Wednesday, January 29, 2014

// // Leave a Comment


The prodigal son returns in the form of Pineapple Pow. In what seems longer than what KT lift lines will be Friday/Saturday or on any good powder day, snow is finally returning to the Sierra. The whole length of the Sierra will receive substantial snow over the next 36-48 hours.

Let's get down to the nitty gritty right away. Mid-mountain and above (~7500 ft.) will receive a solid 2 feet. We said the some isolated areas could see upwards of 30" on Monday. Well, those areas will expand due to the cold air arriving sooner than we saw on Monday. Lake level to mid-mountain will receive about a foot of snow. Two keys for this: 1) the timing of the cold air (sometime tonight the snow level will migrate down to lake level) and 2) The first snow could fall, not stick, and melt thus slowing and delaying the accumulation process. 

So our best forecast is 5-10" near the base, 12-20" mid-mountain-ish, and 2'+ at the very top. Total melt water precipitation will be between 1-3", with some areas near the crest over 3.5" (Figures 1-2).

Figure 1: WRF predicted 72 hour precipitation. Most of the crest expected to receive 2.5"+. This model can overestimate precipitation though.

Figure 2: HPC predicted 5-day precipitation totals. Most areas in the Sierra 1-3".

Due to the great moisture connection (aka atmospheric river), the snow will be very wet, which will be great for filling in the many gaps and bare areas across the Sierra. Precipitable water values will exceed 1" and even approaching 1.5", which is very high (Figure 3). These values are a classic Pineapple Pow signature.

Figure 3: Large quantities of water in the atmosphere getting transported our way. Pineapple Pow!

The ridging is giving way to zonal flow and an extremely positively tilted trough (think a forward slash (/)) over the West Coast. This tilt promotes and maximizes the moisture connection in the Pacific that we currently have (Figures 4-5).

Figure 4-5: Transition from ridging to zonal flow to positively tilted trough in 24 hours. Looking at mid-level heights, winds, temperature.

Also as I mentioned above, cold air is moving in a little faster than was forecasted a couple of days ago. So a quicker change to snow, higher snow to water ratios, and more snow at lower elevations will be the result of the colder air. The longer range model (GFS), is still holding the cold air off a little longer than some of the shorter range models (WRF, NAM).

Figures 6-7: 9-1000 ft. temperature, height, and winds. Temperatures drop this afternoon through tomorrow night. Transition from rain to snow will occur at lake level overnight.

A narrative of the event should unfold like this: 1) Rain starts today (already has) and continues through the afternoon. 2) Snow levels slowly fall through the late afternoon and evening. 3) Snow levels will fall to lake/base level overnight sometime between 10pm - 4am. 4) Heaviest snow/rain will occur late this afternoon and last through tomorrow morning. 5) Finally, snow will linger through tomorrow evening/night.

So go out and enjoy the powder over the next few days. And remember to practice that sick voice or perfect the out sick email today.


Post a Comment