Why has it been so cold?
One of the main reasons; the Eastern Pacific Oscillation /EPO/ which is part of the Pacific Decadal Oscillation /PDO/. To get an idea of upper air pattern thus far this winter, one must look at the hemispheric oscillations affecting it. Years back /Winter of 2002-03/ while researching the cause and affect of that cold and snowy El Nino Winter of 2002-03, I stated the following:
"It appears that the EPO was as much (and sometimes more) of a determining factor to the overall temperature trend of the winter than the NAO in El Nino winters. In some El Niño winters, it was noted that if the winter was dominated by a negative EPO, even when accompanied by a predominantly neutral to occasionally positive NAO, temperatures over Southeast Lower Michigan were generally normal to below while snowfall was normal to above. Logically then, the coldest PERIODS of any of the winters in the study were when BOTH the EPO and NAO were in a negative phase. Therefore, the negative EPO circulation helped set up and "feed" the Arctic air into the circulation of the negative NAO by amplifying a ridge along/near the West Coasts of the U.S. and Canada. Subsequently, the building of ridge along the West Coast encouraged the troughing and cold air delivery over the eastern half of the country."
This past winter was indeed dominated by negative EPO that had its roots in the mid fall time frame. The EPO cycled to and held in a negative phase frequently during the winter with the only predominant positive phase only occurring from mid December into early January. This predominant positive phase coincided well with our "mildest" period of the winter months. Also, even though the EPO was positive during that period, the NAO was mainly NEGATIVE but this still had little effect on our temperatures as readings still averaged above normal. This bears out the findings stated earlier that the EPO was mainly in the "driver seat" this winter.
"When the EPO is dominated by a negative phase (as with the NAO), more ridging develops along the West Coast as higher pressure extends from the Gulf of Alaska south along the West Coast of Canada (opposite of the positive phase). This, in turn, encourages a northwesterly flow from Canada into the middle and eastern sections of the US and thus, the delivery of polar or arctic air."
I also posted a trace of the EPO versus the NAO for that winter, note that while the EPO remained in its negative phase the phase of the NAO which was fairly neutral during the winter, was less affective and thus, we remained cold! After I wrote the above, I found the pattern affected all type of winters whether; El Nino, La Nina (here it would just ampfiy the pattern in place) or Neutral. At that time, I was researching El Nino winters and found the relevance.
Back to this winter...
A very similar pattern developed later this fall into the winter when the EPO went into a moderate to strong negative phase while the NAO (including the subset, AO) has been primarily positive to neutral.
Note the projected EPO and NAO through mid January calls for positive trend to develop in the EPO and continue in the NAO - milder weather afoot, at least through the weekend.
On to the analogues and projections;
Looking back to our initial Winter Outlook; this rough winter pattern didn't just come out of nowhere, it was telegraphed weeks ago by my overwhelming cold analogues. It actually began in November (November's Foreboding) and mid December for storms. Before the excruciating cold and heavy snows of January, were the cold, snow and ice storms of December; a sort of forerunner of things to come, the coldest time(s) of the winter along with the snowiest. Not only was the rough side of the winter projected but also the fluctuating variances in temperatures, from the Winter Outlook...
"In one of the strongest below normal signals I've seen in the 17 years of Outlooks..."
"I can't go against a cold signal this strong and dominant nor from what I'm seeing this fall, therefore below normal it is..."
"There also a strong likelihood of a few mild periods or thaws during the winter given the upper air pattern expected (see in Upper Air/Storm Track section) and the scattering of mild analogue months. Of course, timing will be the issue for milder weather but there is enough of evidence to indicate breaks in the dominant cold. This variance would also be supportive of an amplified upper wind pattern that would be at least, somewhat progressive."
Ok, that's partly in the past, so what are the past similar winters telegraphing for the remainder of the winter, the next several weeks? There were several below normal winters in the analogue selection; therefore I narrowed it down to the best performing thus far...those with similar cold, snow and timing. To give a bit broader perspective on things; I included those with normal to above normal snow along with the cold. In addition; I topped the list with the best performing analogue of this subset thus far /1981-82/ - BUT it is still early in the game and this can change.
First this winter's data thus far (Detroit only);
December 26.8/ -3.3 15.5
January (1st week) 9.2/ -16.8 21.8 (season 38.5/+24.5)
I find it interesting how the rest of the the above winter's played out...
Basically, the analogues moderated (at least somewhat) the remainder of January (remember we are at 9.2 for the first week of January - 'eh let's hope so ;-). Note out of the six analogues posted, three of the six analogues averaged in the seventeens /17.1; 17.2 & 17.3/. The next set /2/ around 20 /19.0 and 21.7/ and the mildest at 24.0. Keep in mind though our normal or average for the past 30 years in January is 25.6 for Detroit. Therefore; the best performing analogues thus far (the six above) project the January average to come in around 19.4 degrees...still well below normal. Also, the average for all of the January analogues came to 22.9. Taking this and the current projections of the EPO, NAO and general upper air pattern, I look for January's average temperatures to come in the upper teens to lower 20s.
Well we've already tied the snowiest January /1880/ with 21.8" so we are now entering uncharted territory for the analogues in January snow territory as this January is now the second highest for any month /Feb 1881 - 28.8"/. This is as good as time as any to remind readers that snowfalls like temperatures don't come neatly packaged in monthly intervals for comparison, therefore I always rely on a trend to estimate. And; with 38.5' already fallen, its almost a certainly we will come in the above normal category for the season />+6.0/ or six inches plus the normal of 42.7 /or 48.7"+/. While I did forecast normal to above snow for the Detroit and Flint area in the original outlook; I went normal to below across the Saginaw Valley and far north Thumb Region.
With the business observed in the storm tracks projected and realized in the Winter Outlook and duplicating the snowiest winters, nearly all areas will see above normal snow this winter; areas of normal to above normal are possible in the Saginaw Valley where less snow has fallen and normals are higher.
Temps & Snowfall;
In 1982; our best analogue to date, February was another below normal month, averaging 20.7 with just over a foot of snow /13.7"/...a bit above normal.
In the best performing analogues above, February's average temperatures were wide ranging. The coldest February /1936/ averaged @ 16.1 (actually the coldest month in the analogues) while the mildest February averaged @ 28.8 in 1981 (and second warmest month in our six analogues). So as you can see, even though December (excluding Dec 1993) and January averages huddled closer together below normal (see above), February's had a wider range. On the colder side of things, the average for the above Februaries comes in @ 22.5 still well below the February 30 year normal of 28.1.
Snowfalls were also wide ranging but as mentioned above; I look for above normal snowfall totals this winter over much of the region. As mentioned in the original Outlook; many of the winters, the second half was back-end loaded with snow. However; there were some (as can be seen above) that had very snowy Januaries.
Next Up; a closer look at the our winter analogue, 1981-82.
Bill Deedler -SEMI_WeatherHistorian