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A summery version of the polar vortex

Climatologically-averaged wind and pressure patterns across the Earth.
Climatologically-averaged wind and pressure patterns across the Earth.
P. Galvin, Indiana University Southeast, New Albany, IN

Just when you thought it safe, the “polar vortex,” has returned for a summery rendition. Not quite as chilly and intense as its wintery cousin, this polar vortex has primarily targeted the northern tier from the Dakotas eastward. The upper Midwest and Great Lake states have been the main recipients of this comforting summer-time weather regime.

As noted in numerous articles this calendar year (see links listing to follow), there is always a polar vortex at high latitudes (in both northern and southern hemispheres). This is tied to the overall global circulation pattern and the favored locations of certain wind regimes and pressure systems (Fig. 1).

The “vortex” only becomes newsworthy when it shifts Equatorward and/or extends a finger Equatorward. This allows unseasonably chilly weather to move into places that are normally much warmer.

This polar vortex (Fig. 2) won’t be as brutal as the one that locked the northern part of the U.S. in the deep freeze this past winter (Fig. 3). However, it is going to bring some rather chilly summer-time temperature readings to places from the Dakotas southward to Missouri and then points east. The greatest seasonal departures will be in the upper Midwest and across the Great Lakes.

Minneapolis, MN, for example, only saw a high temperature on Mon., Jul. 14, 2014 of 65 degrees, thanks to cloudy skies and showery weather. That’s 19 degrees below the seasonal average for the date. Since then (and continuing for the rest of this week), a moderating trend has set in, albeit a slow one. Readings will still be about 8 degrees or so lower than average today and then only moderate to near seasonal average readings by the weekend.

Chicago, IL will see daytime highs today about 15 degrees cooler than average, with nighttime lows around 8 degrees chiller than expected seasonally. Throughout the week (into next Sunday), daytime temperatures will still remain cooler than average, while nighttime lows more quickly warm to seasonal readings.

Des Moines, IA has a similar storyline. Look for about a 10-degree below average day and night cycle today, with a slow warming trend throughout the week. Still, even by Saturday, the temperatures will only be approaching average.

Note that for the first 13 days of July, Minneapolis, Chicago and Des Moines have averaged 2.6, 2.3 and 3.2 degrees below average. This extended cool down will drive the departures for the month to at least 5 or 6 degrees below average for the first 19 days of the month. In January, the same three cities reported departures from average of 7.6, 8.1 and 4.6 degrees, respectively.

As the cooler and drier Canadian high-pressure system covers the northern tier, people can look for crystal clear, blue skies, with only some few fair weather cumulus clouds. However, where the cold air moves across warmer Great Lakes waters, wintry lake-effect showers (and maybe even a few thunderstorms) may develop. This would most likely be across the southern part of lake Michigan where water temperatures are the warmest. This unstable Great Lakes weather pattern might even spawn a few waterspouts.

The following week (Jul. 20 – 25) promises to see markedly warmer readings across the region (Fig. 4) as an upper ridge replaces the “vortex.” However, after Jul. 25, another finger of the polar vortex is expected to snake down across the eastern Great Lakes and the northeast U.S., bringing another sharp temperature drop (Fig. 5).

As is so often the case, when it gets cold somewhere in the U.S., it gets warmer someplace else. With the cold air mass firmly in place from the Dakotas eastward, places to the west (mainly the Pacific Northwest) will see temperatures soar to well-above average seasonal values. Wenatchee, in the eastern part of Washington state, soared to 105 degrees on Tue., Jul. 15. Early Wednesday morning, the temperature only fell to about 80 degrees. These readings were around 18 degrees above seasonal average. Temperatures will remain more than 15 degrees above average today and then undergo a fairly dramatic return toward seasonal averages by the weekend. Meanwhile, in parts of eastern and interior Washington, a heat advisory remains in effect.

© 2014 H. Michael Mogil