Return of the polar vortex: Cold, at times downright frigid for end of January; increasing snow chances

cwg_JunkerThe remainder or January is expected to be colder than normal, sometimes downright frigid.  The pattern is very similar to the pattern during the first week of January when temperatures plunged more than 20 degrees below normal in the so-called polar vortex event.   This time, the cold could stick around for a longer duration.

Temperatures should average well below normal starting around next Tuesday, lasting at least through the end of January.

For the period spanning January 21 (next Tuesday) through January 25 (next Saturday), similar patterns from the past – known as analogs – have resulted in highs not exceeding the mid-20s and lows falling into  high single digits to low teens on some days (colder north and west of the District).  Not every day and night will be that cold, but any warmer than normal period should be limited to a day at a time.

(WeatherBell.com)

(WeatherBell.com)

From January 25 through the end of the month, models suggest  it could be even colder.  Analog patterns (e.g. 2007, 1961, and 2009) all had minimum temperatures in the single digits.  Whether the coming cold will have any day quite that extreme is still uncertain but the pattern looks robust for sustained cold. 

Throughout this period, chances for snow are above normal.  I think light snow events are more likely, but cannot rule out more significant storminess.

Technical discussion

 Why are you expecting the colder than normal weather to last through the end of the month?  

The European model’s (ensemble mean) pattern at high altitudes (500 mb, or around 18,000 feet) through the end of the month exhibits a huge ridge of high pressure over western North America and a big dip in the jet stream over the East Coast.

Referencing the maps below, remember that the high altitude winds parallel the thin black height lines generally running from west to east.   Therefore, weather systems will be diving southeastward from Canada towards the East Coast during this January 21-31 period, with arctic air frequently  following them.

The maps below show the average pattern for the 5-day periods ending January 26 (top) and January 30 (bottom).  Long-range forecasters like using 5-day averages because the longer wavelength features, the ones easiest for the models to forecast, stand out.  In this case, the averages are indicating that the pattern changes very little during the period and that the “nefarious” polar vortex is displaced well southward of its normal position.

High altitude pattern (500 mb or 18,000 feet) for Jan 21-26 (top) and Jan 26-31 (bottom) (WeatherBell.com)

High altitude pattern (500 mb or 18,000 feet) for Jan 21-26 (top) and Jan 25-30 (bottom) (WeatherBell.com)

The pattern favors low pressure systems diving southward out of Canada. Often such lows still end up going to our north giving us a day of more moderate temperatures.  However,  the pattern is also favorable for clipper type systems that sometimes track to our south.

The temperatures around 5,000 feet (850 mb) average well below normal during each of the 5-day periods from January 22 through January 30 (shown below).   The GFS model’s (ensemble mean) pattern for the 5-days ending January 26 (not shown) looks almost identical to the European solution.  The similarities in the mean patterns from the two model groupings lend high confidence to the idea that the remainder of January will average colder than normal with a few really cold days.

Temperature difference from average at 5,000 feet (850 mb) for Jan 22-26 (top) and Jan 27-31 (bottom) (WeatherBell.com)

Temperature difference from average at 5,000 feet (850 mb) for Jan 21-26 (top) and Jan 25-30 (bottom) (WeatherBell.com)

Towards the very end of the month, the various simulations of the GFS model ensemble (GEFS) are not quite as severe with the cold,  but still look colder than normal.  However’s today’s operational run of the GFS model exhibits extreme cold for the period 11-15 days from now.  Throughout much of the U.S., temperatures average 20-30 degrees below average.  This is just one simulation of one run of one model, so reality may not be this severe.  But it fits into the overall idea of a colder than average pattern.

Temperature forecast difference from average for Jan 26-30 from GFS model (StormVistaWxModels.com)

Temperature forecast difference from average for Jan 26-31 from GFS model (StormVistaWxModels.com)

This general pattern shown above is bad news for California and their drought.  Having a strong ridge with the upper level jet displaced well to its north will shunt weather systems well north of the state.  Warmer and drier than normal conditions will rule across the west.

What about our snow chances? 

This is the most favorable pattern for snow that we’ve had all winter and it’s occurring right at the beginning of the period when snowstorms are most frequent climatologically. But it still is not a perfect pattern.

Occasional CWG contributor Matt Ross,  who lives in the District, notes  he has measured at least one 1″ or greater snowstorm during the 1/17 – 1/30 window in 13 out of the last 14 winters. The only winter he didn’t was January 2006 which was a an unusually warm January. Accumulating snow is likely by the end of the month based strictly on climatology and on the fact that temperatures will be on the cold side.

In the 11 analogs (patterns similar to this one) I examined, accumulating snow was often present, but only 3 snowstorms of over 4 inches were recorded and two of the 3 events occurred in the 1961 analog; the other occurred in 1995.  My own feeling is that clipper type systems with light to moderate accumulations are more likely than major winter storms.

However, today’s European model yielded a significant East Coast storm around January 25. So a scenario similar to the analog years of 1995 or 1961, when there was significant snow, is not an impossibility.

The high altitude patterns (at 500 mb or around 18,000 feet) for the 1961 and 1995 events are displayed below.

1961 (left) and 1995 (right) patterns that produced 4 inch or greater snowstorms

1961 (left) and 1995 (right) high altitude (500 mb or 18,000 ft) patterns that produced 4 inch or greater snowstorms

These snow analogs have their blue area (low pressures or heights) a little farther south of Greenland than the European ensemble mean patterns for the January 21-31 period shown above. The above normal pressures (red areas) across Greenland are also a little farther to the south. That subtle difference in pattern makes it easier to hold cold air for any storm

Snow lovers can point out that the GFS ensemble mean (not shown) for the period ending January 26 has a look more like the 1961 pattern so there is hope. 

So what’s the bottom line about snow?  Systems will be diving southward from Canada which will probably offer us clipper type lows making any storms more likely to produce light to moderate snowfall than heavy.  Could we get more than 4 inches from a storm? Yes, but like always, any such storm is a long shot.

RELATED: Nostalgia? The unintentional beauty of the polar vortex 

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