Area Forecast Discussion
Issued by NWS San Diego, CA
FXUS66 KSGX 052138
Area Forecast Discussion
National Weather Service San Diego CA
138 PM PST Mon Dec 5 2016
Temperatures will cool slightly through Tuesday with prevailing
onshore flow over the region. Deepening marine layer stratus
along the coast may bring patchy drizzle tonight. Weak offshore
winds will develop Wednesday-Thursday, with temperatures
returning to near average readings by Friday.
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
At 130 PM, SoCal remained caught between the subtropical ridge over
the East Pacific and a broad longwave trough over the Western US.
This was producing northwest flow in the mid and upper levels, and
advecting elevated moisture (above 500 mb) over the region. As
such, skies were mostly cloudy over SoCal with cirrus streaming
overhead, though scattered low stratus was still hanging on along
the immediate coast and over the coastal waters.
A low amplitude trough will propagate through the Pacific NW
bring increasing onshore flow and promoting eddy development over
the SoCal Bight tonight. This will deepen the marine layer over
the valleys and coast tonight, potentially producing a few
patches of drizzle. Minus some thinning this evening, high clouds
will remain prevalent through tomorrow.
The aforementioned shortwave will shift rapidly eastward through
the Intermountain West tomorrow, as a cold 1046 mb surface high
builds southward over eastern WY/MT by Thursday. This will bring
breezy mountain/desert winds tomorrow and temperature falls,
especially over the mountains and Apple/Lucerne Valleys Wednesday.
Weak offshore winds will develop Wednesday-Thursday. These winds
should be limited to sub advisory levels by a lack of upper level
support and weak surface gradients due to a surface high (around
1024 mb) off the SoCal coast. Temperatures will return to near
average values by Friday.
Onshore flow will return over the weekend as several weak
shortwaves work their way through the Pacific NW and Northern CA.
Prevailing northwest flow aloft may bring a fading plume of
subtropical moisture and increasing high clouds to SoCal by
Sunday, but forcing appears to weak for any light precipitation
chances to be added to the forecast at this time.
052130Z...Coast/Valleys...Patchy stratus will occur along the coast
this afternoon with mostly SCT conditions, along with clouds
at/above 15000 ft MSL. BKN/OVC stratus will become widespread in
coastal and valley areas tonight with bases mostly 2000-2500 ft MSL
with tops to 3500 ft MSL with areas of higher terrain obscured and
slow clearing Tuesday.
Mountains/Deserts...Areas of clouds at/above 15000 ft MSL will
prevail through Tuesday. Local terrain obscurations will occur below
3500 ft MSL late tonight and Tuesday morning due to stratus/fog, but
otherwise visibilities will be unrestricted through Tuesday.
No hazardous marine weather is expected through Saturday.
No fire weather concerns through Tuesday with onshore flow and
increasing relative humidity. Weak offshore winds will bring
lower relative humidity values Wednesday-Thursday. However overall
declines in relative humidity look less pronounce than previously
forecast, with minimum relative humidity values only reaching 20-30%.
With this in mind, any elevated fire weather conditions will be
isolated during this period. Onshore flow will bring increasing
relative humidity Friday, effectively squashing any fire weather
concerns through the weekend.
Skywarn activation will not be needed today.