December: Above Average Temps; Below Average Snow

December 2014, goes down in the record books with above average temperatures and below average to well below average snow.  After a very cold November, the weather pattern reversed and mild weather returned. A reversal of the cold had been forecast, but the persistence of the mild pattern turned out to be several weeks longer than initially expected.  For a good part of the month we had an enhanced Pacific Jet from the east coast of Asia to the west coast of North America.  This was due to the establishment of an upper low in the Gulf of Alaska and over the Aleutian Islands.  At high latitudes, blocking high pressure was centered over the Laptev Sea and lower pressures were over the Beaufort Sea.


Sea this chart for Arctic area geography:


This directed Arctic air east across the Canadian Archipelago to Greenland and the Norwegian Sea.  Mild Pacific and downslope flow crossed west-east across Canada . This kept temperatures mild in our normal cold air source region of NW Canada.

We also had a very strong jet stream and “polar vortex” at the stratospheric level as shown by this image.  Purple/grey tones show the coldest air and greens/yellows/reds show warmer (although still well below zero) air.  The air flows left to right along the white lines.  This type of setup helps contain the cold in the Arctic by assisting in the maintenance of a strong jet stream.


Check out this surface temperature map for December 21st which shows temperatures in the teens and 20s as far north as the northern borders of Saskatchewan, Manitoba and Alberta.   The cold is very well contained.


This really limited access to the cold air for the lower 48.  The Pacific flow was also a cloudy flow with prevailing cloud cover for days on end. This late afternoon visible satellite image from mid month was a typical one for the month.


Oftentimes, clear skies were just a few thousand feet up, as this sounding from Topeka Kansas shows.  Soundings measure the temperature, wind, pressure and moisture as they ascend through the atmosphere.  This plot shows the structure of the atmosphere as the balloon went up.  The red/green lines together showed a saturated atmosphere through around 3,300 feet, with much drier air just above where the lines become widely separated.


The wind barbs on the left tell the other story..very light winds aloft.  The weak December sun is unable to mix the atmosphere very efficiently and you’ll need some sort of mechanical mixing (wind) to bring the drier air into the cloud layer to break up and dissipate the clouds.  That didn’t happen. Those clouds kept days cooler and nights much warmer and the net result was a warm month in the record books, despite not having a lot of clear and mild days.

As the month drew to a close, warming at the stratospheric level was pushing northward and had stretched and deformed the “Polar Vortex” weakening it in the process.


That allowed the enhanced Pacific jet to weaken and retreat back toward Asia and the jet stream to return to a pattern seen in November with a polar low near Hudson’s Bay a ridge of high pressure over the East Pacific and Alaska and a cold northwest flow to return to the U.S.


That helped bring down a 1050MB+ Arctic high into the U.S. east of the Rockies as this chart for December 30th shows.


The surface high pressure area built down the Front Range of the Rockies deep into west Texas with a lesser push to the east. These 6AM temperatures from 12/30 show the cold air (deep blue=32F or less) deep into TX with the shallower eastern edge of the cold banked up against the Ozark Plateau of E OK/AR.


Even the late month chill.. which was not record setting for the district.. did not do enough to take the edge off of the warm month.   The chart below shows the temperature difference from average for December (in degrees C). Most of the U.S. finished above average with only isolated pockets of cool over the CO Rockies/NC outer banks and S FL.



In terms of precipitation — it was a big month.. for the West Coast (rain) and the inland northeast (snow).  For us it was a far cry from December 2013, which opened with big snows for SEMO/SW IL and ended with big rains.  We did see several weather systems of interest which brought rains. The wettest system arrived as the month opened with moderate to heavy rains Dec 4-6:


Another and more modest system brought rains mid month:


Due to the warmth, snow was lacking, although there was minor accumulating snow in the KC area the weekend before Christmas from a mid to upper level system.  A quick burst of snow for SEMO/SW IL had a hard time sticking due to marginal temperatures aloft.

2014-12-18-0240-SATSFCRAD 2014-12-18-0951-NORSCTTLSNWD

No major winter storms impacted the area and ALL areas of the district fell short in terms of snow.

These are the observed precipitation maps for the region for December.  First, the actual totals:


Most significant rains were over east central KS and west central/SE MO and S IL.  Rainfall decreases west and north. This map is a little misleading though.  SEMO/SW IL have a higher average rain total than points W & N so we’ll take a look at the percent of average rain:


The warm tones show areas with below average rainfall, green and blue above average.  There is a bull’s-eye of above average rains over south central IL and wetter than average conditions in west central MO and a good part of east central KS.  A majority of MO/IL had well below average some areas 25-50% of the monthly total.

A wider view shows most of CA had a very good month rain-wise as did the western Plains, Missouri Valley and Northern Rockies.  Much of the Great Lakes, Midwest, Southern Plains, Panhandles, TX, the Ohio Valley and the Northern Plains came up short and in some cases very short. Red colors indicate monthly precipitation which was only 10-25% of the long term average for December.


Here are the monthly stats:

DEC 2014    AVG HIGH  AVG LOW  MEAN     +/-

ST LOUIS:     44.60    33.10    38.90  +4.20
KANSAS CITY:  40.60    28.10    34.30  +2.80
TOPEKA        42.80    29.80    36.30  +4.30    

DEC 2014      PRECIP  AVERAGE    +/-   SNOW
ST LOUIS:      2.72"    2.84""  -0.12" 0.2"
KANSAS CITY:   1.83"    1.53"   +0.30" 2.6"
TOPEKA         2.27"    1.35"   +0.92  2.0"

The US National Weather Service December outlook issued mid November indicated no strong signal for either above/below average temperatures and precipitation.



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