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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 182043

Area Forecast Discussion
National Weather Service San Diego CA
1242 PM PST Mon Feb 18 2019

After a few left-over rain and snow showers this afternoon and 
evening, the cold low pressure trough over the Southern California 
will move east on Tuesday with dry, but cool weather prevailing into 
Wednesday morning. Frost is likely in the colder valleys each night 
before more clouds and moisture arrives with the next trough later 
Wednesday. Rain and snow showers could return as soon as Wednesday 
afternoon, but will be more likely on Thursday into Friday morning. 
This system looks a bit colder with slightly lower snow levels. 
Heavy snowfall will again be possible in the higher mountains. Dry, 
with a warming trend next weekend.




Satellite imagery midday still shows an abundance of cumulus over 
southern San Diego County and mountain areas. Some instability is 
still present and it looks like we have one weak upper-level wave to 
drift south over us later today. The hires models are painting 
additional showers this afternoon and evening near the mts, so POPS 
have been reintroduced into the forecast. This will mostly be snow 
showers this evening, but a few could drift west of the mts with 
sprinkles or isolated showers. This activity should end late tonight 
as the trough slips farther to the east. 

Radar and satellite indications at noon are that showers are 
developing over the mountains, especially Riverside County, so this 
should continue into the evening for mts and adjacent desert slopes 
and some valley areas, especially San Diego County. Some additional, 
light snow accumulations are possible, mainly San Diego County, 
followed by clearing skies and light winds late tonight. More frost 
is expected the next couple of nights so Frost Advisories are in 
effect for the valleys.  

A broad trough covers western N.A. with ridging over the EastPac 
between 140W and 160W today. Shortwave energy will be exiting SoCal, 
but more is topping the ridge, over the Aleutians, and will drop 
south along the British Columbia Coast on Tue, carving out another 
cold low pressure center over CA/NV on Thursday. This will bring 
another surge of cold, continental air south with it and spark more 
showers over SoCal as early as Wednesday afternoon.

This storm system looks even colder than the last, with 1000-500 MB 
thickness values falling as low as 528-530 DM by Fri morning. So 
far, the ECMWF has taken the lead with this system and the latest GFS 
operational runs have been following suit. As is typical with these 
systems, there are numerous waves that move about the center and 
tracking them properly is the key to movement and ultimate weather 
generated. The leading shortwave arrives on Wednesday night, with a 
second system to follow on Thursday night. This is largely an 
"inside" system, but the periphery of the circulation does expand 
far enough west to grab some moisture off the Pacific to enhance 
precip over SoCal. 

Preliminary indications are that precip could be heavier than with 
the system that just moved through. Liquid equivalent QPF range form 
0.75 near the Coast, to just over 1.75 inches in the mountains. This 
one looks particularly good for the San Bernardino Mts to get heavy 
snowfall, although present QPF had to be modified upward to reflect 
this. No doubt we will need some winter weather products for this 
event soon. Besides the precip, it will be quite cold with daytime 
temps averaging around 15 degrees below average.

For next weekend...The flow over the Western Hemisphere becomes less 
amplified, allowing a moderation in temps over the Southwest. This 
should continue into next week with no precipitation expected.


182035Z...High confidence in VFR conditions today and tonight. 
Scattered to locally broken clouds 3000-7000 feet msl through the 
afternoon. Moderate confidence on these clouds lingering this 
evening. Skies slowly clearing late tonight with northerly winds in 
the foothills developing and gusting up to 30 kt after 07Z Tuesday. 
VFR conditions expected to continue through Tuesday. 


Strong northwest winds will create conditions hazardous for small 
craft today and tonight. No hazardous conditions are expected 
Tuesday through Wednesday morning. Strong northwest winds and large 
swell will develop late Wednesday as another storm system arrives. 
Hazardous winds and seas are expected Wednesday night through 
Thursday night.


Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions. Snowfall
reports are especially appreciated today.


CA...Frost Advisory from 10 PM this evening to 9 AM PST Tuesday for 
     San Bernardino and Riverside County Valleys-The Inland 
     Empire-San Diego County Valleys.

     Frost Advisory from 10 PM Tuesday to 9 AM PST Wednesday for San 
     Bernardino and Riverside County Valleys-The Inland Empire-
     San Diego County Valleys.

PZ...Small Craft Advisory until 2 AM PST Tuesday for Coastal Waters 
     from San Mateo Point to the Mexican Border and out to 30 nm-
     Waters from San Mateo point to the Mexican Border Extending 
     30 to 60 nm out including San Clemente Island.