December 2012 in the Top 10 Warmest Decembers; What Happened To Analogue Projected Cold December?

Back in late November, I wrote a blog on this winter's analogue years and what the predominate call was for December.  Clearly the predominant below normal trend called for by the analogues for December was wrong as one can see below it was the opposite; December ended up in the ten warmest Decembers on record at all three climate sites.

Top 20 Coldest/Warmest Decembers in Southeast Lower Michigan
Rank	Detroit Area*				Flint Bishop**				Saginaw Area***
	Coldest		Warmest		Coldest		Warmest		Coldest		Warmest
	Temp	Year	Temp	Year	Temp	Year	Temp	Year	Temp	Year	Temp	Year
1	17.8	1876	40.6	1881	16.1	1989	37.2	1982	16.2	1989	35.3	1923
2	18.0	1989	39.2	1889	16.6	2000	36.8	1923	17.2	2000	34.5	1982
3	19.2	2000	38.1	1877	17.4	1976	35.4	2006	18.8	1983	34.4	2006
4	20.8	1983	37.7	1923	17.8	1958	35.4	1965	18.9	1976	34.2	1931
5	21.5	1976	37.4	2006	20.1	1983	34.7	1931	19.0	1958	34.1	2012
6	21.8	1917	37.3	1982	20.5	1944	34.1	2001	19.1	1917	33.5	2001
7	21.8	1880	37.2	1931	20.8	1963	33.6	1941	19.5	1919	33.4	1941
8	22.2	1985	36.3	1918	21.3	1985	33.5	2012	20.1	1963	33.1	1936
9	22.4	1958	35.9	2012	21.7	1960	33.4	1984	21.0	1924	33.0	2011
10	22.5	1903	35.9	1965	22.2	1945	33.3	2011	21.3	1944	33.0	1965
11	22.8	1878	35.8	2001	22.4	1950	32.8	1939	21.5	1960	32.4	1911
12	23.0	1909	35.6	1891	22.9	1962	32.4	1971	21.6	1985	32.3	1994
13	23.1	1919	35.5	2011	23.0	1995	32.3	1998	22.0	1980	32.2	1913
14	23.2	1924	35.5	1941	23.1	1942	32.2	1987	22.0	1914	32.0	1998
15	23.3	1910	35.4	1994	23.2	1924	32.1	2003	22.2	1962	32.0	1918
16	23.8	1963	35.3	1998	23.9	1937	32.1	1979	22.2	1903	31.5	1959
17	23.8	1914	34.8	1956	24.0	1926	32.0	1994	22.4	1926	31.4	1987
18	23.9	1886	34.6	1959	24.3	2010	31.7	1959	22.5	1950	31.4	1952
19	24.7	1945	34.3	1913	24.3	2005	31.7	1952	22.5	1945	31.4	1920
20	24.8	1944	34.1	1957	24.4	1980	31.5	1956	22.5	1942	31.2	1912

DETROIT...DECEMBER 2012
DY	MAX	MIN	AVG	DEP DAY
1	57	37	47	12
2	57	50	54	19
3	63	47	55	21
4	63	41	52	18
5	43	29	36	2
6	40	27	34	1
7	43	36	40	7
8	42	35	39	7
9	40	35	38	6
10	42	33	38	6
11	36	26	31	0
12	43	24	34	3
13	44	24	34	3
14	48	29	39	9
15	46	32	39	9
16	53	45	49	19
17	45	39	42	13
18	43	34	39	10
19	40	34	37	8
20	46	35	41	12
21	35	30	33	5
22	35	27	31	3
23	38	21	30	2
24	35	24	30	2
25	33	22	28	0
26	32	27	30	3
27	32	17	25	-2
28	30	13	22	-5
29	31	22	27	0
30	30	15	23	-4
31	36	14	25	-1

So what happened? Why did much of the month turn out so mild and the projected pattern so wrong? It really wasn't until the last week or so of December that our temperatures settle down to "normal to slightly below" (see Detroit's December climatic temperature data above).

Oh, the cold pattern was in fact there; in Canada. Much of Canada and Northeast Pacific was cold during December as the cold Polar/Arctic air remained bottled-up north of the border and out over the Pacific Ocean. Ok then; why did it stay there and not blast down like so many other analogue years? An examination of the jet stream or 500 MB pattern holds the key here.

Let's analyze the monthly upper wind pattern for December 2012.

Added>>>Check out this animated 500 MB map. Set to December 2012, start at 00Z on the 1st, 20 day length at 12 hour intervals and let it roll! You will see that the majority of the 20 day animation is ruled by the nearly continuous upper low propagation out of Alaska southeastward. Only around the 9th thru the 11th is there a break with a full lat trough over the U.S. Note at the very end on the 21st, the deep trough that dives into the Midwest and through the Lakes.

