fringe of 
valleys, and on elevated coastal terrain including KCRQ, 10-16Z Mon. 
Elsewhere, VIS 3-5 SM during early morning hours. Terminals clearing 
near 16Z, however patchy CIGS may linger along the beaches through 
the day Monday.

Valleys/Mountains/Deserts...SCT-BKN high clouds AOA 20,000 FT MSL 
with unrestricted VIS through Monday.


Patchy fog this morning with periods of visibility below 1 mile, 
locally down to 1/4 mile. Reduced visibilities will be possible 
again tonight, however should remain above 1 mile for the most part. 


Today will be another very warm, dry day with minimum RH falling to 
10-15% far inland. An afternoon sea breeze will bring gusts of 15 to 
20 MPH at times through early evening. This will result in elevated 
fire weather conditions this afternoon. Further increases in RH will 
occur on Monday as onshore flow strengthens and an upper-level 
trough moves across the State. 

Strong southwest winds will develop over the mts/deserts Monday 
afternoon through Monday night with gusts of 30 to locally 50 MPH on 
the desert slopes and near the passes. These winds will be 
accompanied by higher RH initially, but it will fall to around 15% 
or lower by Tuesday morning in some areas. Critical fire weather 
conditions may develop for a few hours on the ridges and desert 
slopes before the winds drop off.  


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