Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 250430

Area Forecast Discussion
National Weather Service San Diego CA
930 PM PDT Wed May 24 2017

Cooler weather is in the forecast through Friday as a weak trough
of low pressure moves through the region. Low clouds will spread
farther inland each night and morning as the marine layer deepens,
and strong and gusty winds will occur in the mountains and
deserts. The trough may create patchy drizzle in the coast and
valleys overnight into Thursday morning and scattered light
showers over and west of the mountains late Thursday night through
early Friday morning. High pressure aloft will rebuild over the
West this holiday weekend for warmer weather and a shallower
marine layer. Another cooling trend likely to begin next Tuesday
as another trough develops near Southern California.




It was another hot day today in the lower deserts with Thermal
tying the record high for the date of 109F degrees (previously in
1983)! Elsewhere, temperatures cooled as the onshore wind flow had
increased. Marine layer stratus has filled back in along the coast
and has quickly spread inland to the inland valleys. Patchy
drizzle is possible late. No significant changes were made to the
forecast. See previous discussion below for additional forecast

...Previous Discussion (Issued at 128 PM PDT Wed May 24 2017)...

Marine layer stratus has diminished for the most part this
afternoon according to visible satellite, with scattered alto-
cumulus clouds moving overhead from the southwest throughout
Southern California. These alto-cumulus are associated with a
short-wave trough, which can be seen on water vapor satellite
imagery moving northeast over Central California. A low can also
be seen far offshore of Central California, with a stronger trough
over British Columbia and a weakening ridge over Southern
California. The 12Z Miramar sounding shows an 11 deg C inversion
at around 1500 feet MSL. Onshore flow is strengthening quickly,
with San Diego to Las Vegas onshore pressure gradients up to 10.8
mb, whereas they were 2.2 mb at this time yesterday. Despite the
strengthening onshore flow and strong inversion, stratus managed
to clear at the beaches today. Today through early Thursday, the
trough over British Columbia will push south and start to pull the
low far offshore of Central California towards the California
coast while the aforementioned short-wave trough continues to move
northeast into the Great Basin. This increased troughing in
California will serve to further weaken the ridge over us and
continue to increase the onshore pressure gradients, resulting in
strong and gusty west winds along the mountain ridges, desert
slopes, through the passes and into adjacent desert areas
beginning late this afternoon. Wind gusts of 55 mph and local
gusts of 65 mph will be possible in and through the San Gorgonio
Pass and in the San Diego county deserts, where Wind Advisories
have been issued. The lowering 500 mb heights and onshore flow
will also deepen the marine layer and bring a cooling trend for
all areas today and Thursday, with highs dropping to several
degrees below normal on Thursday. Temperatures currently are 5-15
degrees below what they were 24 hours ago over much of the region.
Expect marine layer stratus to move completely fill the coast and
valleys tonight into Thursday morning with patchy drizzle

As the low moves into Southern California Thursday night into
Friday morning, it weakens into a short-wave and will bring
another burst of strong and gusty winds to the mountains/deserts
and further deepening of the marine layer, with stratus Thursday
night/Friday morning filling the coastal basin and moving into
the coastal slopes and passes. Large scale and hi-res models show
some very light precipitation being generated over and west of the
mountains late Thursday night into early Friday morning, resulting
in scattered 0.01-0.05 inch amounts.

The trough will move to our northeast by Friday afternoon, with
the cooler air mass resulting in highs 10-15 degrees below normal
Friday. For Saturday through Monday, an upper level ridge to the
west will move in and strengthen, creating a warming trend and a
shallower marine layer. Highs are likely to reach back to normal
to slightly above normal levels on Sunday and then 5-10 degrees
above normal on Monday. Expect night and morning marine layer
stratus over coastal areas and western valleys during this period.

For Tuesday through late next week, the GFS and ECMWF now show a
closed low (possibly of the cut-off variety) developing near or
south of Southern California. This would likely bring cooling.
However, models don`t show any precip associated with this low at
the moment.


250335Z...Coast/Valleys/Coastal Mountain Slopes...Stratus with bases
1500-2000 ft MSL will continue to fill in west of the mountains
tonight. Bases lifting to 2000-2500 ft by 16Z. Clearing expected to
occur 16-19Z, though coastal airports may remain BKN-OVC through
much of the afternoon. Clouds becoming BKN-OVC along/west of the
mountains after 26/02Z with bases near 2500 ft MSL and tops 4000-
5000 ft MSL. Coastal slopes of the mtns obscured in clouds/fog at
times through Thu night.

Mountains/Deserts...Southwest to west winds with sfc gusts 35-45 kt
along the desert slopes/foothills and through the passes continuing
through Thu night with MOD-STG UDDFS and pockets of LLWS over and
NE/E of the mountains.


No hazardous marine weather is expected through Monday.


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


CA...Wind Advisory until 8 AM PDT Friday for San Diego County Deserts-
     San Gorgonio Pass Near Banning.



PUBLIC...Gregoria (Update)/Harrison
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