Widespread mid and high clouds today will clear from the west overnight. 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 for Friday through the weekend.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
An upper level disturbance brought widespread mid and high clouds to the region today some of which produced rain. Most of the rain evaporated in the dry sub-cloud layer before reaching the ground, although a trace of rain reached the ground in a few places. The disturbance also weakened the marine layer inversion and mixed out the boundary layer so there has been no low clouds or fog to speak of today.
The disturbance will move east tonight taking the mid and high clouds with it and a transient ridge of high pressure will traverse the region through Wednesday. This will allow the marine layer to return, with low clouds and fog in the coastal areas and western valleys each night and morning. There will be only minor day-to-day variations in temperature, with daytime highs remaining about 5-10 degrees above normal each day.
Numerical models are in pretty good agreement through Friday with respect to the evolution of the synoptic pattern. A vigorous low pressure system, with a closed low at 500 mb, will likely impact the region from Thursday through the weekend. Temperatures will begin to lower, clouds increase and onshore flow strengthen on Thursday. Daytime high temperatures will be near or a few degrees above seasonal averages and westerly winds could be gusting to 35-40 mph in the mtns and on adjacent desert slopes.
For Friday through the weekend, model solutions begin to diverge and there is increasing uncertainty in the details of the forecast. In general, clouds will continue to increase through Saturday and temperatures will continue to trend lower. Sunday will likely be the coolest day, with high temperatures near or as much as 10-15 degrees below seasonal averages. This low pressure system will likely be vigorous enough to bring chances for widespread measurable rainfall and some mountain snow. Saturday will be the day with the best chances for precip and the greatest accumulations. Snow levels will start out above 7000 ft before lowering to around 6500 ft on Sunday as the precipitation begins to taper off. The National Blend of Models currently shows a 65% chance of 0.25 or more in the mountains, a 45%-55% chance in the valleys and coastal areas, about a 25% chance in the high deserts and a 10% or less chance in the lower deserts.
The low pressure system moves east by late Sunday and weak upper level troughing remains over the Western US into Monday, maintaining cool weather with highs near or a few degrees below seasonal averages.
061730z, BKN-OVC high clouds above 15000 feet MSL through 18-20Z, becoming mostly clear thereafter. Some of these mid level clouds have become convective enough to produce -SHRA/Virga. Coastal low clouds more likely after 05Z tonight along the immediate coastlines, and then spreading into some of the inland valleys by 09-11Z Tuesday. Bases will be similar to last night, although a little more elevated at around 1000-1500 ft MSL along the coastal areas, then falling to around 500 ft further inland with better coverage expected further inland tonight, with patchy fog being possible for portions of the inland areas. These clouds should begin to scatter from east to west by tomorrow morning by 15-17Z 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.
