Weak to locally moderate northerly Santa Ana winds will develop overnight and continue into Sunday morning, turning easterly for Sunday night into Monday. Weaker offshore flow will follow through at least Wednesday for gradual warming and dry conditions. Confidence in the late week pattern is still uncertain with the potential for a low pressure system to bring some cooling and a return of onshore flow, or for high pressure and offshore flow maintaining warmer and dry weather.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Evening update, Surface pressure gradients are trending offshore but remain weak at this time, around -4.6 mb SAN-TPH. Wind gusts below the Cajon Pass are only around 25-35 mph at this time. Gradients and winds are forecast to strengthen overnight with winds spreading into the Inland Empire and Santa Ana Mountains. A Wind Advisory was issued earlier valid through 1 PM Sunday for the San Bernardino Mountains, Santa Ana Mountains, and Inland Empire, where gusts up to 45-55 mph are expected in the wind prone locations. Otherwise expect remaining low clouds to clear out overnight. It will be cool in the wind- sheltered inland valleys tonight with lows locally in the low to mid 30s.
Previous discussion, North winds weaken Sunday afternoon, then re-intensify out of the east Sunday night as the strong surface high (around 1042 mb) drops down into the southern Great Plains, thereby increasing our East/West pressure gradient. This gradient remains strong through at least Tuesday morning, yielding to periods of strong easterly wind gusts through the Banning Pass and Peninsular Range foothills around 30-45 mph during the early part of the weak.
Very dry air will accompany these Santa Ana winds, dropping humidity down to 20-30% tonight and ultimately down to 10% by Sunday where the winds blow. Morning temperatures will also be on the colder side for Southern California standards, with moderate chances for Sunday morning lows dipping below 35F in the interior valleys and good confidence for below freezing temperatures in the high desert.
Medium range ensembles show high confidence in the redevelopment of a ridge over the mountain West, with a center axis over eastern Washington, Oregon and California on Tuesday. High temperatures will only reach a few degrees above average - in the low 70s for the coastal basin - as high clouds will be passing through intermittently and as mid-level temperatures will remain cool, despite the ridge.
Wednesday, around 30% of WPC model ensemble cluster runs show a closed low developing in the Pacific and moving though Southern California. In all ensemble runs, this trough is weak and embedded within the greater stubborn West Coast ridge. High temperatures may cool slightly Thursday and Friday if this scenario plays out, though the chances for any precipitation are close to zero. Most ensemble clusters then agree that the ridge will re-intensify to a certain extent into next weekend, bringing temperatures back above average.
Weak offshore winds continue through the end of the week if the trough referenced earlier doesn't form, keeping relative humidity low for inland areas, while the coast experiences weak onshore flow.
250400z, Patches of BKN low level clouds along and west of coastal slopes will clear out of most areas by 06Z Sun. Otherwise, gusty north to northeast winds along coastal slopes, and downwind of the Cajon Pass into the Inland Empire through early Sunday afternoon. Gusts 25-40 kts along coastal slopes (locally 45 kts through the Cajon Pass) and 15-30 kts for portions of the Inland Empire. LLWS downwind of the Cajon Pass and VCTY KSNA possible through 12Z Sun.
No hazardous marine conditions are expected through Wednesday.
Ca, Wind Advisory until 1 PM PST Sunday for San Bernardino County Mountains-San Bernardino and Riverside County Valleys-The Inland Empire-Santa Ana Mountains and Foothills.
PZ, None.
