Scientific Forecaster Discussion
fxus66 ksgx 171614
Area forecast discussion
National Weather Service San Diego California
914 am PDT Tue Oct 17 2017
light offshore winds and high pressure aloft will result in
another very warm day today, with scattered high clouds. An upper
level disturbance may bring some sprinkles or light showers to the
mountains Wednesday, with a small chance of thunderstorms. A
deepening trough over the west will bring a cooling trend
Wednesday through Friday with the marine layer returning and
deepening. Expect night and morning clouds over the coast and
portions of the valleys. On Saturday the flow turns offshore as
high pressure builds. That will bring Santa Ana conditions, clear
skies and hot weather, especially Sunday and Monday.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Infrared satellite imagery this morning shows cold cloud tops and
thunderstorms associated with a short-wave trough offshore of
Point Conception, while mid and high level clouds can be seen
starting to stream into Southern California from the southwest.
The 12z Miramar sounding indicated a dry layer from the surface up
through 700 mb, a moist layer from about 660-580 mb, and then dry
air above that. There's no hint of a marine layer returning to the
coast yet, as dewpoints along the coast are only in the 30s-40s.
This is due to offshore pressure gradients continuing across the
region, with the San Diego to Tonopah gradient at 8.8 mb and the
San Diego to Las Vegas gradient at 3.8 mb. On water vapor
satellite, an upper level ridge can be seen over much of the
southwest. This ridge and the light offshore flow will result in
another warm day across Southern California today, with highs 5-15
degrees above normal. The ridge will start to get flattened and
pushed south tonight as a low pressure system from Alaska moves
south off the coast of British Columbia. This will result in
onshore flow returning and likely a return of the marine layer at
the coast. There could be dense fog along the coast late tonight
into Wednesday morning, but confidence is low at the moment as the
hrrr and other hi-res models do not show much.
On Wednesday, the aforementioned short-wave trough gets picked up
by the west flow aloft as the large scale trough off of British
Columbia deepens further. The short-wave trough will move across
Point Conception early Wednesday morning and then across San
Bernardino County mid Wednesday morning into Wednesday afternoon.
The bulk of the moisture and lift will be close to the short-
wave while the best elevated instability will be south into
Riverside and San Diego counties. With those parameters not really
lining up, confidence in any widespread elevated convection
occurring is low at the moment, with best chances for some
isolated thunderstorms being in San Bernardino County and the best
chances for any measurable rainfall being in the mountains where
the rain will not have to go through a deep dry layer.
A cooling trend will begin on Wednesday and continue through
Friday as the large-scale trough deepens further and moves inland
over the Pacific northwest. The coolest day will be Friday when
highs fall to 5-10 degrees below normal. Onshore flow will
strengthen as a result, with strong and gusty west winds in the
mountains and deserts (gusts 30-40 mph, local 50 mph). Finally,
the marine layer will deepen each day, eventually reaching around
3500-4000 ft msl on Friday. Expect night and morning marine layer
stratus clouds over the coast and portions of the valleys. There
may be just enough moisture and lift Friday morning for some
drizzle or isolate light showers over and west of the coastal
slopes of the mountains.
Surface high pressure building over the Great Basin will turn the
flow offshore starting Saturday which, in addition to a building
upper level ridge, will create significant drying and warming. It
will continue to get hotter Sunday and Monday as the ridge builds
further and becomes anomalously strong, with the European model (ecmwf) showing 500
mb heights of around 594 dm on Sunday and 596 dm on Monday.
Santa Ana winds will continue (right now looking to be around
moderate strength) as well, so expect it to get pretty hot even
all the way to the coast.
171615z...scattered clouds at/above 15000 ft mean sea level will prevail through Wed
with unrestricted vis. There is a small chance of thunderstorms Wed,
mainly over the mountains, likely with bases above 10000 ft mean sea level.
Stratus/fog is unlikely in coastal areas tonight or early Wed.
no hazardous marine weather is expected through Thursday. Winds will
increase to 20-25 kt in the outer coastal waters Friday, decreasing
a south southwest swell from 200 degrees will peak today-Wednesday
at 3-4 ft/16-18 seconds. This will produce 3-6 ft surf with sets to
7 ft north of del Mar. Highest surf will occur along southwest
facing beaches. Strong rip and longshore currents will accompany the
elevated surf. Swell and surf will lower Thursday. A beach hazard
statement is in effect for elevated surf and strong rip currents
very warm and dry weather (humidity once again down to 5-10
percent) will continue today, but with light winds. An upper
level disturbance may bring some sprinkles/light showers over San
Bernardino County tomorrow, with a small chance for isolated dry
lightning strikes. Increasingly cool and more humid Wednesday
through Friday as a trough moves in over the west. Gusty westerly
onshore winds in mountains and deserts Thursday and Friday, with
gusts 30-40 mph and local 50 mph.
Another round of moderate Santa Ana winds likely Saturday through
Monday, with day-time humidity falling down to at least 10
percent Sunday and Monday (with poor overnight recoveries) along
with hot conditions and gusty northeast winds. This would likely
result in critical fire weather conditions through and below the
mountain passes and into the coastal slopes and foothills Sunday
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
California...beach hazards statement through Wednesday evening for Orange
County coastal areas-San Diego County coastal areas.