Patchy fog may develop tonight into Monday morning along the coast, with the potential for locally dense fog on the coastal mesas. Low clouds and fog will continue most nights and mornings this week, but the marine layer will become deeper, lessening the chances for dense fog. Temperatures will fluctuate a few degrees each day through mid- week, but mostly stay above average for this time of year. Pattern change expected for the end of next week, with cooler, windier, and wetter conditions expected.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
.Evening Update,
Current satellite imagery shows widespread high clouds over Southern California this evening with more cloud cover upstream. This may hinder low cloud development for coastal areas tonight despite increasing onshore flow. The latest model guidance has backed off on both the probability and coverage of low clouds. If low clouds manage to develop, patchy fog will be possible along elevated coastal terrain.
Otherwise, the forecast is still on track with increasing onshore flow expected through the workweek ahead of a weekend storm system. Gefs and ECMWF ensembles continue to disagree on timing of the heaviest precip as described in the previous discussion. Otherwise, the only notable change is a continued trend toward slightly wetter solutions. Nbm probability of exceeding 0.5" of precipitation is now around 30-40% for most of the coastal basin.
.Previous Discussion (1215 PM Sunday),
Periods of high cloud coverage are expected to continue into Monday. The pressure gradient is trending more onshore compared to noon yesterday with the pressure gradient -1.1 mb from SAN-DAG. The trend is expected to continue to skew more onshore, which may allow for patchy low clouds and fog to develop tonight into Monday morning. One potential hiccup to fog development could be how widespread the high cloud coverage is. More widespread high clouds will lessen fog chances and less will increase them. If fog develops, it may be locally dense. The most likely place for dense fog to develop is near higher coastal terrain. Low clouds and fog are expected to remain present each night and morning for much of this week but a deepening marine layer will limit chances for coastal dense fog.
Highs on Monday are expected to be a few degrees cooler across the area due to a passing shortwave trough. Breezy westerly winds may develop over the mountains and locally into the deserts, but no impacts expected. Weak ridging will build over California on Tuesday, which will bring a degree or two of warming to some inland locations. The ridge will quickly be pushed aside on Wednesday as stronger troughing sets up over the Eastern Pacific.
There remains quite a bit of uncertainty in the ensemble guidance in regards to the timing of the low's passage and expected precipitation amounts. Overall, timing of the onset of precipitation has trended later, but members of the GEFS are still showing a quicker passages of the low with the bulk of the precipitation centered on Friday/Saturday, while the majority of the ensemble members of the ECMWF are showing a slower progression and the bulk of the precipitation centered Saturday/Sunday. Both of the global models have quite a bit of spread in precipitation amounts as well. Nbm probabilities of 48 hour rainfall totals (5 am Friday-5am Sunday) exceeding 0.50" are highest in the mountains at 50-60 percent and around 20 percent for the coast and valleys. The High Desert has about a 10 percent chance with less than 5 percent chance for the low desert. In addition to timing of rainfall, the forward progression of the low will influence when the strongest winds and coldest conditions will occur. Some snow is expected out of this system but there still remains considerable uncertainty in snow levels. The spread in snow level guidance from the NBM ranges from 7000-9000 ft on Friday and 6500-7500 ft on Saturday and Sunday.
060415z, Generally VFR with BKN-OVC clouds AOA 15k ft through tonight. Confidence continues to increase on clearer skies for Monday morning for coastal TAF sites. If any low clouds 1000-1600 ft MSL make it to coastal areas, this would occur between 12-17Z Mon. Highest chance for low clouds is across San Diego County. Vis restrictions 2-5 SM over higher coastal terrain if clouds materialize. High clouds decreasing throughout the day on Monday.
No hazardous marine conditions expected through Wednesday. Winds and seas may increase by Thursday into later in the week due to an incoming weather system with low confidence in forecast details.
Ca, None. PZ, None.
