After a cold October, a warm November, a cold December and early January, a January thaw set in on its normal third week of January schedule. We promised the cold would be back and it returned in week 4.
The cold spells related to stratospheric warmings favored in east QBO low solar years and in years following high latitude volcanoes like we saw in Redoubt, Alaska and Sarychev, Russia. The following link will provide an illustration of the degree to which this phenomenon has blossomed in the past several weeks.
Here is a cross section of mean temperatures between 65N and the pole with height. Warm is above normal. Notice the negative AO when the warming occurs and descends to mid (steering) levels and the recent thaw and a bounce in the AO Index with a polar cooling which causes the polar vortex to strengthen.
Notice how the snowcover over the US peaked on January 9 at 62.7% coverage. Snow flurries fell that weekend in Miami and Naples Florida.
There is a strong correlation between stratospheric warming and developing cold weather in eastern North America. You can see in the above a warming in November 2009 that led to the subsequent widespread cold anomalies of December as that warming propagated down. Notice the AO dropped off the scale (reached well above 5 STD negative). It bounced with the polar cooling but has declined again since as polar warming resumed.
This means more cold for the eastern US and western Europe. In the maps below notice the tendency for more troughing in these locations in negative AO/NAO situations. When positive there is a more zonal flow allowing for maritime air and storms to propagate inland and bring milder conditions and rain as we saw in week 3 of January.
The upper level patterns looked remarkably like the mean 500 millibar height anomalies in other years that featured an El Nino, a weak solar cycle and an easterly QBO (Quasi Biennial Oscillation). All of these interseasonal variables are beyond the scope of operational forecast models and their ensembles. Here is a map of those 500 MB height anomalies for Februaries with those conditions. Note the tendency for blocking with above normal heights near Greenland and below to the south. Above normal height tend also to be found across especially southern Canada.
Now, here’s a look at a recent European forecast for February 7th. On that day in 1978, much of New England was cleaning up from the infamous Blizzard of ’78. This year a major snowstorm occurring across parts of the southern Plains, Tennessee Valley and Mid-Atlantic. In 1978, the storm came from a diving trough out of Canada that caused a cut-off low to form near the east coast that caused a storm to explosively deepen and then be trapped and stall and dump snows for days on the northeast. It was the last in a series of three memorable storms that year.
Here is the European forecast for 500 mb features for the period Feb. 8-14…
Here’s a look at the mean 500 MB height anomalies for that month in 1978.
However that year was an El Nino West QBO winter, which favors more of the storms reaching farther north. The east QBO El Ninos storms are more southern track and out to sea years, exceptions include 1968/69.
After dipping with the thaw, snowcover built again with the southern storm reaching an amazing 69.7% of the United States on January 30.
Globally the snowcover is extensive, including Europe as is common with a negative NAO.
Snowcover for the northern hemisphere has been above normal this winter, as one would expect with a strongly negative AO, NAO.
The El Nino is gradually diminishing. The water beneath has cooled from the 5C anomalies of December.
The Warmest water now has migrated to the dateline with relatively cooler water east. In February’s with that tropical ocean pattern in February, the cold weather extends across the east and central United States across the Atlantic to northern Europe and Asia. There is a strongly negative NAO. The pattern is similar to the ones shown above in analogs and models.
ENSO Models suggest El Nino 2009/10 will continue to diminish to neutral state by summer and fall.