After a very mild start to the month of November in much of the eastern half of the nation, the weather has been primarily colder-than-normal during the last ten days or so. In fact, numerous low temperature records were set last week and earlier this week in what was an unusually widespread and severe cold air outbreak across much of the nation. Looking ahead, it looks like it’ll be seasonal or even warmer-than-normal in the central and eastern in coming days as we close out the month of November, but numerous signs point to the return of a colder-than-normal pattern during the month of December.
The recent chill in the central and eastern US has eased just in time for the holiday and it looks to be close-to-normal temperature-wise in coming days or even slightly above-normal. As an example, high temperatures over the next several days are likely to fall in the 50’s in the DC-to-Philly-to-NYC corridor which is not too far off the normal for the latter part of November. Several signs, however, point to the possibility of the re-establishment of a colder-than-normal weather pattern in much of the central and eastern US as we proceed through the month of December which gets underway next Thursday.
Teleconnection indices (EPO, NAO, MJO), upper-level height anomalies, NH snow cover extent
Two teleconnection indices that we closely monitor this time of year include the East Pacific Oscillation (EPO) and the North Atlantic Oscillation (NAO). The EPO gives us an idea as to the temperature and precipitation patterns across the eastern Pacific Ocean and the NAO does so regarding the North Atlantic region – both of which can be crucial to the weather across the central and eastern US. Both of these teleconnection indices are forecasted to drop into “negative” territory as we head into the month of December and this outcome is generally a favorable one for colder-than-normal weather conditions to develop across much of the central and eastern US.
In the case of a “negative” EPO, the effect is usually strong high pressure ridging that pops up near or over Alaska and this, in turn, enhances the possibility that cold air masses from the polar region will have the ability to drop south and east through Canada and into the central and eastern US. In the case of the “negative” NAO, the effect is usually strong high pressure ridging that forms over Greenland, Iceland and/or northeastern Canada and this, in turn, also promotes the idea that cold air masses can work their way from Canada into the central and eastern states. Given the expectation of the EPO and NAO teleconnection indices to drop into “negative” territory, it is not too surprising to see several computer model forecast maps depicting the formation of strong upper-level (500 millibars) ridging during early December in these two particular locations of near/over Alaska and near/over Greenland, Iceland and northeastern Canada.
Another teleconnection index that we follow here at arcfieldweather.com is known as the Madden-Julian-Oscillation or MJO which tracks a tropical disturbance that traverses the globe on a repeating basis. This index value is tracked by meteorologists on a diagram that displays different phases from 1-8 with the MJO traveling through these in a counterclockwise fashion. The location of the MJO in any of the different phases can be well correlated with certain types of temperature and precipitation patterns around the world depending on the time of year. The MJO is forecasted to push into phase 7 later this month and perhaps into phase 8 thereafter which is a trend we’ll closely monitor in coming days. A push by the MJO index into phases 7 and 8 this time of year is often correlated with colder-than-normal weather across the central and eastern US.
One additional factor that is worth mentioning in a look ahead is the extent of snow cover across the northern hemisphere. It happens to be at its highest level on this particular date (11/21) since 2007. A higher-than-normal snow cover extent across the northern hemisphere is generally a favorable factor for the build-up of deeper and more widespread cold air masses in the source region which can ultimately make their way into the central and eastern US.
We’ll continue to monitor all of these signs in coming days to better determine if, in fact, a colder-than-normal weather pattern will get re-established in the central and eastern US following the milder stretch that is anticipated in the near-term.
Meteorologist Paul Dorian
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