Monday, December 1, 2014

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Another extended round of base building

Although ski conditions remain less than ideal, the recent wet snowfall will only help augment the base requirements for mid mountain and above. Resorts are reporting variable snow amounts since Friday with 3-6"+ at Kirkwood and Mammoth and higher amounts north of the Lake; 9-16" bottom to top at Sugar Bowl and 13" at Squaw/Alpine.
  • Storm will impact the Sierra Tuesday through Wednesday night
  • Warmer, wet storm will generate plenty of precipitation, but with higher snow levels
  • 1-2 ft. of snow above 7500 ft. 
After a brief hiatus for next 18-24 hours, another extended period of precipitation rolls in mid-day Tuesday and continues into early Thursday. Unlike the pervious system, a subtropical plume of moisture will feed across a broad swath of the Sierra (and further south) with liquid amounts exceeding 2.5" (see Figure 1 and 2). However, as before, temperatures associated with this system will be mild yielding wetter snow accumulations concentrated above 8000 ft early on before dropping to 7500 ft during the latter half. Estimated snow amounts by Thursday morning will be 18-24" above 8,000 ft with isolated higher amounts further south associated with higher terrain; 10-15"+/- between 7500-8000 ft; 4-8" between 7000-7500 ft; and couple wet inches near and just below 7000 ft.

Figure 1. Depiction of precipitable water (mm) lifting toward central and southern CA. Courtesy of UHawaii.

Figure 2. 48 hour accumulated precipitation (in) from 12z Tuesday 2014-12-02 through 12z Thursday.  Courtesy of CEFA/DRI.

Like the previous storm, this next system reveals a complicated forecast and may require updates. The overall synoptic picture reveals a closed upper-level low parked off central CA coast (Figure 3). Over the next 18-72 hours, this system gradually pinwheels east prompting a southwest to northeast subtropical fetch into the central Sierra (Figure 1) with precipitation breaking out around mid-day Tuesday and lasting into Thursday morning. As the circulation moves into CA, and transitions from a closed low into an open wave, the associated upper-level jet structure splits driving 2-prongs of concentrated kinetic energy northeast into the central-northern Sierra and another southeast into southern CA (Figure 4). The SW to NE orientation of the northern jet structure, and its location of slightly south of Lake Tahoe, favors good orographic lift and upper-level divergence for the northern-central Sierra.

Figure 3. 250 mb wind (kts). Note the closed low circulation off the central CA coast. Courtesy of UHawaii.
Figure 4. Same as Figure 3 except here the 2-prong kinetic energy jet structures are depicted entering central and southern CA Tuesday evening.  Courtesy of UHawaii.
Although with mild mid-level temperatures (< -5C @ 700 mb), and limited colder air aloft, the nature of the snow accumulation will be wetter and more dense keeping accumulations down. However, this event shall yield an excellent snow base for the Sierra slopes establishing a nice foundation for future Pow (we can hope).

As this storm exits on Thursday, residual moisture and light instability showers will linger during afternoon. The next trough is progged for Friday. Although at this time, the latter appears to take a more northward trajectory.


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