Friday, March 21, 2014

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Winter in Spring?

Well the models have been consistent for past five days or so bringing a trough and some much needed and wanted snow to the Sierra. It will start impacting the Sierra Tuesday evening and last through Wednesday night. The heaviest snow will be Wednesday morning and afternoon. However, we know with the trough being 5 days out, that the timing could change.

Our preliminary snowfall forecast: 12-18" mid-mountain and above and 6-12" below. The snow level will drop down below lake level on Wednesday. This is a pretty cold system so expect some nice powder turns Wednesday and Thursday (Figure 1).

Figure 1: 7-day precipitation forecast from HPC. 0.25-1.5" of precipitation is expected. 

We are at the southern edge of a trough once again, but we may just luck out and get on the cold/north side of the jet. If this happens, we get less wind, better snow-to-water ratios, and better spillover.

Figure 2: Mid-level winds, temperatures, and heights. Valid next Wednesday morning. Cold air advection on the west/back side of the trough will help strengthen the trough and give us more snow.

For a better chance of this, we really need the mid and upper levels to get some cold air advection (movement of air) on the backside of this trough (Figures 2-3). This will deepen the trough and push the jet further to the south.

Figure 3: Upper level winds and heights. Valid next Wednesday morning. We hope we stay to the north of the jet, which means better snow and less wind.

Additionally, if we can get better vorticity (measure of spin in the atmosphere) advection on the backside of the trough, it will help propagate the wave further to the south (Figure 4). A general rule of thumb is thermal advection deepens or weakens a trough, while vorticity advection influences the propagation of the trough/wave.

Figure 4: Mid-level vorticity, winds, and heights. Hoping the vorticity will increase enough to the south to push the trough further south and give us better conditions.

However, the trajectory of the trough and the absence of prolonged strong south-southwest flow ahead of the trough is limiting our moisture connection (Figure 5). Temperatures will be cold and instability associated with the trough will do their best to squeeze out all the moisture possible though.

Figure 5: 9-10000' winds, relative humidity, and heights. Limited south-southwest flow is cutting off our best moisture source.

We will have another post up by Sunday night updating our forecast. So put up the mountain bike this week, and break those skis and boards back out!


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