Friday, February 20, 2015

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Inside Slider Just in Time for Pitchers and Catchers Reporting

Well, I could talk about how depressing the snowpack is or I could talk about the storm coming in this weekend. Let's talk about the relatively good news first. We have a nice inside slider moving into the Great Basin and Sierra late this weekend and early next week that will bring us some snow. Not a lot, but some.

Figure 1: Storm total precipitation via the CNRFC 
  • Snow begins early Sunday morning stretching north to south from Portola towards Bishop and further south
  • Areas around Tahoe will receive 1-4" of snow with some isolated areas near the Sierra Crest and east receiving twice that depending on where convective snow showers develop
  • Kirkwood (4-8") and Mammoth (6-12") should receive the most snow
  • There isn't a lot of moisture with this storm, but it is cold and snow levels will be to valley floors
  • Very dry snow = powder


Figure 2: 1-hour precipitation total color shaded and snow levels color contoured valid Sunday morning via CANSAC

What is an inside slider? I'm glad you asked. An inside slider is an upper level trough that develops just east of a ridge centered over the eastern Pacific. Due to the amplitude (how far north the ridge stretches) of the upper level ridge and the cold air distribution and overall flow pattern across Canada, a trough slides down along and east of the Cascades and the Sierra. It usually possess a positive tilt (axis of trough looks like this: /) and moves north to south along the mountains.

Figure 3: 500mb (~9,000-10,000 ft.) heights, winds, and temperature. Notice the upper level trough across the northern Great Basin and Pacific NW and its orientation as a positively titled trough. This will slide south over the weekend and into early next week.

This storm will be cold therefore snow levels will not be a problem. Due to the trajectory of the storm, there will not be more snow along the crest and west. More than likely there will be more snow along the crest and east due to the upslope flow residing on the east side for this storm. The snow will be very dry and powdery and there is a slight chance of some thunder snow (cue Jim Cantore's reaction).
Figure 4: Water Year (since October 1) Precipitation % of Normal by Basin

Now, the bad news. I will make it brief. The good part of the bad news is that we are near normal or close to it for precipitation since October 1 (I'm fudging a bit here calling 55-65% near normal) (beginning of the water year) (Figure 4). However, as we have said, too much of that precipitation has fallen as rain with the higher snow levels we have seen with our biggest storms this Fall/Winter. We are way below normal for snow-water equivalent (Figure 5). The rain has helped with the drought, but it would have been much better if it had fallen as snow. With a better snowpack, then the water storage lasts longer into the warm season and does more to help alleviate the drought.

Figure 5: Snow Water Equivalent (SWE) % of Normal by Basin

We will have another post up this weekend detailing the upcoming storm and a look into a (hopefully) snowy future.


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