Tahoe Daily Snow
By Bryan Allegretto, Forecaster Posted 6 years ago December 13, 2017
Waiting Game...
Update
We are waiting on the storms to return. We are still about a week out from a pattern that could possibly bring some colder air and a chance of snow. In the meantime the weather is pretty much the same with highs around 50 and lows in the 20's on average across the area. With inversions its a little warmer the higher you go up and colder as you go down below lake level. This weekend we may see some of that mix out with some cooler air for the upper elevations.
The main message I have been posting is that we have changes coming to the pattern, and while those changes bring a better chance for snow than the pattern we have now, they aren't ideal patterns for bringing us snowfall. We basically have to pray for and will the snow to come with the upcoming patterns.
Because the next chance for snow has been more than a week out and especially since the patterns aren't ideal for guaranteed storms, I have been using the ensemble mean runs of the models that take the average result of dozens of members of the models. Each member with slightly different data input to see the possibly results.
The reason we don't just run and look at the normal operational runs of the GFS, European, and other models, is that they aren't very consistent on accuracy more than a week out, and they tend to change run to run more than a week out. Some of you who are running them at home see that. An example last night when the 18z run of the GFS model showed feet of snow from storms undercutting the ridge around Christmas.
The forecast models have been flip-flopping run to run on the storm for the middle of next week and the possible undercutting thereafter, which is not surprsing with how the pattern is setting up, and the fact that it's more than a week out. Let's just look at the two pattern changes coming again, but still not get into any specifics being at least a week out from the first possible snow.
We have been talking about the ridge shifting off the coast and the cold trough dropping down from the North next Wed-Thu. I've explained this week and over the years how the slightest shift in the track of these systems from the North can make a big difference in snowfall. The movement is from North to South over land so there isn't moisture to pick up off the ocean unless the trough or a closed low digs further West close to the coast. If the track from the North comes down just to our East we could miss out on any precip. If it moves right down over the Sierra we could see light snow.
Yesterday I showed the GFS showing a track just to our East with a dusting of snow and the European model with a closed low moving down the coast with the chance for a little more snow. I really shouldn't have shown precip maps yet more than a week out of operational runs, but I was showing two possible scenarios. So let's look at the two models again today vs. yesterday.
The GFS yesterday...
The GFS this morning showing the system dropping down a bit to our East bringing just cold air and now precip.
The European model yesterday...
The Euro today showing a similar track as the GFS today, but a bit shallower and further East with the trough...
Both models are not digging the trough as far West or South as yesterday. They are keeping the snow to the East over the Rockies. The main point of the post today is to show why beyond a week I am using the ensemble mean runs, and to continue to point out the difficulty in getting storms into CA in the upcoming pattern. The pattern we have right now it is impossible to get storms. The new patterns aren't impossible which is why we have been holding onto hope for snow by Christmas.
Looking at the ensemble mean runs of the GFS and European models this morning taking the average of all the member runs, they still show the same thing as they have all week. There is still a chance for light precip and snowfall with the trough next week depending on the track.
So we will keep tracking this system down over the next week. The trend on the operational models today is not looking favorable, but we have time to correct.
We are running into the same issues with the pattern that sets up going into Christmas week. We are heading into what looks like a strong negative EPO(eastern pacific oscillation) pattern that has the strong high pressure centered up closer to Alaska. That could open up the door to storms breaking underneath like we have seen on some of the operations model runs like the really wet GFS run last night, but we aren't seeing the strong extended jet stream across the Pacific like last year, so I haven't been overly optimistic we see storms break into CA.
Like with the system next week, the ensemble mean runs are taking an average of the wet and dry runs of the models to come up with an average that still shows there's a chance we could see storms and precipitation around Christmas or just after.
All we can do is sit and wait until we get these patterns into the one week window so we can look closer at storms. That will start to happen tomorrow for next Thursday if the trough trends back towards digging more South and West.
To beat a dead horse and keep repeating myself all week. The pattern changes coming bring a better chance for storms and snow than the 0% chance in the current pattern. But they aren't the ideal patterns to give confidence that storms will return. On the latest GFS ensemle run 7 of 21 members bring in heavy precip over the next 2 weeks. So that means about a 30% chance big storms return. Better than 0% though!
Hoping for a full pattern shift in January to one that opens the storm door wide open.
Stay tuned...BA
P.S. taking a peak at the new 12z GFS and European runs.... the GFS is still to the East next week but the Euro digs the storm down the coast with a lot of moisture. Continued examples of why we can't rely on run to run of the models, and place our emotions on it...
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