Tuesday 10 a.m.
The warm weather that made it feel more like September than November Sunday and yesterday is now being erased by a cold front that will move off the East coast this afternoon. There will be some snow showers downwind from lakes Erie and Ontario this afternoon, but a large high pressure area will settle into the Northeast and hold through the start of the weekend. This video shows more.
As we go into winter, we look at a variety of indicators for how cold or mild it will be and how much snow to expect. Looking at statistics, both El Nino and La Nina winters are more often warm than cold in the middle and north Atlantic states. This means by default the neutral years are most likely to be colder than average. Since the ENSO (El Nino Southern Oscillation) status has been close to neutral recently, this could be an important indicator of coldness if things stay this way.
When it gets snowy in the Northeast during the winter, we often see high latitude blocking patterns. The effect of these blocks is to force the jet stream south of its usual position. We were in a blocking pattern a couple of weeks ago as a trough from the west and Sandy from the south merged to make a monstrous storm. One indicator for blocking is warming high in the stratosphere. At the altitude where the barometric pressure is only 10 mb, we typically see a vortex (large low pressure area) centered near the North Pole. Sometimes, however, we see the vortex breaking down and being replaced by a weak high pressure area. Studies have shown that major cold snaps in the eastern two-thirds of the U.S. have occurred after that upper-air reversal.
For now, there is a strong vortex in place, as shown on this 10mb map as displayed on the University of Wyoming's weather web.
This map is a rainfall forecast from the NWS Middle Atlantic River Forecast Center located in State College, PA.
The upper-air flow forecast for this evening shows the trough that helps to support rainfall ahead of the cold front.
After reaching the 80s today from NYC to Boston, it might not be that warm again through much of next week.
A noticeable push of cooler air will spread southward from Ontario and Quebec into the eastern Great Lakes and New England between tomorrow and Saturday.
A cold front from eastern Canada will slide southward along the East coast between late Friday and the end of the weekend. For the area from Philadelphia to Boston, where temperatures will reach the summery 80s each day through Friday, it will mean a noticeable change to cooler weather.
Average high temperatures in Chicago and New York City are in the mid-70s now, but for the next several days, temperatures will run 8-15 degrees above those long-term averages. Supporting this warmth is a flow aloft that originates over the Southwest: