Thursday, November 24, 2011

12z GFS Predicts Colder Weather, Potentially Snow

Today's new 12z GFS is predicting colder weather for the US in the long range (11-15 days out), falling into line with the potential for teleconnections to move into a better weather pattern more conductive for snow. The GFS has been having some trouble lately, so we will see if the GFS continues with this pattern before making an assumption about it.

Teleconnections Gearing Up for Winter Pattern Change

Forecast PNA Index.
Positive Phase is Good for Snow and Cold

Forecast AO Index.
Negative Phase is Good for Snow and Cold

Forecast NAO Index.
Negative Phase is Good for Snow and Cold
The teleconnections are looking much better than they have been looking and may be hinting that this winter pattern change is closer than we think. Above are 3 important indices that are commonly tracked and do affect the weather. We have labeled what is good for snow and cold people in each image. Click on the following links for explanations of the indices:
PNA (Pacific North American Pattern) --- NAO (North Atlantic Oscillation) --- AO (Arctic Oscillation)

The PNA has recently been in a strong negative phase, leading to unfavorable chances for snow and cold. However, in the above images, we see the majority of ensemble members are trending towards at least a weak positive phase already in progress but most likely becoming prominent just before November 30 or so. If the PNA is to go positive, chances for snow and cold would rise.

The AO has been in a positive phase most recently. When it is in a positive phase, a vortex around the North Pole strengthens and literally locks in cold air, forcing it to stay up north. However, if the AO is in a negative phase, this vortex weakens and the cold air can escape south, sometimes into the US. The ensemble members forecasting for the AO indicate equal chances for repeating positive spikes or a downward trend towards negative territory. We will have to watch this closely to see what happens.

The NAO has recently also been in positive phase, not a good sign for those who like snow and cold. The ensemble members indicate there is a very substantial potential for the NAO to go negative, increasing chances for snow and cold. The AO and NAO are closely related, so if one goes negative it is possible the other will as well.

In Coming Days

In the coming days we will be seeing a variety of things. One of them will be seeing the ridge of high pressure in the West dissipate and instead be replaced with areas of below average temperatures close to the West Coast. This may be indicative of some storm systems washing shore that would not be able to pump temperatures up. Another big factor that will enhance this dissipating ridge is the potential for another ridge of high pressure to form offshore the US and move towards Alaska. This ridge would therefore defeat the Aleutian semi-permament Low and would have potential to flip things around. This ridge would most likely lead to a trough in the West and a possible ridge in the East if that's how the atmosphere wants to play its games. 

Winter 2011-2012 Update

Winter has not been slow to arrive, even if you think it has. The reason you probably think winter is starting slow is because of the October snowstorm in the Northeast. Let me remind you how that storm was a VERY VERY LUCKY break for the Northeast and has a very low chance of happening anytime soon under normal conditions.

Anyways, this is a briefing on how winter is moving along.
To be honest, it hasn't been starting slow. We  recorded last year O'Hare Airport, Chicago's first snowfall came on December 12th, and they ended up with a historic snowstorm. Just a little bit to tell you.

The La Nina has recently been rumored to be unusually weaker than last year. Well of course it's weaker- it was always expected to be weaker than last year, so the effects will not be just as strong. This leaves more space for other variables to intervene, which they have been doing at the snow lover's disadvantage. At this point in time, we are seeing how some indices will be moving towards better waters:

(Green text is good for snow lovers, red text is bad, yellow text could go either way.)

The PNA will be moving towards positive territory
The NAO will be moving towards more neutral territory rather than positive territory.
The EPO will be dipping into negative territory, then staying around neutral territory.
The AO will be sliding towards neutral territory, but then rebounding into positive territory.
The La Nina should sustain itself or move slightly more negative over the winter.

The big question, of course, is when will winter start to act like winter??
We have gotten some indications that the Arctic Oscillation (AO) will be tanking at some point. We cannot independently confirm this, but we are looking for the certain indices that may be looking at this potential.
Recently, the ECMWF received an upgrade that some have indicated are showing signs of improvement in the model. Again, we cannot independently confirm this, but the long range ECMWF does show the Aleutian Low in Alaska breaking down (see what that does by clicking here), which may give the US  a little taste of winter. The teleconnections are changing, so winter may be on its way!