Back to back storms over the weekend brought several inches of snow to the Tahoe region. Some resorts picked up over a foot of snow on the upper half of the mountain. This continues what has been a fairly active weather pattern for the past few weeks. The one thing that has been missing though is “The Big One”.
In order to get a big storm the atmospheric patterns have to be just right. In a La Nina year like this one you want a negative PNA (Pacific North American) teleconnection pattern, or the MJO (Madden Julian Oscillation) to be in the Western Pacific. Either of those or the combination of will favor a ridge of high pressure to the North of Hawaii and a trough along the West Coast (an Open Storm Door). Then all you need is a big storm to ride the jetstream from the cold Gulf of Alaska down to the West Coast.
We have all of that coming up this week and because of that the forecast has promised the possibility of several feet of snow over the Thanksgiving holiday. But what has happened to storms already this season is about to happen again, the dreaded Split. The jetstream is like a river and when it encounters a large enough “boulder” it has to split and go around it instead of over it or moving it.
One of the things that can cause a “boulder” for Lake Tahoe in the Winter is a strong high pressure to the East over the Rockies. As the storms hit the coast the jetstream feeding them dives under and over the high pressure splitting the storm apart. The image below shows this as you can see the flow splitting around the “H” or high pressure.
If the high pressure is strong enough it can break apart a storm that promises to dump several feet as it approaches the coast. You can see below on the latest satellite image that the jetstream is already diving down the coast and getting ready to make and attempt to take aim at California.
It’s already bringing another pounding to the Pacific Northwest with heavy rain and snow as far South as the Northern CA coast. But as the jetstream pushes towards CA it will hit the back of the strengthening ridge that moved in today behind the weekend snow storms. That will slow it down and then tilt is so that the flow is more South to North. Looking at the forecast models things still look promising up to that point. The map below shows the storm about to reach Tahoe Wednesday afternoon.
The problem is that big “H” over Colorado. That is what is causing the jetstream and the cold front to tilt vertically as the jetstream tries to push around it. By the time the precipitation pushes into Tahoe the jetstream will have dived South all the way down to the Baja. That will cut off the moist flow to the storm and tilt away the East to West orientation needed to produce orographic lift snowfall caused when the jetstream runs over the mountains. Below is the next frame of the weather model showing the storm as it impacts Tahoe Wednesday night.
Basically the storm just disintegrates as it pushes through. Right now it looks like the best we can hope for is an inch or two of snow as the front pushes through. You can see on the map that there is another storm right behind that one but that storm is going to be held to the north by the high pressure ridge building over the Southwest. There is actually a third strong storm that will hith the Pacific NW on Sunday and the same looks to happen with that storm. So the total precipitation through next Monday looks like this.
Sooooo close……but not close enough. It appears that the perfect coditions over the Pacific for big storms will be thwarted by high pressure to the East of Tahoe holding strong. Meanwhile, not that far to the North and West mothern nature unleashes its fury. What is happening to the North would have spelled dejavu for the Tahoe Basin as it would have brought feet and feet of snow for the end of November.
After these storms push through the forecast is for a ridge to build in off the CA coast keeping away storms into
the first week of December. The teleconnections that were favorable this month for snow will flip going into December. The PNA is forecasted to go positive which would support ridging along the West Coast. This is not uncommon for the start of a La Nina season, last year was the exception to the rule.
Stay positive as I am still expecting Tahoe to get pounded with lots of cold and snow once the true Winter sets in. January and February should make up for this dry spell and we should still see cold and snow at times in December. Looking at the last 4 moderate La Ninas during a cold PDO like we have for this season Tahoe has averaged 105% of average snowfall. BA