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Goodbye autumn. Hello winter. | Weather

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Goodbye autumn. Hello winter.

Goodbye autumn. Hello winter. The solstice occurred at 12:30 this morning.  It’s now officially winter.

Maybe that’s been the problem all along with manufacturing snow around here.  It was still fall.  Hmmm.

Well, it would take a colossal (and unexpected) shift in the weather patterns in these final days of the month for December in Rochester and the Finger Lakes to not turn out warmer than normal with below normal snowfall.  What a contrast to the constant cold and record 46.5 inches of snow Rochester received last December.

When we turn the calendar page to January, we will enter the statistically coldest month of the year.

As such, it does not take perfectly aligned and timed weather features to manufacture snow in Rochester.  Sometimes “imperfect” lows can offer snowy surprises.  Sometimes good old’ Lake Ontario can do the trick on weak onshore breezes.

Well, it may take a bunch of imperfect lows and weak lake breezes getting involved to get seasonal snowfall tallies here back up to  normal anytime soon as the pattern is not likely to have changed much when we flip the calendar.

All indications at this point keep the AO/NAO couplet (refer to our first post on definitions of key long range weather terms) in the positive state through the first week of the new year (at least).  This will prevent particularly bitter cold air from dropping south from the arctic into our region with any staying power.

Instead, there will be transient incursions of moderate chill and snow alternating with spells of relative warmth.

So, there are no changes to previous thinking regarding the rest of this month and we are extending that thinking through the first week of January:  while there will be transient incursions of cold air and snow, the temperature for the last week of December and the first week of January will average above normal while snowfall will be below normal. There actually appears a possibility that the first week of January will be milder with fewer snow chances than the last week of December.

Painful news if you are a winter weather enthusiast to be sure.

So what happens after the first week of January?

There continues to be lots of meteorological chatter about a stratospheric warming event.  

To review, the troposphere is the lowest layer of the atmosphere that ranges in height from 25,000 feet at the Poles to 60,000 feet at the Equator.  It is where weather systems reside and is marked by a lowering of the temperature with height.  The stratosphere is the next layer of the atmosphere (separated by the tropopause).  It is marked by a gradual increase in temperature with height.  

When the stratosphere warms, the air expands and its thickness increases.  This forces the tropopause downward.  This lowers the thickness of the troposphere which corresponds to a cooling of the air below.  

When this process occurs near the North Pole this newly cooled air sinks south and often correlates to a change in the state of the AO from positive to negative and often starts the process of high latitude blocking.  This, in turn, causes the delivery of arctic cold into the continental United States which helps to buckle the polar jet stream and induce storm formation.

As you may imagine this process takes time, usually several weeks, for any resulting change in the pattern in the U.S.  It should also be noted that not all stratospheric warming events are created equal as some lead to the development of a major –NAO and some do not.

Regardless, an event is underway and should start to make its presence felt in the way of developing a –AO in the middle of January (the intensity is not reliably predictable at this stage).  It is then that we could start to see an important shift in the jet stream pattern associated with a –NAO that delivers harsher winter weather in the eastern half of the U.S.

So, it would seem that IF Rochester is to experience any truly long-lasting, very cold and very snowy conditions this winter, they will not come before mid January (unless the AO/NAO were to flip negative earlier than expected).  Until then, winter weather fans will have to be content with fleeting periods of snow and cold.  Additionally, we remain of the opinion that the most wintry month this season is likely to be February.  

Regardless of what’s on the weather map when you read this, I wish you and yours a sunny new year.


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