Sierra Snow & Gusty Winds Thursday

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Several days ago it appeared an incoming trough and developing cutoff low would bring some light precipitation to a good portion of northern California. Several days before that, it appeared the system would be well east of our area and keep us entirely dry.

Well, the track has changed again, and while some snow up in the high sierra and points east still looks good, elsewhere things should remain dry due to the fact moisture will be very limited. Cold front looks to slip through during the early to mid morning Thursday, which is when the best chance of precipitation will occur. The best lift and moisture looks to set up from Truckee to north of Mammoth Lakes, and off to the east into western Nevada. This might be a bit conservative, though, as upsloping and some convergence may support some showers along the west slope of the sierra and possibly in the southern motherload foothills, with snow levels around 4000ft. Locally 1 – 3″ of snow seems possible around the Lake Tahoe area into the Reno area, and locally elsewhere along the sierra crest.

Dec 17th 2

This trough will be packing a very strong upper level jet streak, with winds at the 300mb level (approx. 30,000ft) reaching velocities upwards of 150 – 160kts along the western coast of Oregon into northern California.

300mb winds & 500mb heights valid for Thursday morning, modeled from the 12z NAM.

300mb winds & 500mb heights valid for Thursday morning, modeled from the 12z NAM.

While winds that far up surely don’t make it even close to the surface, winds in the lower levels, say around 5,000ft, are forecast between 45 and 55kts, locally 60kts. Behind the positive lift along the leading edge of the trough, towards the back end of the trough, vertical motions become negative, and may drive down some winds somewhere in the range of 20 – 50mph along the west slope of the sierra and portions of the Sacramento valley by mid to late morning Thursday, continuing into the evening. The strongest winds are likely to occur along the west slope of the sierra into the north end of the valley, southwest along the west end of the valley and coastal mountains, with the central part of the valley being in-between the strongest winds. Locally, sustained winds themselves could be in the 25 – 35mph range, would support some 45 – 55mph gusts. These winds could become problematic with some trees and high-profile vehicles, on top of probably causing some power outages due to downed trees/limbs.

850mb winds & heights valid for the later morning Thursday.

850mb winds & heights valid for the later morning Thursday.

10 meter winds valid for late Thursday morning. You can take these and use them for sustained wind estimates.

10 meter winds valid for late Thursday morning. You can take these and use them for sustained wind estimates.

Winds may remain breezy Friday, though not nearly as strong as Thursday, as the exit jet slides overhead as the trough/cutoff low eject east into the southwestern U.S. and so on into the south-central U.S.

300mb winds & heights valid for Friday afternoon, indicating the back edge of the strong upper level jet edging it's way across the state from west to east.

300mb winds & heights valid for Friday afternoon, indicating the back edge of the strong upper level jet edging it’s way across the state from west to east.

One key thing the mixing from the winds will do is (hopefully) mix out the strong inversion currently in place over the valley. This inversion is the cause of warmer temperatures in the foothills and western slope of the sierra, and colder, denser air locked in below around 1000 – 2000ft. This “cap” in place is also trapping a large amount of particulate matter in the valley, creating poor to very poor air quality… especially in the San Joaquin valley.

Sounding for around the Rocklin area (my location) as of Tuesday afternoon. I've added some elevations and pointed out precisely where the inversion is, from just above the surface to just below 850mb/5000ft.

Sounding for around the Rocklin area (my location) as of Tuesday afternoon. I’ve added some elevations and pointed out precisely where the inversion is, from just above the surface to just below 850mb/5000ft.

In addition to some clouds and the sierra snow and gusty (to strong) winds, temperatures will take a fall Thursday by between 5 to 10 degrees in the valley, and 10 to 30 degrees in the sierra compared to Wednesday’s high temperatures! Low temperatures will return to the upper 20s to low 30s in the valley Friday morning (yet again), with teens and single digits in the sierra.

High temperatures on Thursday modeled via the 12z NAM. The sierra won't be getting anywhere close to above freezing through the day.

High temperatures on Thursday modeled via the 12z NAM. The sierra won’t be getting anywhere close to above freezing through the day.

Behind this system, models look quite bleak regarding any potential precipitation. Currently, the week of Christmas looks dry, and could remain that way towards the end of the month or possibly even into early January. This is very far out weather wise and things can of course change, but thus far models have held their ground with this dry pattern.

I’ll of course have an update if any glimmer of hope regarding a new storm arises… which hopefully, is soon.

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