Rain, Sierra Snow Showers, & Colder Temperatures Expected Monday into Tuesday

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A fairly cold trough diving south out of British Columbia/Gulf of Alaska will position itself directly over northern California Monday. As the trough and associated upper level low slides overhead, the -22 to -26c 500mb cold pool will as well, increasing instability, as well as cool off temperatures significantly, as well as lower snow levels.

Graphic depicting Monday and Tuesday's system.

Graphic depicting Monday and Tuesday’s system.

Now, a trough like this wouldn’t do much without moisture, and in this case, it will have access to some .5 – .6″ precipitable water values, which aren’t much, but is more than you’d typically see with a cold with this track. This moisture combined with the upper lows weak to locally moderate strength lift and a 120kt+ jet roaring around the upper low will support isolated to scattered showers beginning Monday morning, increasing in areal coverage and possibly strength through the afternoon, before decreasing in coverage through the overnight hours.

00z NAM's forecast 300mb wind and heights valid early Tuesday morning. The lightest reds indicate some ~130kt winds.

00z NAM’s forecast 300mb wind and heights valid early Monday morning. The lightest reds indicate some ~130kt winds.

Instability won’t be very strong, with surface CAPE values under 300 – 400j/kg for the most part, but lapse rates will be fairly steep. Shear isn’t too great due to us (northern California) being basically directly under the upper level low, but there should be some veering winds with height. Due to the lack of decent instability, I don’t expect any widespread thunderstorm activity, but seeing how there is a touch of it, I wouldn’t rule out a few isolated lightning strikes here and there.

In the mountains, snow levels are expected to start out around 4000 to 5000ft during the day Monday, but overnight could be locally lower, wherever precipitation exists.

Total precipitation through Tuesday morning isn’t expected to be a whole lot, but it does appear that the lower elevations could see anywhere from nothing to a quarter inch of rain. In the mountains, a quarter to half an inch is possible, possibly locally up to three quarters of an inch if any wetter solutions vertify. These liquid amounts would equate to 3 – 6″ of snow in above 4000 – 5000ft… locally 8 – 12″ at the highest elevations.

Temperatures will take quite the tumble Monday and Tuesday, with valley highs in not making it out of the low 60s, with mid 40s to mid 50s in the foothills, and 20s and 30s in the sierra. Tuesday, temperatures may bump up a few degrees, but will remain quite cool. Overnight lows in the valley for a couple nights could dip into the low 40s, with 30s in the foothills and teens and 20s in the sierra.

As the trough and upper level low dive into southern California Tuesday, we here in the north dry out. High pressure should keep us dry and slowly warm things up through the work week, but by the weekend we could see another system. Models differ quite a bit on the strength, timing, and main impact point, but in general most models do indicate a system in the Saturday – late Sunday timeframe. Too early to go into detail right now given the notable model differences, but stay tuned for later posts this week (probably by Monday or Tuesday).

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