Saturday, November 19, 2016

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Nasty Shadowing Today, Weak Spillover Begins This Evening

Sure is a blustery one on the leeside of the Sierra Nevada today! Look at those gusts in the valley! 49mph at the airport!

This is a classic rain shadowing case. The radar and satellite confirms it.
Beautiful shadowing along the crest!

Confirmed. Awesome.

Note the inversion between 690 hPa and 660 hPa. This is the ideal mountain wave/downslope windstorm setup. Not shown, but some serious vertical windshear as well from 850 to 600 hPa. Flights are likely cancelled at KRNO/KTRK. Trashcans are migrated about the neighborhood.

As the upper level jet dips equatorward, it weakens, but this will bring the precipitable water plume into the central Sierra and lead to enhanced precipitation due to the more favorable orientation. Check out the difference in PDub from tha RightThRRR model output from this morning to early Sunday morning:
Snow levels should come down somewhat in the next 18 hours, but still likely will not fall below pass level (at least that's what Chingy's output sez!):
Plot on right shows time series of freezing elevation (in feet) for the gold dot (Tahoe Basin) at right. Currently, snow levels are pretty high!

The waves keep rolling in over the next week, a very nice active early winter pattern to at least get some sort of base going. Moisture tends to be rather anemic with the shorter overwater trajectories and lack of deep coupling with the subtropics/tropical moisture reservoir, but we'll take it!
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Sunday, November 13, 2016

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The Ridge Breaks Down...Too Much

The good news: The ridge breaks down and we get snow.

The bad news: The ridge really breaks down and the main storm track goes south and east of the Sierra...

Figure 1. Snowfall totals through Thursday afternoon.

But hey at least we get some colder temperatures and a little bit of snow. The snow should start falling early Wednesday morning and finish up by Thursday night. But how much snow? A dusting to about 6 inches of snow is expected with the 'higher totals' in the southern Sierra (Figure 1).

 Figure 2. Precipitable water valid Monday late morning.

There is good Pacific moisture associated with the storm (Figure 2), but the trajectory of the storm and a strong push of very cold air on the backside of the trough doesn't allow the deepest moisture to affect the Sierra (Figures 3-4). You always need a good moisture source and strong lifting (orographic, frontal, convection) to dump snow in the Sierra, and this setup just doesn't provide that.

Figure 3-4. Top: Precipitable water valid mid Wednesday morning. Bottom: 500mb heights and winds valid mid Wednesday morning. Notice the strong N/NW winds on backside of trough over the eastern Pacific.

There is a slight chance the storm track/trajectory changes and we get more snow, but I'm not holding out much hope. The Pacific remains active with a few more storms traversing the east Pacific and across the western United States. We'll keep an eye on those and hope for the best.

As many of you know, once the ridge breaks down, we need as much snow as possible before it builds back in. Let's hope the Pacific provides us with a good base during this window of opportunity.

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