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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 171731

Area Forecast Discussion
National Weather Service San Diego CA
931 AM PST Mon Dec 17 2018

The tail end of a storm system moving across California will bring a 
few light showers along and west of the mountains through 
tonight, along with more clouds and cooler weather. A strong ridge
will build in midweek with fair and much warmer weather Wednesday
through Friday. The ridge will break down over the weekend 
leading to cooler, but continued dry conditions. High surf will 
pound the beaches today and Tuesday.




Some echoes on radar, but most of this is virga (rain not reaching
the ground). However, there have been some sprinkles and spotty
light showers. The only sites this morning that have received
measurable rain are: 0.02" at Poway and 0.01" at La Jolla, Del
Mar, and Lake Arrowhead. This trend will continue through tonight
with most areas dry. However, there is a slightly better chance of
a few showers this evening as the tail end of a frontal band, now
over Ventura County, moves southeastward and across the area. WRF
shows a few showers occurring along the coast tonight, across 
Orange County from about 7 PM-Midnight and along the SD coast 
10PM-4AM. This hi-res model shows that some of the showers could
produce 0.10" inches at a few places. No significant changes to
the forecast were made this morning.

...Previous Discussion (Issued at 211 AM PST Mon Dec 17 2018)...

High clouds have thickened once again over SoCal, along with patchy, 
lower marine clouds. At 2 AM PST...The closest precipitation reports 
were from Santa Barbara County. Surface pressure gradients to the 
deserts have been accelerating onshore. Wind reports have been 
increasing as well with peak gusts 30-40 MPH from the SW over the 
higher peaks. 

A trough will move across the State today, and out of the picture 
for Tuesday as a ridge builds east off the Pacific. Onshore flow 
will peak today as the trough passes, and help to build the marine 
layer inland. There may be a few light showers as this occurs, but 
any precipitation that manages to develop is expected to be 
scattered, brief, and very light. A few light showers are possible 
in western San Diego County overnight as well, but by sunup on 
Tuesday, the marine layer will begin to shrink as offshore trends 
return. Daytime temperatures will be lower than the past few days 
due to more clouds and the marine influence. 

By Wednesday...winds will be increasing offshore and may get gusty 
below favored passes and canyons as sfc high pressure gradually 
builds over the Great Basin. At the same time, a ridge will be 
building east, off the EastPac, setting the stage for well above 
average temperatures over SoCal through Fri under fair skies. The 
warmest day looks to be Thursday, followed by slow cooling into next 
weekend as the ridge deflates and drifts east. 

The global models show the main energy in the westerlies over the 
Pacific along or north of 40N for the next ten days. Short waves 
embedded in this flow go zipping by from time to time with little 
consequence over SoCal until about Tue of next week. The 00Z 
operational ECMWF run has taken the more aggressive stance with this 
wave, digging it sharply, while the GFS has been slower, weaker, and 
more inside. Due to the speed of the flow and uncertainty from run 
to run with this feature, confidence is low regarding just what 
affects it may have here and when. Given the track and moisture 
availability though, it does not look like a big precipitation 
producer here, even if the 00Z ECMWF solution were to verify. 


171700Z...Coast/Valleys...Variable mostly BKN clouds will prevail 
through this evening with most cigs 3000-6000 ft MSL layered to 
25000 ft MSL. Local vis 3-5 miles will continue through 18Z in 
fog/haze. A few light showers are possible, mainly 00Z to 05Z. After 
05Z tonight, areas of BKN-OVC clouds will prevail with bases 1000-
2000 ft MSL and tops to 3000 ft MSL.

Mountains/Deserts...Variable mostly BKN clouds will prevail through 
this evening with most cigs 4000-9000 ft MSL in the mountains and 
10000 ft MSL or higher in the deserts. Local mountain obscurations 
will occur in clouds/fog through early this evening. A few light 
showers are possible over the mountains, mainly 00Z to 05Z. After 
05Z tonight, clearing will occur with unrestricted vis.


A large long-period swell from the WNW will continue today and 
Tuesday. It will generate combined seas of 10-13 feet in the outer 
coastal waters, creating hazardous sea conditions near shoals. The 
inner waters could have some hazardous conditions as well, 
especially near jetties and the entrance to Mission Bay. Swell and 
seas will subside late Tuesday and Wednesday. The Small Craft 
Advisory for Hazardous Seas is in effect for the outer waters (30-60 
nm off the coast) through noon Tuesday and will be expanded to the 
inner waters (0-30 nm off the coast) too.


A High Surf Advisory remains in effect through Tuesday evening. A 
west northwest swell of 10-13 ft in the outer waters with a period 
of 18-20 sec from 290 degrees will impact the area today and 
Tuesday. Surf of 6-11 feet is likely with sets to 13 ft possible, 
highest south of Del Mar. Strong rip currents and dangerous swimming 
conditions at all beaches. Swell and surf will gradually lower on 
Wednesday, though another large swell could again bring elevated 
surf Thursday or Friday. 


Skywarn activation will not be needed. 


CA...High Surf Advisory until 10 PM PST Tuesday for Orange County 
     Coastal Areas-San Diego County Coastal Areas.

PZ...Small Craft Advisory for hazardous seas until noon PST Tuesday 
     for Waters from San Mateo point to the Mexican Border 
     Extending 30 to 60 nm out including San Clemente Island.



PUBLIC...Gregoria (Update)/10 (Prev Discussion)