A cooling trend will began today and then turn significantly cooler over the weekend as a cold storm system moves into and over the region. Numerous rain showers and mountain snow in Santa Barbara County and in SoCal are expected between Saturday afternoon and Monday. Brief heavy downpours with small hail are possible with any thunderstorms, mainly Sunday. Dry but cool weather is expected the rest of next week.
Upper Low Brings Showers and Mountain Snow to SB County and SoCal
An upper low will move into the region this weekend bringing with it showers and much colder temperatures along with windy conditions.
Models are starting to shift the low a little farther offshore which is a more favorable pattern for rain locally. However, it's still moving quite slowly so showers may be delayed into Saturday night or early Sunday, especially for LA County. Showers are still expected to be on the lighter side (under a quarter inch per hour) through at least Sunday morning.
First Winter Storm of the Season
All eyes are on the weekend forecast as southern California will receive its first winter storm of the season. This system is not the typical cold front or atmospheric river event. Rather a cold trough will retrograde into the state and develop a cutoff low at the base. The low will be over the Bay Area late Saturday morning and will slowly move down to Pt. Conception by later afternoon Sunday. This pattern will allow for a good amount of over water fetch and the system is getting wetter with successive computer model runs (at least for LA county which will receive the flow pattern that spends the most time over the waters). In a reverse pattern from most winter systems, LA County will see the most rain and SLO County the least.
Showers Expected to Begin Saturday Afternoon
Saturday will be a mostly cloudy day with some morning drizzle but the actual rain will likely hold off until afternoon for the Central Coast. Rain will then develop over the rest of the region in the early evening. There is a decent and fairly long lasting fetch of southerly winds with the system and this will enhance the rain fall amounts across the south slopes. The system is moving a little slower and is a little wetter and rainfall amounts have been bumped up, especially south of Pt. Conception. North of Pt. Conception about a third to a half inch of rain should fall through late Sunday afternoon. LA County will likely see three-quarters of an inch to an inch of rain with the notable exception of the San Gabriel foothills where 1 to 2 inches, locally 2.5 inches of rain now looks likely.
One important thing about this system is that the rain will be spread out over a 36 hour period and peak rain rates are generally expected to be a quarter inch per hour, and for much of the time a tenth of an inch per hour or less. These light rain rates will be very beneficial to the fire weather danger while also remaining under the intensity needed to cause debris flows in the recent fire areas. The only fly in the ointment is the possibility of convection (thunderstorms) which now seems a little less likely during this period since the cold air and instability associated with the upper low will be well to the north of the area for most of the time.
Snow Levels
Snow levels are now forecast to be a little higher than earlier thought. Snow levels now should remain a little above the 4000 ft mark with significant accumulations above 6000 ft. This should reduce the threat to the mountain passes which may see flakes in the air but no real accumulations. With the wetter forecast, snowfall amounts may need to be bumped up some but for now will keep the 5 to 10 inch forecast going but this is just for SoCal Mountains, local totals in Santa Barbara County will be much less. A winter weather advisory is in effect for the mountains of SB County and SoCal this weekend for moderate snow accumulations and some winds.
Temperatures
This is quite the cool system and look for massive drop (6 to 12 locally 15 degrees) in temps. Most max temps will come in a degree or two either side of 60. With even more cooling on tap Sunday, max temps will struggle to even reach 60 degrees. These max temps are 10 to 15 degrees below normal.
Showers Continue Monday
The upper low will move over the area Sunday night and Monday morning. But it should be more for SoCal than here in terms of lingering precipitation.
Looking Ahead
NE flow aloft and weak offshore flow at the surface may bring some offshore winds on Tuesday morning, but right now they look pretty weak. Also the cold air will allow another round of freezing temps to occur in the wind-sheltered areas.
Otherwise weak ridging will be over the state and there will be fair weather with a slow warming trend.
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