Pattern Change En Route?

For the past few weeks or so long-range computer forecast models had hinted that we would see a major pattern change as early as Thanksgiving and we would see much colder weather work into the Northeastern United States.  This, however, has been pushed off but there are some really strong signals that perhaps around the 8th of December this pattern change occurs but the question is for how long?

The latest 8-10 day 500mb height anomaly forecast from the European model and GFS model continue to suggest that we will see strong ridging develop across northern Canada and Arctic region.  While these models have been showing this in the 8-10 day mean for quite some time the "8-10" day never seems to arrive.  However, there is growing consensus that this will indeed occur and perhaps around December 8th.  Ridging in this area would do one of a few things; it would work to further weaken the polar vortex and also allow to to become displaced further to the south which would enhance the likelihood for much colder Arctic air to sink southward into the United States:

 
The support lies within the forecasts for some of the major global teleconnection indices such as the Eastern Pacific Oscillation (EPO), Arctic Oscillation (AO), and North Atlantic Oscillation (NAO).  One of the major teleconnection patterns, the Pacific-North american (PNA) is not very supportive, however, and with this I do think that the pattern change could be rather brief.  Looking into the major global indices, long-range forecasts suggest that the EPO and NAO will really become negative around December 8th.  When the NAO and EPO are negative this enhances the likelihood for really cold Arctic air to work southward into the United States.  The forecasts for the AO though are much more uncertain (as we will see below).  Given the projected 500mb height pattern featured above the more likely scenario will be for the AO to tank negative as well.  The forecast for the PNA is to also become negative and that actually would enhance potential for a trough out in the western United States which can support ridging here in the east.  If, however, the EPO/NAO are quite negative this would suppress ridging:

So what can we expect?  I think there are some strong signals for at least a brief pattern change around the 8th of December and we will have a period of very cold weather.  The big question is does that mean we will get snow?  That is not so easy to answer, however, if the pattern remains as active as it has been this past week there is a good bet we could have our first big snow threat of the season right around this time.  Hopefully (for snow lovers) we can cash in because this pattern change may be brief and we could return to more a more temperate regime.

Saturday Storm System

The GFS computer forecast model is suggesting a fairly impressive little system working through New England during the day on Saturday.  A vigorous piece of shortwave energy associated with an amplifying trough will potentially yield the possibility for a period of showers and perhaps even some thunder, grauple, and strong wind gusts.  Below are two 500mb charts from today's (November 22nd, 2016) run of the GFS computer forecast model.  The graphic on the left we will focus on the 500mb heights and the graphic on the right we will focus on the vorticity field:

 

With the graphic on the left there is an amplifying (strengthening) trough digging into New England and associated with that is a vigorous piece of shortwave energy which will provide a source for some very intense lift.  As the shortwave approaches, we will also see a fairly decent mid-level jet approach the area as well from the southeast placing us in the right entrance region of a 60 knot mid-level jet streak.  This will also work to enhance lift as the system works through:

Another impressive aspect of this system will be the steepening of the mid-level lapse rates (temperature change with height).  With strong cold air advection occurring in the mid-levels of the atmosphere mid-level lapse rates are forecasted to increase to 6.5 C/KM to 7 C/KM!  In fact, overnight Saturday into Sunday they are forecasted to increase to as much as 8 C/KM!!  That is pretty impressive for this area:

The big question is whether or not we will have enough moisture for the system to work with?  the GFS suggests dewpoints will increase into the mid to perhaps upper 40's which should provide enough moisture for this system to work with.

So what can we expect?  As the shortwave approaches we may see an increased risk for some showers (perhaps heavy at times) and given the presence of steep lapse rates enhancing mid-level instability we can't rule out an isolated thunderstorm as well.  Given cold temperatures around 5,000ft, Some grauple could be possible as well along with some strong wind gusts.

Early Winter Talk...Any Cold On the Horizon?

I'm hoping to be able to sit down and actually put forth a long-range outlook for the upcoming winter, however, due to my schedule that may not be able to happen.  Anyways, when composing long-range forecasts there are a slew of variables and atmospheric/oceanic oscillations that must be looked at.  Some winters (such as last winter when we were dealing with a super strong El Nino) are easier to make a long-range forecast for, however, other winters (in fact probably most) aren't very easy.  This is just due to the fact that many of the oscillations looked at aren't very constant and we don't yet (although our understanding of them is becoming much greater) have a full grasp about how they evolve and what causes them to evolve. 

Anyways, when dealing with this winter and what we can expect here in New England one of the biggest keys which could determine how warm or cold of a winter we have overall and how snowy of a winter we have may very well come down to what happens with this area of low pressure which has become established near the Gulf of Alaska:

If you're hoping for cold and a pattern which favors snow here in New England (especially southern New England) this isn't the best of looks at the 500mb level.  Within the highlighted box we have an area of below-average heights (indicating area of low pressure) and those deep blue colors indicate this is a very strong area of low pressure.  This feature is helping to enhance major ridging across much of the United States (indicated by the red shadings across the country).  This posted image is from today's run of one of our computer forecast models (GFS) for about 5-days out.  What this indicates is we will likely see a period of extreme warmth across much of the country and record highs should fly. 

There is some uncertainties though with what happens to this feature moving forward.  Some computer forecast guidance indicates this area of low pressure will sort of weaken which would reduce the ridging over the United States.  However, at the same time some guidance suggests while this will weaken it will just be replaced by yet another area of low pressure and we would yet again see more ridging develop across the country. 

So what is the significance moving into winter?  While this is NOT a forecast we do have to watch this over the next month.  If this pattern remains constant where we see these deep areas of low pressure remain persistent and strong in and near the Gulf of Alaska, cold shots and snow opportunities may be hard to come by here in New England.  What we want to see is this feature either remain quite weak, retrograde up towards the Bering Sea, or just reverse totally and have high pressure become persistent in this area. 

It is still very early and there are some good signals in place if you're a cold and/or snow lover in New England but we do have work to get there.  Stay Tuned!