Friday, December 19, 2014

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A break in the action

After a brief hiatus Thursday, another round of light snow has re-coated the north-central Sierra slopes today, especially just west and north of the Lake (~ 3-6 inches) with lighter amounts east and south. Intermittent light snow showers will continue overnight and part of Saturday with a couple additional inches possible above 6000 ft and slightly more above 7000 ft. Later Saturday, snow showers should become isolated with drier and milder conditions Sunday-Tuesday
  • Intermittent light snow showers overnight into Saturday
  • 1-2" around 6000 ft with a few additional inches above 7000 ft
  • Drier and milder conditions developing later Saturday through Tuesday
  • Milder afternoons with partial sunshine developing, but persistent cloudiness at times will not be uncommon with lingering moisture
The next 24 hours will be a transition period with another weak disturbance this evening ahead of warm advection developing early Saturday. Snow shower activity should be widespread this evening and then gradually taper through Saturday. All of this is a prelude to a developing upper-level ridge that will assume control of our weather into early next week (Figure 1).

Figure 1. Upper-level ridge at 250 mb building into the West coast Saturday evening. From the GFS model; image courtesy of UHawaii.

Looking ahead, the models show amplification of the longwave pattern over the central Pacific on Monday-Tuesday; a reflection of the primary core of momentum associated with the polar jet encountering the upstream West coast ridge. This prompts the polar jet to buckle and break upstream over the central-east Pacific (Figure 2) and preempts a potentially strong storm system affecting the Sierra Wednesday-Thursday (Figure 3).

Figure 2. Rossby Wave break over the central-east Pacific at 250 mb from the GFS model. Courtesy of UHawaii.
Figure 3. Potential storm impacting Sierra Wednesday-Thursday as reflected at 250 mb in the GFS model. Courtesy of UHawaii.


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