Welcome to Meteorological Winter

Winter is here.

I caught up on Game of Thrones this year so you must forgive the countless references to “winter”.  Meteorological winter is indeed here.  Meteorological winter runs from December 1 to February 28 as it is the climatological coldest 3 months of the year.  The past few years December has been warmer than average (save for the mid-December Arctic Assualt of 2016).   My family played football outside in t-shirts on Christmas in 2014 and 2015.  I must also insult your memories because when it comes to the weather they are poor.  It wasn’t that long ago snow and December went hand and hand.  But that is why I am here!  Most of you want snow on the ground on Christmas Day but that is still over 3 weeks away.  Allow me to set the table with some December climatology and history before digging into what to expect this year.

Climatology 

The past 11 years show the wonderful December’s of 2007-2010 as well as good ones inland in 2012, 2013, and 2016.  The 11-year average is above average for all 4 of the Southern New England climate sites, albeit just barely.  If I cut out 2006 and go with the 10-year average SNE is above average for December snow.   The 5-year average is well below average.

Not surprisingly, the past 5 Decembers when averaged together are much warmer than normal.  2014 and 2015 really make this stand out as December 2015 is the warmest on record in Southern New England.  It is one of the most stunning months meteorologically I have ever seen along with March 2012 and February 2015.

NOAA ESRL December Temp Anomaly 2012-2016

Here is 2007-2011.  Just about average in the east, but colder than normal in the Midwest.

NOAA ESRL December Temp Anomaly 2007-2011

So with all that said, none of it matters when forecasting for December 2017.

Meteorological Factors for 2017

There is a La Nina in the tropical Pacific Ocean as we enter winter.  That means there are cooler than normal sea surface temperatures along the equator.

OSPO SSTA 11.30.2017 (image NOAA)

Other things to note are the warm waters to the Northwest of Hawaii, the cool pool in the Gulf of Alaska, and warm water off the Northeast coast.    This is a major difference from the super warm December 2015.  Notice the El Nino that year.

OSPO SSTA 11.30.2015 (image NOAA)

The jet stream in 2015 flooded the United States with mild Pacific air.  El Nino’s are easier to forecast long range for.  La Nina’s are a bit more difficult because history tells us both cold and mild winters are possible when the atmosphere is under the influence of the cooler than normal waters.  Looking at the current SSTA orientation, I believe a ridge will develop in the central Pacific, a trough will develop in the Gulf of Alaska, a ridge in the Western US, and a trough in the East/Central US.  The wildcard is the Atlantic ocean ridge which will be able to pop and perhaps force storm systems inside the Appalachian Mountains until the waters can cool off a bit off the East Coast.

You may be thinking…uh what?

Here is this coming Tuesday, at 7 PM.

12z GFS 500 mb anomaly Tuesday 7 PM (image Weatherbell)

Notice the spacing between troughs (blues) and ridges (reds).  It will be a mild start to December in the east before troughing becomes established towards the end of next week/next weekend.  Here is Friday at 7 AM.

12z GFS 500 mb height anomaly Friday 7 AM (image Weatherbell)

The big ridge out west allows Arctic air to spill into the Central and Eastern US via Central Canada.  Here are the temperature anomalies for next Friday.

12z GFS 850 temp anomaly Friday 7 AM (image Weatherbell)

I think it is important to note that the coldest air will not be centered over New England, but it will be cold enough to get the lakes frozen.  Compared to average, it will be much warmer early next week than cold at the end of the week.  That said, this air mass will help build the snowpack in Southeastern Canada which will allow the next Arctic burst to be colder.   It is unwise to forecast below average temperatures in this clear era of above normal temperatures.  However, the signs are there that December 2017 has the potential to be the coldest and snowiest in the past 5 years or so.

I’ll have an updated winter weather outlook on Monday taking a look at snow chances/temperatures.  Enjoy your weekend.

-Zack Green

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