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John Homenuk

Hype meter rising on potential weekend storm: What’s the deal?

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ECMWF Forecast Model showing a very strong storm system off the East Coast early next week. It is one of a few models showing the potential system in the long range (beyond 6 days).

The meteorological community is buzzing in regards to the potential for a major storm system this weekend, which could affect much of the Eastern United States. Local media has caught on to the potential and is beginning to hype up the potential storm system — as are several weather forecast outlets in the area. However, it is important to remember that the system is still floating around in what meteorologists like to call “Fantasy land” on forecast models. The range where, especially in a high amplitude pattern, models are prone to wild and wacky solutions. In this case, we’re seeing some of the wildest solutions in recent memory — and it’s causing the meteorological community to hype up the potential for a major storm. That being said, taking a look at some of the pieces of the pattern can offer us clues as to where the potential actually lies — aside from the wild model guidance images floating around the internet. We’ll try to answer some of the questions we’ve had emailed/tweeted/facebooked at us throughout the day today.

Why is there a storm threat, where is it developing? One of the major causes behind the threat for a significant storm is the development of a tropical system (Tropical Depression 18) and a large trough over the Central United States, which is then forecast to surge south and east and phase with the tropical system. A phase occurs when two or more pieces of the jet stream interact with one another. In this case, the Central US (northern stream) trough could surge south and east to phase with the tropical entity by next weekend.

Keep reading for a full discussion on the potential and hype…

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John Homenuk

Long Range: High latitude blocking could set off major changes

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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…

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