Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
925 PM PDT Tuesday Mar 31 2015
Synopsis... a series of low pressure troughs moving across
California will bring gusty west winds in the mountains and deserts
through Wednesday night. Marine layer clouds will spread into the
valleys each night. High pressure building aloft on Thursday will
bring warming along with gusty offshore winds to the mountains and
inland valleys through Friday. Over the weekend a low pressure
system will deepen along the West Coast bringing increasing onshore
flow and a cooling trend through early next week.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Onshore gradients have been gradually picking up over the last
several hours this evening. For example the San Diego to thermal
pressure gradient is now up to about 8 mb. Increase will continue
this evening and peak late tonight. Winds will gradually diminish
after daybreak Wednesday morning. The hi-res short fused models
including the NAM...hrrr...and local WRF all indicate the potential
will still exist for gusts 50-60 miles per hour in our most onshore wind-prone
locations...for example through the San Gorgonio Pass and the
northern edge of the Coachella Valley. A Wind Advisory continues for
that reason. Blowing sand and dust will also be possible in the
Coachella Valley and the deserts overnight. A replay of the gusty
winds will be possible on Thursday evening/Friday morning but at not
looking to be quite as strong. No changes were made to the ongoing
forecast database at this time.
..previous discussion (issued at 125 PM pdt)...
An upper level trough will move across the northern Great Basin this
evening bringing a period of gusty winds to the mountains and
deserts tonight. The strongest winds will be in the deserts
where winds will be ramping up this afternoon as surface pressures
fall over southern Nevada. Onshore gradients will be strongest
between 00z-09z when a 1006 mb surface low deepens near Las
Vegas. The strongest winds will surface in the high deserts and
on the eastern slopes of the mountains and into the Coachella Valley
and the San Diego deserts. Gusts from 45 to 55 miles per hour are expected in
these areas with the strongest winds in the San Gorgonio Pass.
The marine layer clouds that retreated off the coast this
afternoon will return this evening. The marine layer will be a
few hundred feet deeper tonight due to the stronger onshore flow and
a developing Catalina eddy. Low clouds and fog will spread deeper
into the valleys Wednesday morning.
A second trough will move inland across south-central California
Wednesday bringing another round of gusty winds late Wednesday
afternoon and evening...but the winds will not be quite as strong.
The trough will move east Thursday and be followed by weak ridging
along the West Coast through Friday morning. This pattern will set
up a brief Santa Ana wind event with gusty offshore winds
surfacing across the mountains and into the inland valleys.
Initially the winds on Thursday will be aligned with the north-south
pressure gradient and mostly impact the Cajon Pass...the inland
Empire and the Santa Ana Mountains. The winds will then
shift from the north to the northeast Thursday night into
Friday morning with gusty winds in the San Diego mountains
and valleys. Downslope compressional warming will send coastal and
valley highs in the 80s to around 90 on Friday. The marine layer
will be forced out to sea.
Saturday through Monday...a low pressure trough will deepen along
the West Coast for gradual cooling and a slow increase in the marine
010345z...coast/valleys...stratus with bases 1500-2000
feet mean sea level and tops to 2500 feet mean sea level moving into the coastal areas through
07z...spreading up to 35sm inland overnight. Local visible 2sm or less
where clouds near higher terrain in the valleys. Stratus and visible
restrictions clearing 15-18z. Stratus will return to the coastal
airports Wednesday evening with slightly higher bases and tops. Otherwise
few-scattered clouds at or above 20000 feet mean sea level and unrestricted visible through Wednesday
Deserts/mtns...few-sct clouds at or above 20000 feet mean sea level and unrestricted visible
through Wednesday evening. West winds 20-30 knots with gusts to 40 knots over
the mountain ridges...through and below passes/canyons...along desert
slopes and into adjacent desert foothills through about 15z. This
will result in moderate-strong uddfs/low level wind shear and possible rotors over and east
of the mountains winds will weaken Wednesday morning...increasing again after
855 PM...combined seas near 10 feet...steep waves...and
northwest wind gusts to 20 knots are possible on Thursday over the
outer waters as a trough moves inland north of the area. Forecast
confidence is moderate. Another large...but short period...northwest
swell may generate combined seas near 10 feet and steep waves for a
short time on Sunday as well.
a building short-period west-northwest swell...peaking
at 8 feet/11 seconds on Wednesday...will be reinforced by a 10
foot/11 second west-northwest swell arriving on Thursday. Although
the period on these swells will be somewhat short...the swell
heights will be large enough to bring at least 4-6 foot surf and an
increased risk of rip currents through Thursday.
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
California...Wind Advisory from 4 PM today to 3 am PDT Wednesday for
apple and Lucerne valleys-Coachella Valley-San Diego County
deserts-San Gorgonio Pass near Banning.