Thursday, April 24, 2014

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How Bout Some Late April Pow

Interesting start to the weekend as a strong polar jet amplifies over the northeast Pacific this evening (see Figure 1) prompting a moderate shortwave pulse to impact the Sierra. Two bursts of precipitation are expected on Friday. The first in the morning associated with strong cold advection, and the second during the afternoon-evening as the upper-level energy works through. Snow accumulations of 12-18 inches will remain concentrated near and above 7,500 ft, particularly further south in around the Mammoth Lakes area, with a coating to a couple inches possible below the 7,000 ft level.

Figure 1 illustrating the location of the upper-level amplification of the polar jet west of British Columbia. 
The mid to upper level cold air associated with this late April storm is quite impressive (Figure 2) and shall enhance the volume of snow accumulation at high elevation. Figure 3 reveals a broad area of 1-2+ inches of liquid precipitation across the Sierra. As cold fropa approaches tonight, winds will continue to ramp up from the southwest. Cold air arrives early Friday morning along with the first burst of precipitation. Intermittent to moderate snow showers continue through the day Friday before a heavier second burst in the afternoon-evening associated with the upper-level vorticity advection. The vorticity advection and central core of the upper trough appear to take a more southern trajectory across the Sierra prompting a bit more orographic enhancement further south along the Crest (Figures 4).

Figure 2 illustrating the cold air advection at 700 mb. Note the perpendicular nature and intersection of the blue-dotted isotherms and black-solid height contours. 

Figure 3 illustrating the broad swath of liquid precipitation across the Sierra through Sunday AM. 

Figure 4 illustrating vorticity advection (red) further south coming into the Sierra. 
As this system exits to the east Saturday, residual snow showers will be frequent with minor additional accumulations likely. Then on Saturday evening, another weak shortwave attempts to press south from the Pacific NW, although the amplifying ridge off the southern California coast assumes control Sunday night. Ahead of this ridge building, a period of convective snow showers associated with a pulse of mid level moisture will be the feature on Sunday. 

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