Very patchy fog may develop this Monday morning along the coast. Low clouds and fog will become more widespread each night and morning through the week as the marine layer deepens. Temperatures will fluctuate a few degrees each day through mid- week, remaining around 5 to 10 degrees above normal. Much cooler with gusty onshore winds and chances of precipitation late in the week.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Widespread high clouds continue to move across the region this morning. With the thick high cloud cover, coastal fog has struggled to develop. Expect any fog this morning to remain sparse. A weak short wave trough moves through today, bringing widespread cooling for the mountains westward with little change in the deserts. Weak transitory ridging then follows for Tuesday and Wednesday. Overall expect minimal day-to-day changes through Wednesday with highs around 5-10 degrees above normal each day. Once high clouds move out tonight, marine layer low clouds and fog will have a better chance of developing across the coastal areas. With persistent onshore flow the next few days, the marine layer will become more established with high clouds spreading into the coastal areas and far western valleys each night.
Ensembles have come into much better agreement with regards to the progression of an upper level low, which is currently situated well off the coast of Northern CA. This low will slowly approach the coast Thursday and Friday, bringing more noticeable cooling and high temperatures returning to near normal. The approach of this low will also generate gusty southwest to west winds in the mountains and deserts, as well as a deeper marine layer with low clouds extending into the valleys each night. The GEFS remains on the faster end of the eastward progression of the low, bringing precipitation as early as Friday morning. However, this only accounts for about 18% of the total ensemble space. Around 50% have precipitation by Friday afternoon, but all are showing the heaviest precipitation occurring Saturday morning through Saturday evening with light showers lingering into Sunday.
There is still a lot of spread in precipitation amounts, though in general ensembles have been trending slightly wetter. For the 48 hour period from 4 AM Friday - 4 AM Sunday, the probability of 0.25" or more has increased to 50-70% for the coast to the mountains, 20-25% for the high desert, and 5-10% for the low desert. Chances of 0.50" or more range from 20-40% for the coasts and valleys, 40-60% for the coastal mountain slopes, 10% for the high desert, and less than 10% for the low desert. Instability increases Saturday afternoon as the cold core moves overhead, bringing a 15% chance of thunderstorms. Any thunderstorms that develop could produce locally heavier rain.
The upper low moves out by late Sunday and weak troughing remains over the Western US into Monday, maintaining cool weather with highs a few degrees below normal.
060930z, BKN high clouds above 15000 feet MSL through 16Z, becoming mostly clear thereafter. 20% chance coastal low clouds 12-17Z; any cigs would be patchy and intermittent, and around 500-1000 feet MSL with vis 2-5SM on higher coastal terrain. Coastal low clouds more likely after 05Z tonight into Tuesday.
No hazardous marine conditions are expected through Friday. Wind gusts in the outer waters could exceed 20 knots Wednesday night.
Ca, None. PZ, None.
