Scientific Forecaster Discussion
fxus66 ksgx 241555
Area forecast discussion
National Weather Service San Diego California
855 am 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 day as the marine layer deepens, and strong
and gusty winds will occur in the mountains and deserts starting
late this afternoon. The trough may create scattered light showers
late Thursday night through early Friday morning over and west of
the mountains. 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 along the West Coast.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Marine layer stratus is covering the coastal waters, coast, inland
Orange County and western San Diego County valleys this morning
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 west of
Point Conception moving northeast into 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 mean sea level. Onshore flow is strengthening
quickly, with San Diego to Las Vegas onshore pressure gradients up
to 7.1 mb, whereas they were 0.9 mb at this time yesterday. With
the strengthening onshore flow and strong inversion, clearing of
stratus may be slower than usual this morning and the beaches may
not clear at all. 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 50 mph and local gusts of 60 mph will be possible in the
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. Expect
marine layer stratus to move completely fill the coast and valleys
tonight into Thursday morning.
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, models tend to show a trough
moving in along the West Coast, bringing a cooling trend. However,
the amount of cooling is uncertain due to model differences in the
strength of the trough.
241553z...coast/valleys...widespread low clouds with bases 500-1000
feet msl with tops to near 2000 feet mean sea level extending inland to just west
of kont and krnm. Areas of vis 3-5 sm over higher coastal terrain
and down to 1sm on coastal mesas/western valleys. Stratus clearing
back to near the coast midday. Risk of bkn low clouds at least part
of the afternoon at ksan and ksna is moderate so confidence of bkn
low clouds at times into the afternoon is moderate as well for
ksna and ksan.
Mountains/deserts...a few high clouds and unrestricted vis through
Thu morning. Onshore winds becoming strong after 00z this evening
with gusts through the mountains and along the desert slopes up to
45 kts along with MDT up/downdrafts...and patchy blowing dust/sand
in the deserts.
no hazardous marine weather is expected through Sunday.
Skywarn activation will not be needed today.
California...Wind Advisory from 5 PM this afternoon to 8 am PDT Friday for
San Diego County deserts-San Gorgonio Pass near Banning.