Polar vortex déjà vu: How cold and for how long?

Starting around next Tuesday, a long duration cold wave will envelop Washington, D.C. and much of the eastern two-thirds of the nation. We cannot yet say if it will be as intense as the cold around January 6 and 7, but we have reasonable confidence that once the cold gets here it will stay relatively cold for a lengthy period of time.

In short, this cold wave may be more memorable for its persistence, rather than its intensity.

How long a cold wave are we talking about?

Averaging together the different simulations of the GFS model (ensemble), it shows the D.C. area staying below freezing for an incredible 9 straight days – from January 21 to 30.

Forecast highs and low by average of GFS model run simulations (ensemble) over next 16 days.  (The u symbol indicates the average temperature for each day.) (WeatherBell.com)

Forecast highs and low by average of GFS model run simulations (ensemble) over next 16 days for Washington, D.C. (The u symbol indicates the average temperature for each day.) (WeatherBell.com)

When you average together a bunch of simulations like this – you tend to smooth out both the extreme high and low end of possible temperatures. In other words, it’s likely on some days temperatures could be warmer or colder than shown above.

Staying below freezing in D.C. for nine straight days would put this cold wave in historic territory, so I’m somewhat skeptical we won’t have a day or two in this span where we get into the mid-to-upper 30s or even briefly into the 40s. On the other hand, this average of simulations probably also doesn’t do a great job conveying how cold it may get on the coldest days.

The bottom line is that the period from January 21 to January 30 looks extremely cold, overall. After that, temperatures may moderate a little before the cold pattern possibly recharges around/after Groundhog Day (though this is a low confidence outlook).

How cold might it get?

The first several days of this cold wave -from January 21 to around January 25 – will be cold but won’t share the intensity of the polar vortex event of January 7 and 8. In other words, expect highs in the 20s to low 30s, with lows in the teens (perhaps single digits on the coldest mornings). (Readings would be slightly colder than this north and west of the District.) These temperatures would be about 10-20 degrees below normal.

The potential for a more severe cold snap exists between around January 26-28. That’s around the time when both the operational GFS and European model show a huge piece of the polar vortex crashing south into Canada with tentacles extending deep into the U.S., not dissimilar to the January 7-8 event.

The European model shows a piece of the polar vortex centered over eastern Canada with its influence extending deep into U.S. on January 26. (WeatherBell.com)

The European model shows a piece of the polar vortex centered over eastern Canada with its influence extending deep into U.S. on January 26. (WeatherBell.com)

The GFS model shows a piece of the polar vortex centered over eastern Canada with its influence extending deep into U.S. on January 26. (WeatherBell.com)

The GFS model shows a piece of the polar vortex centered over eastern Canada with its influence extending deep into U.S. on January 27. (WeatherBell.com)

However, different simulations of these same models – with the data going into them tweaked – don’t all predict such an intense event. Thus, all we can say right now is that period from around January 26 to January 31 is likely to be cold, possibly downright frigid, but we can’t yet give specifics.

What about snow?

As Capital Weather Gang’s winter weather expert Wes Junker said yesterday, there is likely to be some light snow in this stretch – both because it’s going to be cold enough and it frequently snows in D.C. in this part of January.

Models continue to simulate a few clippers scooting through the region in this cold January 21-31 period – offering quick-hitting opportunities for light snow – but not a lot of accumulation. These clipper all originate from the northwest in Canada and have little moisture to work with.

If we’re going to get a more significant snowstorm, we need to involve some Gulf of Mexico or Atlantic moisture. A time period to keep on eye on is around January 24 or 25, when a possible disturbance tracking across the south could tap some of these moisture surfaces. If this disturbance can team up (or phase) with a one of the clippers coming in from the northwest and further develop along East Coast, then the snow excitement factor would increase. But such a scenario requires the right timing of these different features so they can come together. For now, models are waffling back and forth on what will happen, so it’s wait and see.

European model simulates two separate weather systems on January 25.  If they were to combine in our vicinity, it might make for a significant storm.  The latest simulation keeps them separate. (WeatherBell.com)

European model simulates two separate weather systems on January 25. If they were to combine in our vicinity, it might make for a significant storm. The latest simulation keeps them separate. (WeatherBell.com)

Of course, if we can build-up some substantial snow cover over the region, that will increase the likelihood of severe cold – as snow acts like a refrigerator, helping arctic air masses from the northwest maintain their character.

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