Tuesday, December 09, 2014

Don't Be Fooled. Lots of Winter Left!

Winter arrived very early this year across much of the US. Per NOAA, the average contiguous U.S. temperature was 39.3°F, which is 2.4°F below the 20th century average. This ranked as the 16th coldest November in the 1895-2014 record, and was the coldest November since 2000.

US snowcover for the month was the most since 1966, a 48% increase from last (2013) November's snowcover.
November temperatures in northern Ohio were the 8th coldest on record at 37.1 degrees with a stretch of 9 days below 40. November snowfall was the most since 1996. We are ahead of last year  when we count days with at least ONE INCH of snow cover at 12 (only 9 as of Dec 9th last year).
Many people falsely believed that the arctic cold would continue into December. Look at the top 16 coldest Novembers (16th is this year) in the chart below. Then look at the Decembers that followed. The temperature ranks are no where as cold relative to normal. The Decembers that followed had an average temperature rank of....48th!  Only 2 cracked the top ten coldest.

Here is a comparison of November vs December through the 9th.  December temperatures are not as cold relative to normal. The coldest air has lifted back into central Canada.

So what does this mean looking into our winter future?

All of the factors that went into our winter weather outlook are still present and valid. Nothing has changed.

1) Central Based El Nino has developed or is developing depending on which index you use. This central based Pacific El Nino favors colder than normal temperatures across the eastern US. Note the warm pool along the equator and the smaller cool pools flanking it. The cool pools are indications of a Modoki along with warm pool extending along the west coast of North America.
Are their similar years to what we see above?  Yes.  Below is a blend of those similar years. The scale is a bit different but the warm and cool pools are in similar locations and intensities. They're not exact matches but they are close.

2) Warmer than average water off of the west coast along with the cool pool north of Hawaii strongly favor ridging in the western US, troughiness in the eastern US. The Pacific North American Index (or PNA Index) measures this tendency. In most Decembers where the PNA is more than +1, the temperatures across the eastern US are usually below normal.

 The forecast continues to favor a POSITIVE PNA into mid December.
3) The wild cards are always how the Arctic and North Atlantic will behave.  The Arctic Oscillation and North Atlantic Oscillations are indices that measure changes in the pressure patterns over each location. The AO is more of a player earlier in the winter, the NAO during the second half. If the indices are negative, the pressure patterns favor a more variable jet stream with a tendency for more cold air outbreaks.

4) The QBO (another wild card), read about it here, are stratospheric winds over the tropics which alternate direction easterly to westerly every 2 years. These easterly winds have been shown to disturb the Polar Vortex over the arctic making it more unstable. The propensity for cold outbreaks is higher across North America but not guaranteed. We monitor warming in the stratosphere over the arctic. Sudden warming can be a sign of a weakening of the jet stream and colder outbreaks. A good description of these warming events is on this site. Check out this site for more on Sudden Stratospheric Warming events and their potential effects on our winter weather.

Each one of these variables should not be overstated. One factor doesn't make-it or break-it. Our outlook is a blend of the combined influences that each one of these variables bring to the table. All told, our Winter Weather Outlook from early November still stands