*Annual Rainfall is based on the California Water Year,|
which runs from October 1 through September 30.
FXUS66 KSGX 252159
Area Forecast Discussion
National Weather Service San Diego CA
200 PM PST Wed Nov 25 2020
Areas of low clouds will form over the coastal areas this evening
and spread into the valleys tonight ahead of a trough of low
pressure moving across the Great Basin. After the trough passes,
gusty Santa Ana winds will move into Southern California Thursday
and Friday. Skies will be mostly clear over the weekend with warm
days, cool nights and low humidity.
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
Skies were generally clear this afternoon. Pressure gradients were
trending onshore this afternoon ahead of a trough over the Pacific
Northwest that will move across the Great Basin Thursday. Marine
layer clouds will reform over the coastal waters this evening and
push into the coastal zones and western valleys overnight.
Skies will clear in the Inland Empire early Thursday morning as the
dry northerly flow kicks behind the passage of the trof axis. Skies
will clear more slowly in San Diego County as the dry offshore will
be slower to reach that area.
The pending Santa Ana winds event will be moderate in strength with
the strongest winds Thursday into Friday morning. Northerly winds
will surface in the San Bernardino County mountains late tonight,
then move down the slopes into the Inland Empire and the Santa Ana
mountains through midday. Strongest winds arrive late Thursday
afternoon and continue through early Friday morning in these areas.
The ECMWF ensemble projects 60 mph wind gusts at Fremont Canyon in
the foothills of the Santa Ana Mountains. Projected gusts at Ontario
is 32 mph with the NBM.
The winds arrive later in San Diego County in this northerly flow
event, reaching the San Diego County mountains and foothills
Thursday evening through Friday morning. The winds will not be as
strong as they will be farther north, with wind gusts from 40 to 50
mph in the mountains and foothills Friday morning. While the winds
will not be as strong, the ensembles show moderate strength easterly
winds persisting longer in San Diego County, continuing into
Temperatures: Northerly flow will draw in a cool High Desert airmass
into SoCal Thursday, so Thanksgiving will be a cool day in most
areas. The coldest areas will be the High Deserts (highs in the mid
50s) and the San Bernardino Mountains (highs 35-45). The warmest
areas will be inland Orange County where the offshore flow will push
highs into the low 70s. The Lower Deserts will also see highs in the
low 70s. Friday will be a warmer day in all areas, with the most
significant warming west of the mountains from the Santa Ana
downslope flow. Both Saturday and Sunday will be sunny and warmer.
Long Range Outlook for next week: There are differences in the
operational GFS and the ECMWF runs, which isn't unusual, so we have
sided with the ECMWF ensemble that project general ridging over the
Southwest. On Monday a shortwave moves across the Pacific Northwest
and then across the Great Basin Tuesday and Wednesday, with a high
amplitude ridge over the West Coast. This pattern suggests above
average chances of continued offshore flow over the Southwest
through the middle of next week.
252200Z...Coast/Valleys...Very isolated patches of low clouds will
occur across the immediate coast this afternoon. Mostly SCT
conditions with isolated BKN, bases 1500-1700 FT MSL. BKN-OVC to
develop after 03z Thu across the coast and then spread inland. Bases
2000-3000 FT MSL, extending 15-20 mi inland. FEW-SCT by 16-18z Thu.
Mountains and Deserts...Mostly clear with unrestricted vis through
this evening. BKN-OVC with bases 2000-2500 FT AGL to develop 08-10z
Thu and persist through 14-16z Thu. Breezy north to northeast winds
to develop after 16z Thu, with winds 15-20 kts and gusts to 30 kts.
.MARINE...Gusty northeast winds will develop through Thu
afternoon, with the highest winds Thu night through Fri morning.
Expect winds in excess of 20 kts at times. Winds will diminish below
20 kts Fri afternoon, but breezy conditions will persist at times
through Fri night.
The Fire Weather Watch was upgraded to a Red Flag Warning today. The
Red Flag Warning was shortened by a few hours due to the drop off in
winds earlier than previously thought. The RFW is in effect from 2
PM Thursday to 6 PM Friday for Orange, San Bernardino and Riverside
Counties. In San Diego County the RFW is in effect from 9 PM
Thursday to 9 AM Saturday.
Northerly winds will surface in the San Bernardino County mountains
tonight and then spread downslope into the Inland Empire and into
the Santa Ana mountains Thursday. The strongest winds will arrive
Thursday evening through Friday morning with gusts to 60 mph. This
will be the period of greatest concern for rapid fire spread of any
fires that do start. Weaker winds are forecast Saturday.
In San Diego County the northerly flow pattern will delay the onset
of the winds until late Thursday afternoon or evening, with a period
of gusty northeast winds Thursday night into Friday with gusts from
40-50 mph. Another round of slightly weaker winds follows on
Lowest humidity on Thursday afternoon will fall to 10-20% for
Orange, western Riverside, and southwestern San Bernardino Counties,
followed by lower RH of 8-12% Friday. This drier air will spread
south into San Diego County Thursday night and Friday.
Weaker winds may continue into Sunday and Monday. Little recovery
in humidity for inland areas is expected on Sunday. On Monday, a
recovery in humidity near the coast could begin to spread inland
for Monday and Tuesday.
Skywarn activation will not be needed Thursday.
CA...Red Flag Warning from 9 PM Thursday to 9 AM PST Saturday for San
Diego County Inland Valleys-San Diego County Mountains-
Including The Palomar And Descanso Ranger Districts of the
Cleveland National Forest.
Red Flag Warning from 2 PM Thursday to 6 PM PST Friday 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 Gorgonio Pass Near Banning-Santa Ana Mountains-
Including The Trabuco Ranger District of the Cleveland