Cooler conditions with increased cloud coverage is expected through early next week. Periods of rain are expected Friday through next week. The first round is expected Friday morning, with chances of isolated thunderstorms over the coastal waters locally into the beaches. The next chance will be Friday evening into Saturday, with more widespread light to locally moderate showers. In addition to periods of rain, winds are expected to increase over the mountains, deserts, and coastal waters. Sunday could see mostly dry conditions with our next chances of precipitation and increased winds Monday through Wednesday.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
At noon today, temperatures were running at least a few degrees cooler than noon yesterday for most locations. Even with the cooling, highs will be 5 to 10 degrees warmer than average. Additional cooling is expected for Friday, which will bring temperatures in most places to below average. Below average temperatures are expected to continue through next Thursday. The pressure gradient has begun to trend onshore ahead of an approaching upper level low. This will lead to increased cloud coverage across much of the coastal basin for Friday. Periods of clouds can be expected through the weekend into early next week.
The aforementioned upper level low will dig south and move east into Northern Baja on Friday. This will bring periods of rain to the area by early Friday morning. The highest chances of rain for this first period of rain are along the coast. Additionally, there is potential for isolated thunderstorms to develop over the waters, locally into some coastal communities. The thunderstorm chances are largely focused to coastal waters and the coast due to the temperature difference between sea surface temperatures (running well above average for this time of year) and the cold temperatures associated with the passing low pressure system. High resolution guidance is picking up on an increase in instability with CAPE values increasing to 500-1000 J/kg early Friday morning. There is some potential for thunderstorms to locally reach coastal communities but atmospheric instability drops drastically the further inland you go, which will limit thunderstorm potential. Where thunderstorms develop, expect erratic winds and locally heavy rain that may lead to localized flooding. Rainfall Friday morning is generally expected to be on the lighter side for inland areas. Isolated rainfall rates of 0.40-0.60 inches per hour are most likely to develop along the coast where thunderstorms or heavier rain develops. Chances of rain decrease during the day Friday, but isolated light showers at times cannot be ruled out. Through Friday afternoon, rainfall totals will largely be 0.20 inches or less, but some isolated totals over 0.50 inch are possible along the coast.
Rainfall chances increase again Friday evening as another upper level low moves east through Central California. This will bring another round of rain Friday evening into Saturday, which is expected to be more widespread across the coast, valleys, and mountains. Peak rainfall rates of 0.20-0.30 inches per hour are expected during this time. Rainfall totals for the coast and valleys will be largely 0.20-0.50 inches, with totals in the mountains closer to 0.50-1 inch. Less than 0.10 inch is expected in the deserts. Rainfall is expected to taper off from north to south late Saturday morning through the early afternoon. Dry conditions look likely for Saturday evening into Sunday. Snow levels Friday into Saturday are expected to stay above 8000 ft, with up to 2 inches of snow above 8500 ft.
Another low pressure system is expected to dig south along the West Coast Monday and east late Monday into Tuesday. Ensemble guidance is in good agreement in the timing of this trough, with lower confidence in how far south the low will dig. The ultimate depth of the trough will be a big influence on rainfall totals and snow levels. Nbm chances of rainfall totals exceeding 0.50 inch from Monday morning through Wednesday morning are 30 to 45 percent for the coast and valleys and 60-70 percent for the mountains. By Tuesday morning, spread in NBM snow levels ranges from 5200 ft to 8300 ft.
Winds are expected to increase over the mountains and deserts with each passing system. Expect a slight increase in winds this evening, with a more noticeable increases Friday evening into Saturday. Peak gusts Friday evening into Saturday will be mainly 35 to 45 mph, with gusts up to 55 mph through wind prone passes. Winds are expected to increase again early next week with the next trough passage, with the track of the low the main determining factor in expected wind speeds. A more southerly track would bring us stronger winds compared to a more northerly track.
Additional precipitation and winds are possible for the latter half of next week, but there is low confidence in details. Continue to monitor the forecast for more information.
121830z, Coast/Valleys, Mostly SKC and VFR conditions today. Low clouds based around 3000-4000 feet MSL to gradually develop over the coastal basin after 04z tonight into Friday. Increasing chances of -SHRA overnight into Friday with cigs down to 2500 feet MSL in RA. Widespread BKN-OVC high clouds AOA 8,000ft MSL. VCSH will continue for much of Friday, even as the heavier RA comes to an end after 17z.
Mountains/Deserts, Mostly SKC today with increasing clouds AOA 10 kft after 02Z.
Southerly winds increase tonight and Friday, turning westerly and strengthening late Friday into Saturday. Swell and wind waves will increase considerably at the same time. Hazardous boating conditions are likely Friday into Saturday afternoon, and a Small Craft Advisory is in effect. Winds and seas diminish late Saturday and Sunday, before increasing again on Monday as the next weather system arrives. Additionally, there will be a threat for storms Friday morning with lightning and gusty and erratic winds all possible.
A series of swells (14-15 seconds) from the west-northwest (270-280 degrees) will generate elevated surf Friday, with higher surf and sets to 8 feet Saturday and Sunday. A High Surf Advisory is in effect for these two days. Surf begins to wane Sunday afternoon but may remain elevated for west-facing beaches into Monday morning. Another northwesterly swell will arrive beginning Tuesday. Additionally, there is a threat for lightning near the beaches Friday morning with isolated thunderstorms over the waters.
Ca, High Surf Advisory from 4 PM Friday to 10 AM PST Sunday for Orange County Coastal Areas-San Diego County Coastal Areas.
PZ, Small Craft Advisory from 10 AM Friday to 4 PM PST Saturday for Coastal Waters from San Mateo Point to the Mexican Border and out to 10 nm-Waters from San Mateo Point to the Mexican Border Extending 10 to 60 nm out including San Clemente Island.
