Too Cold for Snow in Tahoe?

Too Cold for Snow in Tahoe?

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Too Cold for Snow in Tahoe?

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That may be the case but not because of the air, because of the Pacific Ocean.  November was an active month although many of the storms split as they hit the coast or they were cut-off lows that grazed Tahoe.  That led to multiple small storms dropping several inches at a time.

The final splitting storm moved through on Thanksgiving, and in its wake we have a ridge building over the West.  We have shifted into a postive PNA (Pacfic North American) teleconnection pattern.  When there is a positive PNA there is usually a ridge in the Eastern Pacfic with a trough near the Aleutians.  That sends the jetstream and storm track up into Alaska and Canada.  This pattern will block storms from heading towards the West Coast.

Here is a map of the pattern for this week showing the ridge in the Pacific off the Pacfic NW coast and the cold trough dropping into the center of the country and digging Southwest.

This ridge is setting up pretty far North along the West Coast and helping to drive the cold air from Canada down into the U.S.  Sometimes this can actually be a great setup for Tahoe to pick up a lot of snow.  As you can see there is an opening under the ridge into CA.  If we could get the subtropical jetstream to come underneath the ridge and merge with the cold air digging Southwest towards CA it would mean the snow hose is turned on.  This has happened before including one of the 100 inch weeks last season.

The problem this season so far is that there is no sub-tropical jetstream, just the polar jetstream that is stuck up in Alaska over top of the ridge.  All that we have over the Pacific off the coast is sinking air thanks to the colder than average water temps.  Look on this map at the extent of the below average water temps between California and Hawaii.

Without a jetstream to bring moisture under the ridge we are just stuck with dry conditions down the West Coast until the ridge moves.  This week we will see colder air move into the Tahoe area on Thursday and Friday as the cold air coming down the Rockies backs West a little.  There may even be a little light snow as the cold front moves through.  The European model suggest a possible cut-off low forming and bringing several inches of light snow as it stalls to the South.  You can see on the forecast map below that the light precip moves in from the East instead of the West.

 

One of the things that could get us some moisture under the ridge is the MJO as it enters the Western Pacific.  Looking at the latest Wheeler Henderson phase chart the MJO is getting really strong over the Maritime Continent.  If it should continue into phase 7 and 8 in the Pacific at this strength it could mean some fun in December about a week after it moves into the Western Pacfic.  That is still a few weeks away.

The other thing that could change the pattern back towards a trough on the West Coast and storms is the PNA switching back to negative.  La Nina favors the -PNA so it will be back.  Looking at the latest forecasts for the PNA they do suggest a return to the negative phase around mid-December.

Until then it looks like it is going to be quiet with shot of cold and only some light snow possible over the next couple of weeks.  BA

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