Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service San Diego California
1145 am PDT Monday Apr 27 2015
Updated for extension of Wind Advisory until 2 PM. West/W/a section
a ridge of high pressure off the California coast will move inland
bringing a warming trend through Thursday. Surface high pressure
over the Great Basin will continue to produce locally strong and
gusty north to northeast winds below the Cajon Pass and near the
coastal foothills of the Santa Ana Mountains this morning. Onshore
flow will return during the middle of the week and continue into
next weekend...with a return of night and morning marine layer
clouds and fog to the coast...and eventually the valleys as well.
Cooling will develop near the coast on Thursday and spread inland
Friday into Sunday as a weak trough of low pressure approaches and
slowly moves inland.
Previous discussion issued at 857 am...for extreme southwestern
California including Orange...San Diego...western Riverside and
southwestern San Bernardino counties...
Moderate Santa Ana winds are occurring this morning...with north to
northeast wind gusts mainly in the 30s/40s through and below the
Cajon Pass...and up into the high 50s in the windiest spots of the
Santa Ana Mountains and foothills. In fact...Pleasants Peak in the
Santa Ana Mountains reached 61 miles per hour at 630 am. There are also some 30-
35 miles per hour wind gusts in the San Diego and Riverside County
mountains...and also parts of the Coachella Valley. Currently...the
offshore pressure gradient is 7.7 mb from San Diego to Tonopah. The
decent offshore pressure gradient and some upper level support in
the form of 40 knots winds at 850 mb...will continue to bring the
moderate Santa Ana winds through the morning hours today. The upper
level winds and pressure gradients slacken off pretty quickly this
afternoon...resulting in quickly diminishing surface winds.
Thus...the Wind Advisory will likely be allowed to expire at noon.
Water vapor imagery shows a large low pressure system over eastern
New Mexico...with a circulation large enough to help bring that
aforementioned upper level support for the offshore winds. A ridge
intruding into California is also evident on satellite. The ridge
and offshore winds will produce significantly warmer conditions
today in the coast and valley areas...with highs today 5-10 degrees
above normal in those areas. The ridge will continue to build in
over the region through Thursday...resulting in a continued warming
trend. By Wednesday and Thursday...day-time high temperatures are
likely to reach 5-10...and locally 15...degrees above normal for
most locations. Flow should become pre-dominantly onshore by
Wednesday night...which may bring the night and morning marine layer
clouds back to the coast by Thursday morning.
The ridge starts to break down on Friday as a weak trough develops
off the West Coast and slowly moves inland. This should produce
gradually cooler conditions...with a deepening of the marine
layer...and an increase in the onshore flow Friday through Sunday.
271530z...strong northeast wind is the issue across the forecast
area this morning into tonight. However the pressure gradient is not
strong enough to overcome the sea-breeze and northwest surface winds
will kick in at coastal airports around 271800z at 8 to 10 kts...
then subside shortly after 280000z. The areas of stronger winds and
directional wind shear will produce pockets of strong up/down drafts
and local low level wind shear over and west of the mountains. This will most likely
occur in the northern inland Empire and Orange County. VFR
conditions expected through Tuesday.
815 am...no hazardous marine weather conditions are expected through
surface high pressure over the Great Basin will continue to bring a
period of some strong and gusty north to northeast winds through the
passes and into the coastal foothills this morning...especially
below the Cajon Pass and near the Santa Ana Mountains and foothills.
The winds...combined with plummeting humidities...could bring a few
hours of near critical fire weather conditions this morning for
those areas. Minimum humidities this afternoon look to fall into the
single digits...but winds will already be diminishing by that time.
Poor to locally moderate humidity recoveries tonight...then
continued dry tomorrow with more warming and weak offshore winds.
Humidity will increase Wednesday through the rest of the week as
onshore flow returns. However...it will still be warm Wednesday and
Thursday for the valleys and deserts before temperatures start to
decrease Friday into the weekend.
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
California...Wind Advisory until 2 PM PDT this afternoon for Orange County
inland areas-San Bernardino County mountains-San Bernardino
and Riverside County valleys-the inland Empire-Santa Ana
Mountains and foothills.