We *MAY* just catch a break this weekend

If you hadn't noticed, Chicagoans, May has been very cold. After an April that averaged almost 5 degrees above the long-term climatological mean, May has been downright chilly. At O'Hare, the temperature has only gone above 60 once since April 28. And looking at the plot of all high (red) and low (blue) temperatures in Chicago since January 1 you can see that warm temperatures in mid-April gave way to well below-average for a prolonged period in early May:

These below average temperatures (13 days in a row below average, btw) are due to two major factors. The first is that May near the lake is often cold because of cold lake temperatures, and when the wind blows from the east, it can be downright chilly. Here's a look at lake temperatures at the Michigan City buoy showing high 40s (8-10 degrees Fahrenheit):


The second and more significant factor is a the development of a prolonged standing-wave atmospheric pattern in the Northern Hemisphere known as wavenumber-6 pattern (see here: the jet stream is weird rn) which has brought weird weather to almost all of the northern hemisphere (severe thunderstorms in the south and Denver, rain and chilly temps in LA in mid-May, searing heat in Alberta province Canada, etc). Chicago has been on the western fringe of a pronounced (and v. deep) cold trough that invaded the eastern half of North America about a week ago which has kept our temperatures well below normal.

But this stubborn standing-wave resonance pattern looks about to shift eastward. The first cracks will come tomorrow as a powerful storm edges northward bringing a surge of 60s to the region. But the trough should break wide open this weekend as 70s come roaring back into the upper Midwest. Here are the 500mb height anomalies for today, Sunday and next Tuesday showing the stubborn trough (blue blog) in the Eastern US slowly being shoved off toward the east making way for a dirty ridge (light red) which will grow by next week (3rd panel):

And here are the 925mb temperatures for Sunday afternoon showing very warm temperatures exceeding 20 degrees Celsius (translating to surface temps of 75-80) returning to the region :)

The finer details are still sketchy, but the pattern shift appears to be real. Our days below 60 (the average high for May 9 is 67.5F by the way) may be numbered.

(But in the meantime, more rain. Ugh. More on that later)

Stay tuned.


x Mick


Mika Tosca1 Comment