11/4/14: Signal for Big Early Season Chill Still Looks Good

On November 2nd, I commented (on Facebook) that there was a strengthening signal for a period of unseasonably cold weather across the Midwest, Great Lakes, Ohio Valley, and Northeastern US starting late this week and persisting for at least two weeks. As we move closer, that cold signal is still present on our models. Before we get into the specifics, let’s look at the large scale pattern expected to play out starting this week:

GEFS telleconnections

The GFS ensemble teleconnection graphs (above) are showing 3 of our major teleconnections all giving a cold signal for the eastern half of the US starting very soon. The NAO (North Atlantic Oscillation) is expected to drop negative this weekend, before slowly rebounding later next week. This corresponds to upper level high pressure over the Northwestern Atlantic. Upper level high pressure here causes the jet stream to “buckle” over the eastern US and drop south, bringing cold south with it. The AO (Arctic Oscillation) is expected to drop strongly negative this week, before slowly rebounding towards mid-month. This corresponds to upper level high pressure over the polar regions, which forces the jet stream south, and brings cold south with it. The PNA (Pacific North America) is expected to go positive tomorrow and remain positive for the next two weeks. This signifies upper level high pressure on the West Coast, causing the jet stream to go over that high pressure, before dropping south into the eastern US. All three of these “teleconnections” point to the jet stream dipping well south over the eastern US and bringing cold with it starting late this week, and peaking next week.

ECM ENS 120

The European ensembles (above) show a very similar pattern, with a +PNA, strong –AO (the position of that high pressure over NW Canada can also qualify as a –EPO too), and –NAO, with a large upper level trough with cold weather over the eastern US this weekend. The extremely strong low pressure over the Bearing Sea is going to be driven by a recurving Typhoon this weekend, and is what will drive the ridging over western North America.

This cold will come in a couple of waves; the first one this weekend, and the second one during the first half of next week.

ECM ENS 72

The first cool down (above) won’t be the most intense, as the strong –AO/-EPO, that will allow truly arctic air into Canada, will still just be getting established. Regardless, much cooler than normal weather will return to parts of the Midwest, Great Lakes, Ohio Valley, and Northeast by Friday. There is still some possibility of a low pressure system developing in time to give parts of northern and eastern New England some accumulating snow as this low passes by Thursday night into Friday, especially in the higher terrain. As can be seen on the left side of the image, the “trough” associated with the cool down is neutrally tilted (north-south orientation)…however, you can see two pieces of energy in this trough nearby, one just north of Lake Ontario, and one over western New England. If these two pieces of energy combine right, or “phase,” then a more significant winter storm would be possible for parts of northern and eastern New England. It may get cold enough for some accumulating lake effect snows in the eastern Great Lakes as well for a time Thursday night through Friday.

A clipper system may reinforce this cold air a bit on Sunday across the region, and perhaps bring some light snows to parts of the Upper Midwest, northern Great Lakes, and New England in the process. The big cold shot however will come during the first half of next week, and likely persist through most of the work-week. This is when the ridging over the western parts of the continent will be most enhanced by the recurving Typhoon over the western Pacific.

ECM 850 6-10

The European ensembles (above) show a large area of MUCH colder than average weather across all of the Upper Plains, Midwest, Great Lakes, Ohio Valley, Northeast and into the Mid Atlantic in the 6-10 day timeframe (next work-week). This type of cold would possibly spark off the first significant lake effect snows of the season off of all of the Great Lakes, however key details such as wind direction, which determines exactly who gets the lake effect snow, are still to be determined. The GFS, Euro and Canadian models are also suggesting that a winter storm may develop over parts of the Midwest/Great Lakes next Monday and Tuesday as this larger surge of cold initially dives into the Plains and Upper Midwest. This isn’t impossible, due to the cold air diving in to the west of the region (a storm often forms south and east of cold air diving into the US), so we will keep tabs on potential snow in those areas early next week too as the cold air dives down.

Lots to talk about, so keep checking back in! –Jim Sullivan

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