Mid to Late February Pattern Relaxation…But is That It?

The winter regime that has been dominant since November will be breaking down over the course of the next 5 to 7 days. However, does that mean winter is ending? I will be providing evidence to the contrary in this discussion. First, let’s not ignore the storm threat immediately in front of us. After a period of light snow later today (on the order of coating to 1″ amounts for most), most of the models are signaling the potential of a phased system Wed-Thurs along the east coast. The mid level energy associated with it is quite potent, but the lack of downstream NAO blocking could be problematic in terms of precipitation type for our area. Nevertheless, there is a signal for a strong surface low riding SW-NE near the coast mid/late week. Whether the precipitation falls as snow, mostly snow, rain to snow, or rain, is uncertain at this juncture. As always, the track of the low is crucial. Polar air will be rapidly retreating mid week, after another shot of bitter chill arrives Mon-Tues, with overnight lows Wednesday morning probably widespread in the single digits across suburbia. Snowpack and clear, calm conditions will aid the temperature plummet. Beyond mid week, the airmass becomes “stale” cold, and our Wed-Thurs system must intensify such that it can cool the lower boundary layer dynamically. A weak low is not going to produce accumulations in the mid week pattern. We need a strong storm to our east. There’s time to work on this situation, but let’s move along to the pattern thereafter.

This has been the 500mb / mid level pattern since December 1st. As anticipated the negative EPO / NPAC high pressure feature has been the mainstay of the winter, providing a funneling mechanism for Siberian cold into North America. The NAO has been predominately positive, and the AO, though strongly positive in December, has been closer to neutral values for the heart of the winter.

Some of the analogs noted at the beginning of the winter have held quite well. Below is the 500mb composite for December 1st-early Feb of 2008-09.

Here’s the 500mb composite for Dec-early Feb of 1983-84, another strong analog from the outset of this winter.

And finally, the 500mb composite for the 1993-94 Dec 1st to early Feb period.

Notice the same features were present in those seasons, namely the negative EPO / Alaskan block, some height rises into the arctic, a mainly +NAO signal, and the trough centered in the Mid-western US with the SE ridge just offshore of our coast.

Going forward, it’s interesting to see the similarities in the evolution of some of those Februaries in comparison to the current pattern progression.

By late this week (14th/15th), the EPO should reverse to strongly positive values, for the longest duration that we have seen thus far this winter. Synoptically speaking, this means a retrogression of the arctic vortex into Alaska and a development of Pacific-maritime air flow into the CONUS. In other words, say goodbye to the arctic air across the USA for the February 15th-20th period, possibly longer, through the 25th of the month.

Take a look at the similar breakdown that occurred in the years of 1994 and 1984 with the +EPO development in mid February.

1994 – Feb 10th to Feb 20th, +EPO, +NAO, +AO.

1984 – Feb pattern – similar progression, into a +EPO and continued +NAO, yielding a monster torch in the Northeastern US

2009 was a slightly warmer than normal February, following the bitter January of 2009, and 1984 was a blowtorch February following the bitter January of that year. 1994 however ended up colder than normal in February even with the pattern breakdown Feb 10th-20th. Where will we end up this year?

Well, given the progression from late week into the second half of February, my confidence is high we will see a moderation to normal / above normal temps for probably a 5 to 10 day period, which would take us into the last several days of February. Will be interesting to see how much that warm period erases the cold departures we have built up thus far. But the analogs have always indicated that February would be warmer than January relative to normal. I wouldn’t expect a prolonged torch like 1984, but a warmer than normal period is probable for the latter part of this month.

Beyond that, how did those analog years trend into late Feb and March?

Well, 1994 was colder than normal in the Northeast, 2009 was near normal, and 1984 was very cold across most of the northern USA into the Northeast. So none of my main analogs featured a warmer than normal March.

Note the return of the negative EPO and winter regime for March 1st-15th of 1984:

March 1994 put the trough back in the Northeast via a +PNA pattern.

It’s interesting to note that the warm pool of SST’s was present south of Alaska in both February of 1984 and 1994, in fact the entire winters.

Note the current SSTA profile continues to have very warm SST’s south of Alaska, suggestive of the resurgence of the -EPO pattern like 1994 and 1984.

2009 had colder SST’s immediately south of Alaska, which probably aided in that March’s less impressive return to winter in the Northeast.

Conclusion:

1. Light snow later today, minor accumulations.

2. Severe cold Wednesday morning with an arctic high, but the airmass fades quickly east.

3. Storm threat Wed-Thurs with a strong surface low – could be rain or snow depending upon track (stay tuned!)

4. Pattern relaxation of normal to above normal temps for the 15th-25th period of February.

5. Suggestion by analogs and longer term guidance of a likely return to the -EPO regime (late winter style) for the end of Feb into the first half of March. Right now, a warm March does not appear likely.