Scientific Forecaster Discussion
fxus66 ksgx 221530
Area forecast discussion
National Weather Service San Diego California
830 am PDT Tue Oct 22 2019
hot and very dry conditions will continue today with gusty northeast
winds near the coastal slopes of the mountains. It will not be as
hot on Wednesday as the easterly wind weakens and turns back onshore
briefly. Stronger offshore flow is expected again late Wednesday
night through Friday morning, with more hot and very dry conditions.
A large trough over the western states will bring cooling for Sunday
into early next week with a slight chance showers along and west of
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
It was clear over the forecast area this morning. Easterly winds
were gusty along the coastal slopes and below passes with peak gusts
mostly in the 25-35 miles per hour range in the offshore, wind-prone spots. The
sfc pressure gradient to ksan was near 10 mbs from SW Nevada but about
neutral from the lower deserts. Where winds were mixing the
atmosphere, temperatures were in the upper 60s and 70s, with some
cool spots closer to 50f. The 12z Miramar sounding had a sfc-based
inversion with NE winds between 10 and 20 kts from near sfc to 20k
feet. A heat advisory is in effect for the coast/valley areas today.
No forecast changes this morning.
From previous discussion...
High pressure aloft and weak offshore flow will bring additional
warming today, especially for coastal and valley areas. Higher
terrain in coastal areas, inland Orange County, and much of the
San Diego County valleys will warm into the mid 90s to 102 today
with a heat advisory in effect from 11 am to 5 PM today. Gusty
east to northeast winds will continue along the coastal slopes of
the mountains into adjacent foothill areas and below passes and
canyons this morning with strongest gusts to around 40 mph. More
localized east to northeast winds will continue through tonight
into Wednesday morning.
With weakening of high pressure aloft and offshore flow, there
should be at least slight cooling on Wednesday for coastal and
valley areas with 5 to locally 10 degrees of cooling in some
inland coastal and western valley areas.
Another round of stronger offshore flow is expected to develop
late Wednesday night and peak Thursday, with offshore flow
continuing for Thursday night into Friday. There remain
differences among the global models with how much and how quickly
the winds will weaken on Friday. The GFS and its ensemble members
are faster with the weakening of the offshore flow with the
current forecast leaning toward the models that are slower with
the weakening. The offshore flow will weaken further on Saturday
with cooling beginning to spread inland as weak onshore flow
returns near the coast.
For Sunday into early next week, high pressure over the eastern
Pacific will amplify with a downstream trough of low pressure
over the western into the central states. This will bring stronger
onshore flow on Sunday with greater cooling spreading inland.
There remains a decent amount of spread in the details of the
trough of low pressure over the western states for early next
week. For Southern California, it should be cooler. But beyond
the cooling, the range of solutions is anywhere from dry and
breezy to breezy with a chance of showers along and west of the
mountains. The current forecast mostly uses the nbm pops for
Monday and Tuesday to account for the uncertainty with a slight
chance of showers along and west of the mountains.
221500z...local northeast wind gusts to 25 kt will continue through
late morning, mainly in foothills, below passes and canyons, and
locally into the inland Empire. Low level wind shear and moderate up/downdrafts are
possible over and west/southwest of the mountains. Otherwise, clear
skies will continue through tonight.
no hazardous marine weather is expected through Saturday.
..Fire Weather Watch late Wednesday night through late Friday...
Weak to locally moderate offshore flow will continue through about
midday below the coastal slopes of the mountains/passes, then weaken
this afternoon, but relative humidity will be around 10 percent. Strongest winds
this morning will be around 25 miles per hour with local gusts 35-40 miles per hour.
Weaker winds on Wednesday will turn back onshore west of the
mountains in the afternoon, setting up a modest recovery of relative humidity
over coastal and western valley areas.
Another round of stronger offshore flow is expected late Wednesday
through Friday morning. Northeast winds 20-30 miles per hour with gusts around
50 miles per hour should be underway over northern, wind-prone areas areas like
the Cajon Pass early Thursday morning, but will increase over all
the coastal slope/foothill areas through the day. Minimum relative humidity of 5-
10% looks likely over most of the area. The greater area of higher
wind speeds and lower relative humidity should result in extended periods of red
flag conditions both Thu/Fri, prompting the Fire Weather Watch.
The winds will weaken Friday afternoon but very low, single-digit relative humidity
is likely to continue into Saturday in many areas. By Saturday
afternoon the recovery will begin as onshore flow increases,
spreading higher relative humidity inland through Sunday.
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
California...Fire Weather Watch from late Wednesday night through Friday
evening for Orange County inland areas-Riverside County
mountains-including the San Jacinto Ranger district of the
San Bernardino National Forest-San Bernardino County
mountains-including the mountain top and front country
Ranger districts of the San Bernardino National Forest-San
Bernardino and Riverside County valleys -the inland Empire-
San Diego County inland valleys-San Diego County mountains-
including the Palomar and Descanso Ranger districts of the
Cleveland National Forest-San Gorgonio Pass near Banning-
Santa Ana Mountains-including the Trabuco Ranger district
of the Cleveland National Forest.
Heat advisory from 10 am this morning to 5 PM PDT this afternoon
for Orange County coastal areas-Orange County inland areas-
San Diego County coastal areas-San Diego County valleys.