***Multiple cold air outbreaks in an active pattern...some snow likely in southern Mid-Atlantic region on Friday...a possible east coast storm signal for down the road***

Additional cold air outbreaks in coming days assure colder-than-normal conditions on average for the ten-day time period from Wednesday, December 3rd to Saturday, December 13th. Map courtesy ECMWF, BAM Weather (X), Weather Bell Analytics

Overview

The month of December has gotten off to a cold start across much of the eastern half of the nation and there will be multiple cold air outbreaks in coming days. Two signals that have foreshadowed a cold period include an unusually early season stratospheric warming event and the movement of a tropical disturbance into a position that favors colder-than-normal weather across much of the nation. In fact, there appears to be a second burst of stratospheric warming in the offing for later this month with a “stretched” polar vortex, and the tropical disturbance will likely stay in the same “cold” position” for awhile...both of these favor the notion of additional cold air outbreaks as we progress through the month.

The cold pattern has been quite active as well and multiple storm threats are likely during the next few weeks. On Friday, low pressure will develop over the Tennessee Valley and head to the northeast, and it is likely to produce some accumulating snow in the southern Mid-Atlantic region later in the day. Another storm system may threaten the southern Mid-Atlantic region during the early part of next week. Later next week, it appears multiple “clipper” systems will have to be watched as they’ll drop southeast from southwestern Canada into the Great Lakes region and then to the Northeast US. Finally, the change in recent days of an index value known as the “Southern Oscillation” is raising a red flag to me that the chance of an east coast storm will be on the rise somewhere down the road with a likely activation of the sub-tropical jet stream.

Computer forecast models continue to have trouble seeing the cold air outbreaks more than a few days in advance...this "delta" in temperatures is quite dramatic for the 5-day period of Dec 12-Dec 17 between yesterday morning's 12Z (02 Dec) Euro EPS model run and last night's 00Z (03 Dec) run...much colder trending for most of the US and Canada. Map courtesy BAM Weather (X), ECMWF, weathermodels.com

Near-term Arctic cold…numerous records may be broken on Thursday and Friday

Temperatures this morning across the northern states from Montana to Wisconsin are below-zero in many areas and this Arctic air mass will head southeast to the Mid-Atlantic region and Northeast US by later Thursday night and Friday. In fact, there may be numerous low temperature records set on Thursday and Friday across the northern US from the Upper Midwest to the Northeast US. Temperatures early Friday are likely to bottom out at the lowest levels of the season so far in the I-95 corridor with the middle or upper teens in many suburban locations from DC-to-Philly-to-NYC (single digits in and around Boston). Daytime highs on Friday will be well below-normal for this time of year with the low-to-mid 30’s likely in DC, Philly, NYC as clouds thicken up (mid-to-upper 20’s in the Boston metro region).

There has been an unusually early Stratospheric Warming (SW) event across the polar region of the Northern Hemisphere and it appears quite likely for another “burst” of warming later in the month of December. This second SW event is a favorable sign for additional cold air outbreaks from Canada into the US as we progress through the latter stage of December and into January. Map courtesy NOAA, Weather Bell Analytics

Stratospheric Warming, Madden-Julian Oscillation

Two factors that have been pointing to an extended cold stretch this month include the unusually early stratospheric warming event that took place over the polar region of the Northern Hemisphere, and the movement of a tropical disturbance (MJO) to locations (or “phases”) which favor colder-than-normal weather in the eastern half of the nation. In fact, these two factors are “doubling-down” on their foreshadowing of colder-than-normal weather as we progress through December with a second burst of stratospheric warming likely later in the month resulting in a “stretched” polar vortex, and the rather persistent positioning of the tropical disturbance (MJO) in a cold “phase” 8 into the second half of December.

A tropical disturbance known as the Madden-Julian Oscillation (MJO) has moved into a cold location or “phase” 8 (left plot) in recent days and when this is combined with a weak La Nina - as we are currently experiencing - a 500 mb trough typically sets up across central Canada and the central US with “blocking” to the north over the polar region. Plot (left) of MJO courtesy NOAA, ECMWF; plot (right) of 500 mb height anomaly courtesy NOAA, BAM Weather

Snow threats late this week and perhaps again early next week

In terms of storms, the overall pattern has been quite active in recent days with a significant Great Lakes snow event late last week, a weekend snowstorm across the Midwest, and then an interior Mid-Atlantic/Northeast US snowstorm on Tuesday. All indications are that this active weather pattern will continue well into the month of December with several threats likely in coming days.

Low pressure could bring some accumulating snow to at least the southern portion of Mid-Atlantic later Friday (i.e., southern and central Virginia, Delmarva Peninsula), and perhaps as far north and east as DC, Philly and NYC…likely a light-to-moderate event. Map courtesy ECMWF, Weather Bell Analytics

On Friday, low pressure will develop the Tennessee Valley head northeast into the very cold (and dry) Arctic air mass that will be in place across the Mid-Atlantic region. Snow is likely to break out across the southern Mid-Atlantic region later in the day on Friday in such places as southern and central Virginia, and the Delmarva Peninsula. The snow shield on Friday certainly has a chance of extending north and east into the DC, Philly and NYC metro regions...overall this looks like a light-to-moderate event. Looking ahead, another storm system could become a threat in the same southern Mid-Atlantic region by the early part of next week and then multiple “clipper” systems will have to be monitored later next week as they drop southeastward from western Canada into the Great Lakes/NE US.

The Southern Oscillation Index (SOI) has dropped into “negative” territory in recent days (left table lists daily values) and in the month of December, this is usually associated with an upper-level trough over the central US (shown in blue/green) and “blocking” across Canada (shown in yellow). Table (left) courtesy Queensland Government Australia; Plot (right) courtesy NOAA, BAM Weather

Southern Oscillation Index and a possible east coast storm signal down the road

Another factor of interest that I am monitoring closely is known as the Southern Oscillation Index (SOI). This particular metric evaluates pressure differences in the Southern Hemisphere (specifically between Darwin, Australia and Tahiti) and it provides an assessment of the strength of the El Nino Southern Oscillation (ENSO) phenomenon be it El Nino (warmer than normal water) or La Nina (colder than normal water) which we are currently experiencing in the equatorial Pacific. When this index value falls into negative territory for a sustained period of time, it usually indicates El Nino is strengthening or, in this case, it suggests La Nina conditions are weakening. The movement of the Southern Oscillation Index value in recent days into deeper “negative” territory suggests to me that indeed La Nina is about to undergo some weakening, and this should, in turn, should activate the sub-tropical jet stream which, in turn, should raise the chance for an east coast storm system somewhere down the road...we’ll see how all of this plays out in coming weeks.

Meteorologist Paul Dorian
Arcfield
arcfieldweather.com

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