Inland areas will warm this weekend while coastal areas begin to cool on Sunday as onshore flow returns. Onshore flow will strengthen and spread cooling inland on Monday as a low pressure system moving into California brings cooler and breezy conditions for the middle of next week, along with a slight chance of precipitation Tuesday afternoon into Wednesday morning. Cool weather and breezy onshore flow persist for the remainder of the week.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Hazy sunshine this afternoon under a ridge of high pressure, with variable high clouds ahead of a weak upper level trough approaching from the west. Temperatures are close to what they were yesterday in the coastal areas while inland they are as much as 10-12 degrees higher. Surface pressure gradients remain weakly offshore and helped to prevent low clouds from spreading inland last night. In the mountain passes and foothills, northeast to east winds are still occasionally gusting 25-35 mph but will continue to weaken while a sea breeze is developing in the coastal areas and valleys.
The high pressure ridge will shift eastward over the next several days as a vigorous low pressure system moves in from the northwest. Temperatures will generally remain several degrees above seasonal average through Monday as we remain in the warm sector ahead of the approaching low pressure system. Highs will be in the low to mid 70s along the coast, upper 70s to mid 80s for inland coastal areas and valleys, upper 50s to upper 60s in the mountains, mid to upper 70s in the high desert, and upper 80s to around 90 in the low desert. A return of onshore flow will spread cooling inland west of the mountains, and bring more widespread low clouds along the coast tonight into Sunday morning.
Onshore flow strengthens Monday through Wednesday as the low pressure system moves inland through California. Gradual cooling takes place through this period, with highs decreasing to around 3 to 8 degrees below normal by Wednesday. Wednesday will likely be the coolest day as the upper level trough axis moves inland and colder air filters into SoCal from the northwest. Gusty southwest to west winds develop over the mountains, deserts, and through the passes each afternoon and evening, peaking on Tuesday as the closed upper low moves inland to our north. Wind gusts around 30-45 mph will occur along the desert mountain slopes, below the passes, and into the deserts Tuesday, with locally higher gusts below the San Gorgonio Pass.
The passage of the upper level trough will bring a slight chance of showers sometime late Tuesday into Wednesday. Forecast details are still somewhat uncertain but most likely we will be on the southern periphery of the moisture with limited potential for precipitation. The National Blend of Models probabilities of measurable rain (at least 0.01") for the 24 hr period ending at 5 am Wednesday range from around 20% for Orange County/far west Inland Empire and the mountains to 10-15% for the remainder of the Inland Empire, high desert, and San Diego County coasts and valleys, and less than 5% in the low desert. Chances of at least 0.10" is around 15%-20% along the south slopes of the San Bernardino Mountains and less than 10% elsewhere. A second short wave moves through sometime late Wednesday, but is currently forecast to be to dry and unlikely to produce any precipitation.
We will remain under broad troughing aloft through the end of next week, continuing the cooler weather with high temperatures a few degrees below normal and breezy conditions in the mountains and deserts each afternoon. The marine layer will also be fairly deep during this time, but the marine inversion will likely be so weak that low cloud coverage will be patchy in the coastal areas and valleys once the main trough axis passes on Wednesday.
190600z, Mostly clear skies and VFR conditions continue this evening, with low clouds lurking about 50 miles southwest of San Diego at 10 PM through this evening. These low clouds will gradually spread northward over the next several hours, with higher confidence to cover areas of coastal San Diego County between 09-16z. CIGs near 1300-2000ft MSL with a ~70% chance of BKN conditions at VCTY KSAN after 10z.
Increasing westerly to southwesterly winds across the desert slopes into the deserts Sunday afternoon/evening near 20-30 kts.
No hazardous marine conditions are expected through Tuesday. Elevated onshore (northwesterly) wind gusts near 20-25 knots are possible Wednesday into Thursday.
Ca, None. PZ, None.
