Tag Archives: Jet Stream

SO WHERE’S THE SNOW?

I should preface this post by saying that I enjoy trying to predict snow rather than dealing with it after it has fallen. Pretty when it falls, a pain afterwards.

That said, I’m being asked, ‘ so where’s the snow?’

I’ve mentioned since the end of November that my sense of the weather pattern so far this winter is what I like to call “a lack of very cold air” to our north.

Here’s the current GFS statistical ensemble forecast (GEFS average) expected minimum temperatures for next week—

GEFS statistical average minimum temperature forecast for next Friday.  (Click on image for a larger view.)

Well, maybe you’re thinking, -24ºF seems awfully cold to me? The truth is, when things get going, temperatures are as low as -40º to -50º in the same areas above, especially in Greenland. (Greenland is looking awfully warm this year.)

Contrary to what many weather entertainers on TV seem to suggest, the jet stream doesn’t just do it’s own, arbitrary thing. The jet stream, a fast flow of winds at about 35,000 to 40,000 feet in winter, is a river of air actually flowing in the three dimensional “valley” between air masses.

It is the density and shape of the cold air mass that determines the shape of the valley and of the jet, not the other way around. In essence, the jet flow shape in winter depends on the “sag” of very cold moving southward, as depicted below with the white line—

GEFS forecast Jet Flow (300 mb winds) next Friday (Click on image for a larger view.)


Notice that the white line is tilted slightly eastward instead of straight southward.  The relatively “not so cold air” up north doesn’t have the mass density to dive straight south.     The jet stream flow (red arrows) isn’t suppressed directly southward, but rather southeastward.  

There also hasn’t been much of a blocking effect of a large cold air mass over Greenland and the Northern Atlantic to cause the same jet flow to bend back north up the coastline. 

As a result the two “jet streak” impulses (numbers 1 and 2 above) develop low pressure systems, but these move out to sea.  

So we need colder air in Canada, and colder air in Greenland and the North Atlantic.

The time period depicted above (January 15th-20th)  does show some positive trends towards snow here:

  • Temperatures are colder in Canada than they’ve been.
  • The jet flow next week will be “less split” than it’s been.   It does appear that some phasing may occur between the northern jet flow shown (disturbance #1 above) and the southern jet flow (just visible on the map from Mexico and near #1)

So, some colder temperatures and some action is possible January 15th – January 20th, although not currently captured on the models.  Stay tuned

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WEEKEND WEATHER OUTLOOK

The snow-free winter continues…

A cold front moves through on Thursday and cold high pressure moves over us for Friday and Saturday. The National Blend of Models (NBM) is showing highs of 33° for both Friday and Saturday although some statistical models (ensemble models) show highs to be a bit colder.  Skies will be sunny both Saturday and Sunday.

The pattern is fast moving and progressive; temperatures rebound into the mid 40s on Sunday as winds shift to southerly with the departure of the high.

The Extended Range GFS model attempts forecasts 384 hours (16 days) into the future. No potential snow storms are currently predicted, except a possibility the last day in February.  Don’t change your plans.

This winter has been characterized by a lack of large, dense, cold air masses sinking straight south in the middle of the country.  

There is a  common misunderstanding, perpetuated by TV weather entertainers, that the jet stream determines where the air masses go.  Actually, it’s really the other way around— the size, shape and temperature-density of the cold air masses in winter determine the position, shape and configuration of the jet stream. The ‘valley’ between the polar air mass and the tropical air mass is where the jet stream is positioned and where major low pressure systems develop and track.

The current pattern has been with us for almost three months. Like bull stock markets, weather patterns don’t last indefinitely. Weather patterns tend to change as the seasons change.