Slightly warmer and drier weather is expected tomorrow through Saturday, with a few showers possible over the mountains tomorrow afternoon. A more robust weather system will enter the area Sunday through the early part of next week. This will bring more widespread rainfall, mountain snow, and windier conditions. Cooler temperatures will prevail for much of next week.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
, New Aviation Discussion for 18Z TAFs,
Areas of low clouds continue over parts of the inland valleys this morning with bands of high clouds moving northeast across the region. Morning water vapor imagery shows a nearly stationary upper low over Central California and another low in the base of the long wave trough around 850 mi southwest of San Diego. The low in the base of the trough will continue to dig south today before progressing eastward on Friday. At the same time, the northern low within the trough will drop south into So Cal on Friday. For today, this will maintain cool but dry weather with highs near seasonal normals. As the trough axis moves across late tonight into Friday morning, there is a slight (15-20%) chance of showers, mainly over the Riverside and San Diego County Mountains and adjacent desert slopes. Weak offshore flow briefly develops behind the trough, providing a little warming. Transitory upper level ridging follows for Saturday with highs increasing to around 2 to 7 degrees above normal.
A large upper level low drops out of the Gulf of Alaska and towards the Northern CA coast this weekend. The initial trough is forecast to move inland across CA late Sunday through Tuesday morning. Ensembles are showing a weak Atmospheric River with this trough sometime Monday or Monday night when there is 80-90% chance of IVT of 250 kg/m/s or more, though there are still timing differences on when this will occur. Unlike previous AR events this winter, this storm will be much colder with snow levels somewhere between 5500 and 6500 ft during the heaviest precipitation. In terms of rainfall amounts for the 48 hour period from 4 AM Monday through 4 AM Wednesday, the NBM has a 75-95% chance of 0.50" or more for the mountains westward, a 30-50% chance over the high deserts, and a 15-30% chance over the low deserts. Chances of 1" or more are around 40-65% for the mountains westward, locally up to 95% on the south slopes of the San Bernardino Mountains, and a 5-10% chance in the deserts. For this same 48 hr period, there is around an 80% chance of 6" or more of snow above 6000 ft, and around a 40-50% chance down to 5000 ft. Chances of 12" or more are around 60% above 6000 ft and 25% down to 5000 ft. This period will be much cooler with highs around 5-10 degrees below normal. Gusty southwest to west winds are also expected across the coastal waters, mountains, and deserts with this system.
Additional troughs will move through into at least Thursday, though as time goes on the ensembles become slightly out of phase, leading to lower confidence on when additional rounds of precipitation will occur. In general, these subsequent troughs are currently forecast to have less moisture and lower precipitation accumulation associated with them compared to the early week system. The probability of precipitation fluctuates between 20-45% for Wednesday and Thursday, highest Wednesday morning and late Thursday. These systems are also colder, with a 15-35% chance of snow levels of 4000 ft or lower, with the highest probabilities across the San Bernardino Mountains. It is not out of the question that resort levels could see close to 2 feet of total snow accumulation by the end of the week.
121800, VFR conditions expected through this evening at all TAF sites. Patchy low clouds will move in along the coast, mainly in San Diego County, after 10Z. MVFR conditions will be intermittent with Cigs around 1,500-3,000ft MSL. Coastal low clouds are expected to clear around 16-18Z. Otherwise, intermittent high clouds AOA 20,000 ft.
No hazardous marine conditions are expected through Sunday. An approaching system will increase southerly winds and wave heights beginning late Sunday night into early Monday morning. This will likely generate hazardous winds and seas for early next week.
Ca, None. PZ, None.
