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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 160129 AAA

Area Forecast Discussion
National Weather Service San Diego CA
627 PM PDT Sun Sep 15 2019

After another very warm day today, Monday and Tuesday will be 
noticeably cooler with more clouds 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 below 
average inland, but near late Summer normals along the Coast. Gusty 
westerly winds will peak over the mountains and deserts Monday 
night. This prevailing onshore pattern will rebuild the marine layer 
and associated low clouds and fog inland nights and mornings.



Evening Update:  

The forecast was updated to add a slight chance of dry lightning for 
Monday during the daytime for all areas. The threat is very low at 
about 20 percent or less, but nonetheless an incoming weak upper 
level disturbance currently over the Eastern Pacific could trigger 
sprinkles and a few low precipitation thunderstorms just about 
anywhere tomorrow. No other changes to the forecast at this time.

Previous Discussion issued at 114 PM PDT:

At 1 PM PDT...high clouds had thinned over the region but were 
extensive offshore and drifting NE. Lower clouds that were visible 
over the coastal waters were patchy. The sfc gradient readings we 
have are very weak, but trending onshore to the high deserts. 
Strongest wind reports were over SW San Bernardino County with peak 
south winds gusts of 25-35 MPH. 

The higher clouds are forecast to thicken and lower some on Monday 
as some moisture is drawn north from Hurricane Kiko ahead of a weak 
300 MB trough. A few sprinkles are possible, along with gusty and 
erratic winds if these showers form. Elevated instability predictors 
will be monitored for thunderstorm and dry lightning potential, but 
for now the chance looks too small for inclusion in the forecast. 
The prevalence of high cloudiness may inhibit much low cloud/fog 
formation again tonight, but at least patchy low clouds/fog are 

Higher confidence does exist for relief from the heat though, as an 
upper-level trough drops south along the British Columbia Coast 
today, and swings inland across CA on Monday with increasing SW to 
west 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.

The ocean sfc temps over the CA Bight are currently in the lower 
70sF, so the cooling power of the sea breeze 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.


151945Z...Coast/Valleys...VFR conditions will prevail through 
midnight. 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. SCT-BKN high 
clouds AOA 20,000 FT MSL will continue through Monday, and could 
inhibit some low cloud coverage tonight, resulting in intermittent 
and patchy CIGS. Moderate to high confidence in areas of VIS 1-3 SM 
along western fringe of valleys, and on elevated coastal terrain 
including KCRQ, between 10-16Z Monday. Elsewhere, VIS 3-5 SM during 
early morning hours. Low CIGS should dissipate near 16/16Z with VFR 
conditions prevailing thereafter, however variable SCT-BKN clouds 
with bases near 10,000 FT and tops to 20,000 FT can be expected 
throughout the day.  

Valleys/Mountains/Deserts...SCT high clouds AOA 15,000 FT MSL 
through Monday.  


Small patches of low clouds may reduce visibility to near 1 mile 
tonight, however confidence is low. Visibility is not expected to 
drop below 1 mile.  


It is another hot, dry day inland with minimum RH values of 15% or 
less far inland valleys/mts. 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. Further increases in RH 
will occur on Monday as onshore flow strengthens and an upper-level 
trough moves across the State. Some instability aloft may produce 
sprinkles with gusty and erratic winds at times. At this point the 
dry lightning threat looks to be 15% or less on Monday.

Winds have been increasing over southwest San Bernardino County 
today, with peak gusts around 35 MPH. These winds will become more 
widespread and turn westerly Monday and 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 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 midday Tuesday.


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