Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 171548
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
848 AM PDT Wed May 17 2017

.SYNOPSIS...
Slight warming today but still cooler than normal as a low
pressure system moves through the Great Basin. This low will
continue to create strong west winds in the mountains and deserts
today before winds turn weakly offshore tonight. Marine layer
clouds over and west of the mountains today should at least
partially clear this afternoon before filling back into the
coastal and valley areas tonight into Thursday morning. Warming
for Thursday through the weekend under building high pressure
aloft and weak offshore flow, with night and morning low clouds
and fog possible at or near the coast. Likely cooler for the first
half of next week as lower pressure develops over California.

&&

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

Morning visible satellite imagery shows marine layer stratus
covering most of the coastal basin, the coastal slopes of the
mountains, and the mountain passes, while mostly clear skies exist
in the deserts. A coastal eddy developed overnight, which helped
to bring in the stratus clouds. However, local WRF model shows the
eddy dissipating by this afternoon with partial-to-good clearing
of stratus clouds. Water vapor satellite shows a closed low
moving over the Great Basin, with northwest flow aloft over our
region and a ridge over the northeast Pacific. Looking at the 12Z
Miramar sounding from this morning and comparing it with
yesterday`s 12Z sounding, a few degrees of warming throughout most
of the profile is indicated. The air mass has been warming due to
yesterday`s trough having moved into the Great Plains and
replaced with higher heights. Thus, highs today should be about
3-7 degrees higher than yesterday for the most part, although they
will still be about 5-15 degrees below normal for this time of
year. The low over the Great Basin has created stronger onshore
pressure gradients and stronger winds aloft as well, which is
resulting in strong and gusty west winds in the mountain ridges,
desert slopes, through/below the passes, and into adjacent desert
areas. Latest San Diego to Las Vegas onshore pressure gradient is
up to 11.7 mb which is strong. Winds are currently gusting in the
30s/40s in those areas, with White Water the peak gust (60 mph).
Winds will continue strong in those areas through about mid-
evening before winds start to die down as gradients turn offshore.
A Wind Advisory remains in effect for those areas through 9 PM.

Local WRF Model shows a coastal eddy re-developing tonight, which
will help to bring marine layer stratus clouds into the coast and
valleys, but not quite as far inland as this morning due to
developing weak offshore flow. The offshore flow and slightly
higher 500 mb heights will create additional warming for Thursday
of several degrees. The ridge to the west will continue to build
into the region through Sunday, with offshore flow turning onshore
by late Saturday. The result will be a continued warming trend
through Sunday for all areas, except the coast will likely cool
slightly on Sunday due to the return of onshore flow. Highs look
to be 5-10 degrees above normal on Sunday, with highs in the low
90s in the Inland Empire and High Deserts, low 100s in the lower
deserts, upper 80s in the San Diego county valleys, mid-to-upper
70s at the coast, upper 70s/80s in the lower elevations of the
mountains, and 50s/60s in the higher elevations of the mountains.
The marine layer will also become much shallower, with night and
morning low clouds/fog mainly along/near the coast.

Confidence in the temperature forecast decreases for the first
half of next work week, as the models differ regarding the
strength of a northeast-to-southwest oriented trough that develops
over California. Chances are that there will be a cooling trend
starting Monday but just how much remains to be seen.

&&

.AVIATION...
171520Z...Coast/Valleys/Coastal Mountain Slopes...Low clouds with
bases 1800-3000 ft MSL and tops to 4000 ft MSL, will likely clear
coastal airfields by midday, but mainly BKN clouds much of the day
coastal slopes and locally into the valleys. VIS of 1 SM or so will
occur where low clouds interact with higher terrain. Expect variable
midday clearing at the coast with most areas SCT by about 20Z-22Z.
Risk of low clouds remaining at KSAN during the mid afternoon is
low, but an eddy circulation may still affect KSAN, so confidence in
SCT clouds at KSAN in the TAF for mid afternoon is moderate

Desert Mountain Slopes/Deserts...West winds with gusts of 35-45 kt
will result in areas of mdt to strong up/downdrafts and wind shear
over and east of the mountains through this evening. Otherwise FEW
clouds AOA 20000 FT MSL and unrestricted vis through early evening.
Areas of blowing dust and sand possible in the deserts this
afternoon and evening with locally reduced visibilities.

&&

.MARINE...
Northwest winds with occasional gusts of 25 kt in the outer waters
this afternoon and evening. No other hazardous marine weather is
expected through Saturday.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory until 9 PM PDT this evening for Apple and Lucerne
     Valleys-Coachella Valley-Riverside County Mountains-San
     Bernardino County Mountains-San Diego County Deserts-San
     Diego County Mountains-San Gorgonio Pass Near Banning.

PZ...Small Craft Advisory for winds from 11 AM this morning to 10 PM
     PDT this evening for Waters from San Mateo point to the
     Mexican Border Extending 30 to 60 nm out including San
     Clemente Island.

&&

$$

PUBLIC...Harrison
AVIATION/MARINE...Small



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