Pineapple Pow Part Deux

Another Pineapple Express is on its way to the Sierra! But first we have a colder albeit drier storm coming through late tonight and through tomorrow. So now we have two storms! The mega ridge of death is slowly dying after its mid winter peak and giving way to the more progressive late winter and spring pattern.

Figure 1 (top): 9-1000' temperature, wind, and height valid Thursday evening. 1st storm is colder with snow level down to around 5000'.
Figure 2 (bottom): 24-hour liquid precipitation forecast valid Wednesday night through Thursday night. Over 0.5" expected along the crest near Tahoe with higher amounts to the south and north.

I know everyone wants the snowfall (rain?) totals for these storms, so let's get right to it. The first storm will drop about 4-8" of powder, possibly hitting double digits at the highest elevations around Tahoe with higher amounts to the north (Cascades) and to the south (southern Sierra) (Figures 1-2). The 2nd storm is a doozy. It is forecasted to dump upwards of 8" of liquid water in the northern Sierra and widespread values of 4-6" of liquid water to the rest of the Sierra Friday night through Monday morning (Figure 3). 

Figure 3: HPC QPF (precipitation forecast) for the next 5 days. Lots of precipitation is coming to the West Coast this weekend. Expecting 5"+ along crest of the Sierra through the weekend.

HOWEVER, it comes with a caveat. Snow levels will be high and stay high. Right now the best case scenario is if the snow level sets up at 7000', worst case is at 8500', and most likely scenario is at 7500-8000' (Figure 4). Either way, snow to water ratios will be low (5-8 range). We are forecasting about 2' of snow above 8000' with a range of 18-30" due to the uncertain, but probably high snow level. The 7-8000' level is the tough forecast since we are confident it will be mostly rain below 7000'. The 7-8000' level could be split and range from almost no snow accumulation to more than a foot. We will keep an eye on this. 

Figure 4: 9-1000' temperature, wind, and height valid Saturday afternoon. Snow levels forecasted to be at ~7500'.

So for those who like to listen to us detail why this is happening, keep reading and hopefully learn something. The upper level jet will be over the area for the next few days (Figure 5). It will always not be favorable for upper level lifting to support the snowmaking process, but usually the jet over us is a good sign for snow. This also means it will be windy most of the weekend so we could see some lift slowdowns or closures.

Figure 5: Upper level winds and heights. Upper level jet over the area which means strong winds at times through the weekend.

The pattern change is similar to the last storm. An upper level trough pushes west due to cold arctic air and there is an approaching trough in the central/eastern Pacific. These troughs help breakdown and split the ridge, but this time it occurs further west. The ridge then builds back and moves east slightly, putting the juicy, moisture rich southwest flow right over us.

So this time we will start out cooler with the first storm. This storm essentially splits and misses the central Sierra. A center of vorticity (measure of spin in the atmosphere, good proxy for small troughs in the flow and instability) moves to our south in northwest upper level flow while the main trough impacts the Cascades (Figure 6). This is the storm that will provide the initial snow.

Figure 6: Mid-level heights, winds, and vorticity (shaded). Area of vorticity oriented NW/SE over west coast split from the trough to the north. This split minimized the snow for Tahoe area but to the north and south will see better snowfall totals.

We get a quick reprieve Thursday night through early Friday morning before the unstoppable force (Pineapple Express) meets the immovable object (Sierra) and starts dumping lots of precipitation on the Sierra (Figure 7). The heaviest precipitation looks to be late Friday night through Sunday. We get warm air advection (warm air moving in) at lower levels and essentially neutral temperature change at mid and upper levels, which means we should have some decent upward motion in addition to the terrain lifting. This is because colder air over warmer air, means the air below can continue to rise (warm air rises), which helps us make more snow.

Figure 7: Precipitable water (measure of total liquid water in atmosphere). Lots of water is coming, some in the form of rain and some in the form of snow. Let's hope for more of the latter for the Sierra.

This storm is very wet, which also keeps it relatively warm in the absence of a large, cold trough. The thermodynamic properties (condensation=heat) will keep the snow levels higher. Also, just the sheer amount of moisture will raise dewpoints (temperature of condensation), which will act as a barrier to drop the snow level. Unfortunately, the air temperature cannot be lower than the dewpoint temperature and with dewpoints in the upper 20s to mid 30s in the Sierra, there will be rain.

So all in all we have to be mostly happy with the snow and moisture (we need it, DROUGHT) we are going to receive. However, if we get 5" of rain with the snow level staying around 8000', that could spell disaster for mid to lower mountain snowpack. If we had more snow, there would probably be more concern about a rain on snow flood event. However, we don't have that much, but isolated flooding could be a possibility.

So go out and enjoy some snow (and rain...sigh) this weekend. Be sure to wax those skis and boards because it will be some sticky Pineapple Pow.