Drier and warmer through Saturday. Rain chances, gusty winds, and cooler weather return Sunday afternoon, increasing Monday and Tuesday with heavy precipitation at times and a slight chance for thunderstorms. Snow levels fall through Wednesday, with elevations above 5000-6000 feet likely to see heavy snow and significant impacts next week.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Mid-level water vapor satellite imagery shows a broad positively tilted trough stretching from Central California down to around 800 miles southwest of San Diego. A much tighter trough off the coast of Canada starts to push southward today, kicking the more broad trough eastward over the region, where it will quickly move through on Friday before advancing into the Plains. Newest model guidance predicts the low to close briefly as it moves directly over the area, which, combined with lingering low and mid level moisture from the past day, could be sufficient for a brief round of showers (20% chance) in the San Diego County mountains Friday morning. Otherwise, ridging will build in behind the trough, with warming temperatures for Friday and Saturday to slightly above normal and dry weather prevailing.
By Sunday, a significant pattern change will be underway with the biggest winter weather event of the season thus far. The aforementioned Canadian trough will become a closed low and move towards the California coast beginning Sunday afternoon and propagate across the state through Wednesday. Heights fall quickly Sunday and into Monday, with high temperatures 5-10 degrees cooler day over day. Increasing wind gusts for nearly all locations, including the marine zones, peaking Monday and Tuesday as this system moves through. Gusts will be especially prevalent through the mountain passes, which may gust above 60 mph at times, making travel hazardous.
A weak Atmospheric River will arrive in conjunction with this next trough, although models are struggling to resolve the timing of the main IVT. At this point, GFS and ECMWF both are moving towards the Monday afternoon/night time frame, with the GFS around 6 hours faster than the ECMWF. More agreement is found in the temperatures with this next system, which will be significantly colder than previous events. Snow levels begin Monday around 6000-6500ft, falling to around 5000-5500ft Tuesday morning and closer to 4000ft by Wednesday. As previously mentioned, precipitation may begin as early as Sunday afternoon, with higher precipitation for Monday and Monday night. Elevations above 5000ft will see significant snowfall next week, regardless of the timing of the heaviest precipitation, with elevations above 6000ft likely to see the most significant impacts. At this time, snowfall totals for the San Bernardino Mountains for elevations above 6000ft are likely to reach a foot or more, with 10th percentile (lowest reasonable amount) being 4 inches and the 90th percentile (highest reasonable amount) being 29 inches for Big Bear with the mean right at 16 inches for totals Sunday through Thursday. Further south in Riverside County Mountains, the spread for snowfall totals in the Idyllwild region ranges from 2 inches (10th) to 20 inches (90th) with mean being right around 8 inches through Thursday. The range remains quite large next week for snowfall accumulations through Thursday generally due to uncertainty with the snow levels as well as the main IVT timing, although numerous rounds of precipitation looks likely through Thursday as snow levels fall regardless of the timing of the first round. Either way, expect significant impacts for mountain roadways for nearly all of next week, especially elevations above 5000ft. Now is the time to prepare for snow, and ensure that you have proper equipment stored in your homes and cars, and avoid travel during the worst conditions.
Outside of the mountains, significant precipitation is expected, especially for the coastal basins and foothills of the mountains. Similar to above, there will be numerous rounds of precipitation with heavier bands possible both Monday and Tuesday. As the system moves through during that Monday night timeframe, a few rumbles of thunder will not be out of the question, primarily for the western areas. Rainfall totals are also quite spread for next week for the same reasons mentioned above, although totals through Thursday could reach a few inches. Depending on the rainfall rates next week, localized flooding may be a concern, especially for more flood-prone spots in urban areas and near burn scars. Stay tuned as the picture for next week becomes clearer.
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.