While more subtle way up at 18,000 ft (map above), the areas of lower mean heights are reflected from Alaska southward out of the Pacific. Several times during the month, the cold air that pooled over the Arctic and headed south into Alaska spread southward (in the form of an upper low pressure) and plopped down over the northeast Pacific Ocean. Meanwhile; what air that did come into the country; modified over the ocean and spread east over a weak ridge of high pressure situated over the Mountain states before shooting off to the east across the Midwest, Great Lakes and East (white arrow). The remaining air over the northeast Pacific pushed up into western Canada and met with the main Polar/Arctic jet and continued east southeast across southern Canada. Until a change late in the month; much of the Midwest, Great Lakes and Ohio Valley was in "no-man's land" (which leads us to our vector wind map below).

This vector wind flow map shows the wind in vectors at 500 MB (18,000 ft). It also shows where the wind flow was stronger THIS December (the anomaly) compared to the past 30 year average. It is easy to see where the jet streak prevailed. The main core of wind vector (relative to normal or average) shot across the Aleutians (large red arrow) southeastward across the northeast Pacific, down across California (while weakening) and then spread out across the western and northern U.S. into central Canada. Another anomaly reflects the upper low out over the western Atlantic. Note the very light, almost nondescript wind over the Great Lakes. Our next map below, shows where the anomalous low and high heights that were found relative to normal.

It is seen clearly here where the 500 MB heights were well below normal and well above (relative to the 30 year average) while reflecting the below and above normal temperatures aloft. One last map on this December below; we'll look at is the 850 MB mean which shows closer to the earth (5,000ft) the mean heights. No doubt here, the below normal heights (compared to average) extended Alaska, across western Canada and into the west and south-central portions of the U.S. Above to well above normal heights at 850 MB show up well over north and eastern Canada.

Ok, that was December's upper wind pattern compared to average. Now; let's look at the analogue  500 MB mean wind flow from the Winter Outlook for the month of December. Note the difference, the BIG difference in the trough over eastern Canada and the second trough just poking south from Alaska.

The trough over Alaska and points south into the northeast Pacific Ocean (below) was much stronger and deeper this past December (reflected by the below normal heights). In other words; much of the cold that was headed for the eastern Canadian trough was diverted southeast over the northeast Pacific Ocean and then modified; in sort of a pseudo El Nino pattern, or more zonal pattern across the States. The cold air that flooded at times southeast from Alaska and the Arctic (shown by the upper low and arrow) was modified and then was somewhat "exhausted" as it tracked southeast over the ocean and phased with the Pacific jet. That is shown in all the maps whether it be the 500 MB heights, wind vectors or 850 MB heights. At the same time; the trough over eastern Canada contracted and pivoted northward and thus; pulled the coldest of air northward along with it.

Ok; then what happened the last week of December? Temperatures finally fell to normal to slightly below normal into early January. Looking at the following three maps during late December; note the deep trough pivoting out of the Desert SW and into the Midwest and Great Lakes while phasing with the eastern Canadian trough. Meanwhile; a newly formed ridge of high pressure develops and strengthens over Alaska and the northeast Pacific. This pattern is just the opposite from the one(s) depicted above that was seen much of December. In response to this change; the colder Arctic air that's been bottled over much of Canada surges south into the U.S.! This distinct change in the December pattern was culminated by our snowstorm on the 26th.

Epilogue:
So now what? Basically our pattern expected to develop earlier in December is now here...albeit three weeks late. Remember what I stated in the December analogue outlook blog?

"In regard to the above mentioned trends of winter; very rarely do you see these trends distributed nice and neatly in each winter month. On the other side of the coin; trends usually don't come all wrapped up nicely in individual monthly sequences; whether it be colder, warmer; snowier or snow-less than average. Many, many times prevailing trends of the month tend to overlap before or after the month. Meteorological oscillation patterns tend to prevail in a two to three week dominance. Therefore; when we look at composites for monthly sequences during the analogue years they a reflecting only the anomaly for the particular month not the dominant trend."

The first three weeks of December were ruled and dominated by a very mild pattern caused by the meteorological variables (and more) I stated above. We are now into the more seasonable pattern expected, with normal to below normal temperatures. Latest model trends indicate a continuation of the cold until a moderating trend starting this weekend (Jan 5th->) into next week. Another plunge of Arctic cold is suggested later the following weekend (12-13th time-frame). Welcome to winter and stay tuned!

Oh, BTW... three of our analogue winters DID start out with a mild or above normal December...all three had a cold or below normal January (but ya didn't hear that from me). ;-)

Making weather fun while we all learn, Bill Deedler -SEMI_WeatherHistorian