Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 201638

Area Forecast Discussion
National Weather Service San Diego CA
938 AM PDT Wed Sep 20 2017

A deep low pressure trough will bring cooler weather through
Friday, along with mostly cloudy nights and partly cloudy days.
There is a slight chance of showers Thursday. Strong westerly
winds are expected in the mountains and deserts especially
Thursday. Higher pressure and offshore flow develop over the
weekend and next week leading to clear skies and warmer weather.




Coastal eddy evident in satellite imagery and this really has the
coast to inland valleys socked in with a stratus deck. Some sun is
shining through in Orange County. Clouds will likely hang tough
especially along the San Diego coast and valleys, though partial
clearing is possible for a few hours this afternoon before status
re-forms this evening. Winds will be picking up in the mountains
and deserts by tomorrow and will be assessing whether a Wind
Advisory may be needed for these areas for tomorrow and tomorrow
night with latest guidance coming in.

.Previous Discussion (Issued at 218 AM PDT Wed Sep 20 2017)...

Low clouds have settled into most of the coastal basin this
morning, but curiously the massive cloud deck north and west of
our region has disappeared. This is probably in response to the
cooler air aloft that has begun to erode our marine inversion,
which makes it hard for low clouds to form. Clouds will generally
break up today, but could persist in some areas. Clouds will
gather quickly this evening. An unseasonably deep trough will
drop out of Washington down the West Coast today and Thursday.
This will increase the onshore flow and strengthen our coastal
eddy. There is a weak front-like feature that pushes through
Southern California Thursday late morning. It looks capable of
producing light showers anywhere from the coast to the mountain
crests. Some foothill locations might get more than a tenth inch,
but only a hundredth or two is most likely in the lower
elevations. Winds will crank up as well in the mountains and
deserts Thursday afternoon and evening. Top gusts in the top
spots, such as through mountain passes and adjacently downwind,
will exceed 50 mph, but most areas will stay under 45 mph.
Temperatures will be mostly in the 70s, but 80s in the lower
desert Thursday and Friday. That`s 15-20 degrees below average for
inland areas. On Friday the marine layer clouds will not be as
solid, and even less so into Saturday. Most clouds on Friday will
migrate inland and into foothills. Mother Nature is monitoring the
calendar as we head into fall and has appropriately ordered up
some fall weather with an offshore flavor. As the trough axis
moves through California Friday and Saturday, dry northerly flow
aloft develops and surface pressure gradients align into the
offshore setup. That should take care of any residual clouds this
weekend. Northeast winds will develop Sunday through and below
the mountain passes into the inland valleys, mainly below Cajon
Pass. Winds don`t appear overly strong, although top spots around
Rialto could gust to 40 mph during the mornings through Tuesday.
Skies will turn crystal clear and humidity will go way down.
Temperatures will climb each day next week, into the 80s and lower
90s at lower elevations by Tuesday. The warmup will be most
notable west of the mountains as would be expected with offshore
flow, but the dry air and the cool nights will make the warm
weather fairly comfortable.


201530Z...Coast/Valleys/Coastal Slopes... BKN-OVC025-035 Tops 045
feet through 18Z. Stratus will be slow to clear today due to strong
coastal eddy that will bring south winds to the coastal airports.
Some areas may only partially clear this afternoon.

From 21/0000-1500Z, BKN-OVC025-035 Tops 045 with possible -DZ/-SHRA
and mountain obscuration of coastal slopes.

Mountain Ridges/Desert Slopes/Deserts...Mostly SKC with P6SM vis
through tonight. After 21/0000 UTC, west winds 20-30 kt with gusts
40 kt will create mod-stg uddfs/llws. Surface winds at KPSP and KTRM
could gust to 30 kt at times.


Northwest wind gusts increasing to 25 kt Thursday afternoon through
Friday. This, in addition a steep west-northwest swell of 6-8 ft/7-9
seconds, will likely result in hazardous conditions for small craft
in the Outer Waters. Northwest winds will weaken this weekend with
seas falling to 2-4 ft.


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




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