Long Range: High latitude blocking could set off major changes

GFS Model Ensembles 500mb Standard Height Anomalies, showing high latitude blocking from the North Atlantic, through the Davis Straight, and north of the Aleutians.

It has been a while since the meteorological community has been given the chance to analyze some high latitude blocking, at least blocking that looks to have a fighting chance at actually coming to fruition. The last major high latitude blocking event in the fall or winter season came in January-February 2011, and was one of the major causes of an unusually cold and snowy winter in the Eastern US. Since then, many blocking episodes have been modeled too strongly in the medium range, only to appear weak and meager in reality. This time, however, teleconnections support the development of strong blocking not just in the Atlantic, but from north of the Aleutians as well. Medium range ensemble guidance supports positive height anomalies (+3 to +4 Std. Anomaly of height at 500mb) by 72 hours, or the middle of this upcoming week as you can see in our lead image right above this text.

The three major areas of positive height anomalies at 500mb are fairly classic — one from the North Atlantic (which will eventually surge westward towards Greenland), one over Central Canada west of the Davis Straight, and the aforementioned major block with very impressive positive anomalies north of the Aleutians. The three would effectively keep the high latitude well “blocked” through the medium range — something we have only seen in spurts during the past several months.

GFS Ensemble Mean temperature departures on Day 6 (Saturday 10/27/12) showing an outbreak of cold air over the Central US heading east.

With the high latitude blocking development, most if not all of medium range model guidance has -NAO values through the long term with no major rise in the near future. The GEFS mean NAO forecast plume shows values of near -2.0 through the end of the period. You may be asking yourself, after a broad brush of the pattern — despite this, most of the forecast models are showing a ridge in the east and above normal temperatures. So what gives? The answer is a strongly -PNA with the Aleutian/Alaskan ridge correlating well with a big trough over the West Coast by Day 5. The southeast ridge flexes its muscles in the east allowing for positive temperature anomalies to develop. However, in such a pattern…it is only a matter of time before a major shot of cold air is forced southward underneath the blocking into Central Canada and eventually the Central/Eastern US. This will likely come in the form of a large upper air trough by Day 7, easily visible on the GFS temperature departure means to the left.

Keep Reading for a full discussion on the long range, including a cold air surge and potential tropical systems…

So while the airmass this upcoming week may be warm (in fact, it could be quite warm with medium range models forecasting temperatures 5-10 degrees above average in our area), the shot of cold looms by late week/next weekend which brings us to Halloween Weekend and the end of October. With the high latitude pattern supporting sustained cold air into the Central US at that time, temperatures falling below normal behind the cold front shouldn’t be hard to imagine, especially given the state of some of the major teleconnectors including the aforementioned -NAO and neutral and/or negative PNA (a positive PNA would support an even deeper trough in the east).

What catches the long range forecasters eye around Day 5-7 is the forecast development of a tropical system in the Caribbean, which could drift north into the Southeast Atlantic by Day 7. Such a

GFS Forecast Model showing a very strong low pressure system and high impact event in the Northeast US on 10/31 (Day 10).

development doesn’t seem overly concerning at first — but the mid and upper air pattern rings some serious alarms for long range forecasters almost immediately. Although variance at this range is a major factor — forecast models don’t usually get a good grip on the pattern for several more cycles — the blocking adds an extra level of intrigue to the pattern development. Should the tropical system head north and not east (also a big question mark) it could be drawn into a phase with the large upper level trough and aforementioned cold front over the Eastern US. We saw some very dramatic model solutions today, including the GFS/Canadian/Euro indicating large surface lows with potential significant impact to the Northeast US.

All in all, much of that remains fantasy (hence the term “fantasy land” for this time frame on forecast models) given the degree of uncertainty in the pattern. That being said, the high amplitude pattern suggests that with the right timing, the event could become a high impact one for our forecast area. The probability of it remains very low — but the overall pattern seems to be trending towards one of high amplitude…from the beginning of the short term period and a large ridge, to the end of the medium range with a big trough sweeping through the Central US and potential interaction with a tropical system.

Interesting times seem to be ahead of us.

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