Monday, February 15, 2010

Enjoy the Sun While it Lasts.........

It's been 3 weeks now since the last big storm cycle and about 4 days of sun and highs in the 40's. It's about that time that people start bugging me about when it is going to snow again. Don't worry, this will not be like 2008 where it was a decent season until the end of February and then everything stopped. If you figure that our snowiest 4 month period is normally mid December through mid April, then we are only halfway through winter as of today.

The weather will be the same, sunny and 40's on the mountains through Friday, so let's talk about what's coming after that. I believe the main driver in the pattern to be the strongly negative AO. Take a look here. Notice how the two times this winter the AO went negative, the end of December and the end of January, correspond with our two extended dry periods this winter. The cold air over the arctic is displaced Southward over the continent of North America when the AO is negative. With the El Nino bringing a strong jetstream across the Southern part of the U.S., it also caused the available cold to the North to dive South due to the lower pressures across the South. The jetstream buckled sending storms up the East coast and forming a block out in the North Atlantic holding the cold air over the East and blocking up the pattern. This caused the ridge to sit locked in over the West. Notice the AO is forecasted to start heading positive going into the end of the month.

Once that happens you just have to wait for two things, either the ridge over the West to shift slightly Northward allowing the jetstream, displaced South by El Nino, to come into CA, or the air over the East to moderate and the block to weaken allowing the ridge to slide to our East.

Starting the end of this week the pattern is going to do both in that order. The ridge is going to shift North allowing a low off the coast of CA to send waves of moisture into CA over the weekend. The ridge shifting North may also allow the cold over the East to shift West as well bringing us much colder air along with the snow. The storms don't look all that strong but with the cold air and 3-4 waves of moisture coming through in a 5 day period it may begin to pile up. Still have to watch the next couple of days to see where along the coast the low sets up as to where the heaviest precip will be. Either way it looks much colder with snow.

Next thing to look for is the blocking to weaken over the East. A sign of this is watching the NAO to see it go towards positive. Take a look here. Once the NAO is above -1 it is hard for the block to hold and the pattern should begin to progress going into the last week of Februray. Second sign that is good for storms to return to CA is for the PNA over the Pacific to go negative. This usually means less ridging along the West coast with a flatter jetstream into the West coast. Take a look here. The PNA is beginning to head negative and should be negative the last week of February.

Both of those factors should be enough to allow the El Nino jetstream to head back into CA, but there is a bonus on the table going into March. If we can keep the NAO positive and PNA negative we can add in the MJO. Take a look here. The MJO is forecasted to move back into the Western Pacific and strengthen over the next week. Usually the affects are an enhanced jetstream and storms into CA about a week later. The MJO looks to stay in the Western Pacific for about two weeks, so as the El Nino fades this will be the factor to look for to help our chances of stormy weather going into the first two weeks of March.

Everything is lining up for a stormy weather pattern to take over here and hopefully it lasts into March. Would love to see another March of 2006. The models are catching onto the idea of the stormy pattern returning as well. Take a look here. Notice how we start off small with some storms this weekend into Tuesday, but then the middle of next week into the first week of March the pattern gets flowing and big storms ride along the jetstream and slam into CA. These maps can change alot this far out, but they take a blend of models and at least we can see they are indicated a snowy pattern with several feet of snow. Stay tuned..........BA

No comments: