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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

                            
000
FXUS66 KSGX 160439 AAB
AFDSGX

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

.SYNOPSIS...
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.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

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 
expected.

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.

&&

.AVIATION...

160430Z...Coast/Valleys...Mostly BKN high clouds at/above 15000 ft 
MSL will prevail through Monday, decreasing Monday evening. There is 
a slight chance of TS with little or no rainfall Monday with CB 
bases above 10000 ft MSL. Patchy stratus/fog will occur near the 
coast 07Z-16Z Mon though confidence is low for occurrence/timing at 
any specific coastal airport, including KSAN and KSNA. Bases will 
likely be 600-800 ft MSL with tops to 1000 ft MSL with local vis 
below 3 miles. Stratus will more likely develop Mon evening.

Mountains/Deserts...Mostly BKN high clouds at/above 15000 ft MSL 
will prevail through Monday, decreasing Monday evening. There is a 
slight chance of TS with little or no rainfall Monday with CB bases 
above 10000 ft MSL. Vis will likely remain unrestricted.

&&

.MARINE...
There is a slight chance of cloud to water lightning Monday. 
Otherwise, no marine weather hazards are expected through Friday.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.
&&

$$

PUBLIC/FIRE WEATHER...10/JMB
AVIATION/MARINE...Maxwell