We saw this transitory blocking in September when our first autumn cold blast sent temperatures down to near record cold territory. Now; latest indications are that another blast of cold air will dive out of Canada late this first week of October; prompting my previous Frost/Freeze blog. A strong ridge/trough blocking pattern is being forecast by the majority of models for the first half of October commencing this upcoming weekend. This pattern would bring a cooler than normal October to Southeast Lower Michigan if it were to dominate during the month. But have heart; an amplified upper air pattern during the fall months of October and November also brings the likelihood of a spell or two of Indian Summer weather. What do I mean by blocking pattern? See for yourself; below. Note the strong ridge of high pressure along the west coast of North America and the huge, cold low pressure trough that extends from Siberia, across the Pole into the eastern half of North America. A Blocking Pattern means just that; the atmosphere upper wind pattern gets "blocked" and therefore, the pattern stalls and remains constant over the same region for several days or even weeks..
Sunday Night October 7th 2012 - Polar Projection of North America
This strong blocking pattern setting up this October bears watching for any further developments into the fall as a possible hint to the coming winter. Years back, I did an in-house research study on cold or below normal Octobers and their possible relevance to a colder than normal upcoming winter. The paragraphs below come from my Winter 2002-03 Outlook which was also, like this upcoming winter, at the onset of another weak-moderate El Nino winter.
An important change came in October (and again, one of the few cool Octobers in the study)
when abnormally cold air surged into the region mid month. While the timing of the polar blast
duplicated most in the study its strength and dominance was atypical. The monthly average
temperature for October 1977 was a cool 47.9 degrees /normal 51.9/ at Detroit. Curiously,
this same pattern was noted in the fall of 1972 and again, this abrupt change surfaced in
October. October of 1972 not only turned out to average well below normal, but with
nearly the same average temperature as October 1977 with 47.3 degrees.
While October 2002 remained warm until mid October, the recent abrupt change to a deep
500 MB trough over the eastern half of the country resulting in much colder weather bears
watching, especially if it represents an intermediate trend change to the past several
months. In addition, the near-term projections of the EPO and NAO both indicate, at least
short term, negative phases and this too bears watching for any overall, longer term trend
October 2002 was much like the other notably cool Octobers mentioned in the research as ironically, October 2002's weather also abruptly changed from a balmy summer-like beginning to a late fall-like ending; changing in mid-month. During the first half of the month, the temperature averaged 60.4 degrees at Detroit, but by the end of the month, the average temperature had dropped to 50 degrees at Detroit (and 47.6 at Flint) or 1 to 2 degrees below normal. One of the analogue winters I mentioned I was researching in my Autumn Outlook for possibly using for this winter, was the Winter of 1976-77. From my Autumn Outlook:
Weak to even moderate El Nino conditions along with a NAO-/AO- in the winter
are notorious for active weather patterns across the country. One of my
earlier studies presented such a case (Winter of 2002-03; a moderate El
Nino). A new set of analogues are already being decided upon and
watched for similarities to this autumn into winter pattern projected.
In regards to a weak El Nino winter; one possible candidate of many is
the Winter of 76-77 but it is much too early to let those "cats" out of
the analogue bag just yet. One must see how the "birth" of our new El
Nino plays out this fall and how he interacts with the phase(s) of the
NAO/AO. Stay tuned.
The Winter of '76-77 was an exceptionally brutally cold winter across the Great Lakes and actually much of the country east of the Rockies as the strong blocking pattern discussed above remained in place for basically several months; only occasionally breaking down. The upper and surface wind pattern THAT fall was one of progressively colder air masses spilling down from Siberia across the Arctic into the eastern half of the country. And; if memory serves me, it was when the phase "Siberian Express" was first coined. It was very early in my career and I remember the seasoned lead forecasters just amazed at the unusually cold core upper lows that dropped out of the Arctic and became displaced and lodged much further south than typically seen...oscillating between James Bay, Canada and the Great Lakes. From month of October; onward into the winter months of December and January, the below normal departures were larger than the month before so that by January; the monthly below normal departure handily exceeded 10 degrees!
1976-77 Fall-winter monthly averages at Detroit:
Sep Oct Nov Dec / Jan
Does this all mean my outlook for the Winter of 2012-13 is likely to call for a colder than a normal winter? Not necessarily as the discussed above pattern change has yet to be confirmed. This is just one more card that mother nature may be showing us from her elusive meteorological hand. It IS however, I believe an important card and possibly a trend to watch for as we advance through the remainder of autumn. As stated above when researching in 2002; more of than not, a confirmed abrupt pattern change in October may very well foretell of things to come. We shall see!
Making weather fun while we all learn,
Bill Deedler -SEMI_WeatherHistorian