I hope that everone is having a great holiday weekend aside from the lack of snow. At least the driving conditions were really good. I have haven’t posted in 2 days due to last minute shopping, wrapping, and putting together toys that come in 300 pieces in a box. I have however been doing the normal weather routine and scouring the forecast models and looking at the patterns. I have found a little time to write this evening as the only activity is my daughter cooking me wooden meals on her new toy kitchen.
Although there is a lot going on in the weather and a lot of it contradiction, not much has changed in the forecast for the next 2 weeks. Everyone wants to know what is causing the difference between last year and this year. One of the big differences is that the Nino index for the La Nina this time last year was already strongly moderate at -1.4. We are only at half of that so far at -.07. In a La Nina season we are only 50/50 above an below avg. precip here in Tahoe because we are on the Southern edge of the strong jet stream that hits the Pacific NW in a La Nina.
Hisorically we average more precip the stronger the La Nina with 4 out of the past 5 strong La Ninas including last season above average. We are 50/50 for the past 4 moderate La Ninas which is the strength we should peak at in January this season. Then in weak La Ninas we average below average precip. The difference with the past 4 moderate La Ninas is that during a cold PDO which we have this season the 2 moderate La Ninas averaged slightly higher precip. Remember this map from the Fall showing the shaded areas of above average precip in a La Nina in a cold PDO.
Since historically we average above average in a moderate La Nina and a bit more in the cold PDO I went with the average of the last 4 moderate La Ninas for my Winter forecast of 105% of average snowfall for this Winter. But history only works for making an educated guess on the upcoming Winter in the Fall. Now that we are starting the Winter and we are off to a very slow start we have to look at what is happening. Although the La Nina is becoming moderate in strength it is acting more like a weak La Nina still.
We can tend to get a late start in La Nina seasons because the pattern doesn’t usually take over until Winter sets about now and going into January. One of the things that got us an early jump start last year was the AO went strongly negative which set up blocking in the North Pacific and sent the jetstream straight into the West coast. We have been watching for the AO to go negative in January with the stratospheric warming occuring and forecasted to shift over Alaska and Western Canada. This can cool the troposphere underneath and disrupt the polar vortex send cold air South and sending the AO negative. Many times this cold can funnel into the East but during a cold PDO it tends to come South first down the West coast. Here is an example from Jan. 1967.
That can force the trough into the West Coast and push the jet stream further South. It can also mean that blocking sets up in the North Pacific. But the main driver for out weather as far as teleconnections is the PNA. Blocking in the North Pacific can help get us a negative PNA, but the PNA can be postive or negative during the negative AO. Last year we had a negative PNA in mid-November that may have been kick started by the -AO at the same time.
But as you can see the PNA went positive in January while the AO stayed negative and then back negative in February as the AO went positve. Do you remember the 6 weeks without snow last January and into February while the PNA was positive? We had big snow last year whenever the PNA was negative, in Nov, Dec, Feb, March…. So while the AO may have helped to kick the PNA negative it doesn’t control the PNA and the PNA is what correlates to us being wet or dry which is why I have it at the top of the site.
We had a negative PNA at the end of October and the first half of November and that is when we picked up the 2-3 feet on the mountains that we have been stuck at since it went positive in mid-November.
In a La Nina the PNA tends to spend more time in the negative state with a weakened East Asian jet stream, blocking in the North Pacific, and a split Pacific jetstream. We don’t have the blocking yet which is the hope if the stratospheric warming can cause the AO to go negative. The East Asian jetstream has been strong and not split but it started to weaken and split about 10 days ago. One of the things that could be keeping the Asian jet stream extended and the ridge closer to the coast is the Sea Surface Temps further East and warmer North of Hawaii this year. Here is last year.
And here is this year…
You can see too that the La Nina conditions along the equator are not as strong this year. Indian Ocean is also warmer which could be changing the pattern this year as well. So if you are wondering why it is not dumping in a La Nina like last year those are the several things that I see that are opposite or different from last year.
The forecast for the next 10 days keeps the ridge off the coast of CA but further South than it has been. This will open up the storm door to the Pacific NW. Here is the precip map for the next 7 days from the Euro today.
You can see that the pattern is changing as the areas over the Southwest that have been getting all the snow almost like an El Nino will now be dry and the Pacific NW will get a lot of rain and mountain snow. That will keep the cities of Seattle and Salem from breaking all-time low precip records from December as they get hit hard the last week of the month. But as you can see it’s the typical weak/moderate La Nina pattern where we are right on the edge.
It looks like we could pick up a few rain/snow showers from the storms on Wednesday and Friday but I’m not expecting more than a dusting on the mountains if we get anything. The ridge will shift further out in the Pacific around the 1st and then the 5th of January to around 150-160w before quickly shifting back off the coast. This will allow the storms for the 2nd and 7th to shift further South which we have been watching for since it showed up last week. Right now though the brunt of the storms still look to stay North of the area with only light amounts of snow.
We will have to watch the first week of January to see if we can get the storms to shift further South, but for right now it’s +PNA and +AO for the next 2 weeks and I don’t think a big storm gets in yet. Another factor that has not been on our side this month is the MJO. It was strong and progressing toward the Western Pacific at the beginning of the month and then just stopped at the edge of phase 5 in the Maritime Continent and has been sitting there all month.
That is not in a good position for our weather pattern. Look at the right sidebar though at the new forecast for the MJO. It is forecasted to start moving again towards and through the Pacific over the next 2 weeks. The convection moving into the Western Pacific could mean the ridge beginning to retrograde back towards Hawaii the first week of January opening the storm door. The MJO tends to have a bigger affect on the pattern during a weak/moderate La Nina.
So the bad news is that there are still no big storms showing up in the long-range forecast, but the good news is that the pattern is changing to more of a La Nina pattern with the jet stream hitting just to our North. Now we just need help getting rid of the ridge off the coast so we can get the jet stream further South. We will continue to watch for the MJO to move into the Western Pacific, the AO to go negative, and especially the PNA to go negative. Until then you’ll need to take a road trip North. Stay tuned….BA
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