Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 182040

Area Forecast Discussion
National Weather Service San Diego CA
140 PM PDT Wed Oct 18 2017

A deepening trough over the West will bring a cooling trend and a
deepening marine layer through Friday. Expect night and morning
marine layer clouds over the coast and western valleys tonight and
Thursday morning and then over the coast, valleys and coastal
slopes with possible drizzle Thursday night and Friday morning.
Locally strong and gusty west winds also likely in the mountains
and deserts Thursday and Friday. Santa Ana winds and a building
strong ridge of high pressure will bring significant warming and
drying to the region Saturday through Monday. Hot conditions
possible all the way to the coast Sunday and Monday, with weather
almost as hot on Tuesday.



Afternoon water vapor satellite imagery shows a weak short-wave
trough moving east over Southern Nevada/San Bernardino county with
a few high clouds over the deserts associated with this trough.
Humidity observations at the coast indicate dew points in the
50s/60s, which shows that the marine layer has finally returned.
The 12Z Miramar sounding shows a surface based marine layer
inversion and a moist layer from around 700 mb to 400 mb. Pressure
gradients continue to trend onshore this morning, with the San
Diego to Thermal gradient up to 4.5 mb onshore, while 24 hours ago
it was only 2.3 mb onshore. With the return of onshore flow and
large scale trough deepening over the Pacific Northwest,
temperatures at 1 pm are around 5-10 degrees lower than 24 hours
ago, although highs today will still be several degrees above
normal. Local WRF shows that marine layer stratus will likely
develop and deepen tonight in the coast and western valleys, with
help from a coastal eddy which appears to already be developing
according to coastal wind observations.

The cooling trend will continue through Friday as the trough
continues to deepen and move inland, with possibly enough lift and
moisture for some drizzle and/or light showers over and west of
the coastal slopes Friday morning. Expect highs 5-10 degrees
below normal by Friday. Onshore flow will strengthen as well, with
onshore pressure gradients forecast to reach around 10 mb from San
Diego to Las Vegas on Friday, so expect west winds gusting 35-45
mph with local gusts to 50 mph in the mountains and deserts
Thursday and Friday. The marine layer will continue to deepen as
well (up to 4000-4500 ft msl), with stratus filling the coastal
basin and likely moving into the passes and coastal slopes
Thursday night/Friday morning and again Friday night before
offshore winds clear clouds out from the north Saturday morning.

On Saturday, surface high pressure over the Great Basin will build
in the wake of the trough that moves over that area. This will
create offshore pressure gradients at around 7 mb from San Diego
to Tonopah, resulting in weak Santa Ana winds that day. A building
ridge from the southwest, in addition to the Santa Ana winds, will
result in significant warming that day in the coast and valleys,
as well as clearing out the marine layer. The ridge will will
continue to build over the region Sunday through Monday, becoming
anomalously strong by Monday with 500 mb heights reaching 596 dm
according to the ECMWF and Canadian models (GFS is weaker at
around 595 dm). Although they do not look strong at the moment
(offshore pressure gradients from San Diego to Tonopah increasing
to 10 mb with 850 mb winds of only 10-15 kt), the Santa Ana winds
will continue as well, bringing hot weather all the way to the
coast. The ECMWF also shows 850 mb temperatures reaching 24 deg C
on Monday, which translates to 101+ degree surface temperatures
west of the mountains. Once again, the GFS isn`t as warm, showing
850 mb temperatures around 23 deg C. Although we start to trend
back onshore on Tuesday, the ridge could be almost as strong
which means that inland areas may be almost as hot as they were on
Monday, and in the case of the mountains and deserts they may be
even hotter.


182000Z...FEW-SCT clouds above 15000 ft will continue this
afternoon, then mostly decrease tonight.

Near the coast, stratus will likely return this evening and continue
until Thursday morning. Bases will likely be somewhere between 700
and 1200 ft MSL, though cig height and timing are somewhat low
confidence. There is a chance of fog with visibilities below 3 miles
over higher coastal terrain and coastal valleys late tonight and
early Thursday.


No hazardous marine weather is expected through Thursday. Winds will
increase to 20-25 kt in the outer coastal waters late Thursday night
and Friday, decreasing by Saturday.


A south-southwest swell from 200 degrees from 3-4 ft/16 seconds will
continue this afternoon. This will produce 3-6 ft surf with sets to
7-8 ft over Orange County and northern San Diego County. Highest
surf will be on southwest facing beaches. Strong rip and longshore
currents will accompany the elevated surf. Swell and surf will lower
Thursday but some surf to 6 ft with strong rip currents will
continue. The Beach Hazard Statement continues for this reason until
5 pm Thursday.


Increasingly cool and more humid Today through Friday as a trough
moves in over the West. Gusty westerly onshore winds in mountains
and deserts Thursday and Friday, with gusts 30-40 mph and local
50 mph.

Weak to moderate strength Santa Ana winds likely Saturday through
at least Monday, with day-time humidity falling down to at or
below 10 percent Sunday and Monday (with very poor overnight
recoveries) along with hot conditions and gusty northeast winds
(15-25 mph gusting 30-40 mph and locally 50 mph). This would
likely result in critical fire weather conditions through and
below the mountain passes and into the coastal slopes and
foothills Sunday through Monday, and possibly into Tuesday as
well. Also, late Saturday there could be critical fire weather
conditions near the Cajon Pass.


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


CA...Beach Hazards Statement through Thursday afternoon for Orange
     County Coastal Areas-San Diego County Coastal Areas.




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