Monday, January 27, 2014

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And We're Back But, Don't Call it a Comeback!

It is with great pleasure to reintroduce Powdiction back to the wild. We stepped away because we knew this winter wouldn't be good (WE KID!...but seriously...haha). Anyways we decided to say, 'hey day jobs and school, the world needs Powdiction and so do we'. So as Randy Quaid said while helping save the world in Independence Day, I'm (we're) baaaaacccckkkkk! (And no we weren't abducted and probed by aliens).

Well well, we finally have some snow in the forecast. Some light rain/snow is falling in the northern Sierra now due to a very nice moisture connection with the Pacific (Figure 1). This moisture is helping to destabilize the area, which is a fancy way of saying, helping to kill the ridge of death.

Fig 1: Amount of water in the atmosphere. And there's a lot for winter, Pineapple Pow! Valid Tuesday.

Some other factors are helping to squash this ridge. An approaching Pacific trough is helping, and the ridge built so far north and wide that it is actually a party to its own demise. This is how. The ridge is thinning over the west coast of US and the overall blocking pattern is allowing and helping bring very cold air from the arctic south (POLAR VORTEX WHAT UP?!)(Figures 2-3).

Mid-level temperatures. Very cold air coming south form the arctic with some of the air pushing SW.


Figs 2-3: Mid-level spin/instability in the atmosphere. Vortex cores moving south from Arctic with core approaching west coast.

Well this cold air is pushing south-southwest, which is helping to break down the ridge from the east. The current moisture and approaching upper level trough in the eastern Pacific are breaking down the ridge from the west and the ridge kind of splits with a small component going over NW Canada and the rest of it pushing south (Rossby Wave Break in da house?!). This is happening now and over the next two days (Figure 4-5).

Figs 4-5: Trough off the PNW coast with very cold air coming from the Arctic. This flattens the ridge and brings much needed precipitation to the Sierra.

Due to the strong moisture connection (atmospheric river) and the ridge softening the blow of the cold air and the effects of falling heights with the approaching trough, this event will start Wednesday and continue into Friday night. Rain/snow will begin to fall Wednesday afternoon/evening with the snow level near the top of the crest. The snow line will slowly fall to base/lake level by Thursday afternoon (Figures 6-7).

Figs 6-7: Fig 6 shows rain/snow starting Wednesday afternoon/evening. Fig 7 shows one of the heaviest 6-hour rain/snow periods during the event, Thursday morning/afternoon.

So to summarize (and for those who just scroll to the bold/italics text), rain/snow begins Wednesday afternoon with snow levels around 8500-9000 feet. It takes about 24 hours for it fall to and below lake level by Thursday afternoon. So about half, if not more of the precipitation will fall as rain below 7000 feet. However, the timing of the snow level drop is still tentative and the amount of snow below mid mountain elevation (7500-8000) is very tenuous.

The heaviest rain/snow will be Thursday morning through Thursday night. Total precipitation for the mountains will range from 1-3" for the event (Figure 8). Snow totals will range from 1-2' with high peaks and snow loading areas possibly seeing upwards of 30". Rain to snow ratios will be lower in that Pineapple Pow range of 6-12 to 1.

Fig 8: Total amount of liquid precipitation forecasted to fall over the next 5 days.

The snow will be some fine Sierra Sticky Icky or Pineapple Pow if you prefer, with a good atmospheric river connection. So it will help cover the rocks and pretty bare slopes around Tahoe and the Sierra. Also, remember as LL said, DON'T CALL IT A COMEBACK!


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