Saturday’s comfortable weather to yield in coming days to formidable late season heat

August’s final full weekend is to be a sun-drenched affair here. But the comfortable temperatures and humidities predicted Saturday aren’t likely to last.


The heat currently predicted Monday and Tuesday is likely to see daytime high temperatures surging to within a few degrees of each day’s record and heat indices surging to 105-degrees—the level at which heat becomes dangerous.


The first in a series of 90+-degree temperatures arrives Sunday afternoon; A 96-degree high in the days which follow would be Chicago’s hottest late season temp in 22 years


Southeast winds off Lake Michigan will temper the degree of warming which occurs Saturday. Temperatures are expected to be eminently comfortable. But, the southwest flow which takes over Sunday eliminates lake cooling and allows temperatures to surge to 90-degrees or higher for the first time in what’s likely to become a multi-day string of 90-degree highs.


It’s possible temperatures may surge to 96-degrees Monday and/or Tuesday. If true, the reading would become the hottest late season temperature on the books here in 22 years.


Predicted mid 90-degree readings Monday and Tuesday would put highs within a few degrees of the records


The mid-90-degree temperatures predicted Monday and Tuesday would rank among 2013’s hottest readings and would place each day’s peak readings within striking distance of records.


The 97-degree record highs for each of those days were each set during a 1973 hot spell.


Excessive heat watches already up for Minneapolis where a triple-digit temperature is not out of the question in coming days


Building heat this weekend is to produce temperatures approaching triple-digits from the northern Plains east into Minnesota Sunday and into early next week. That’s quite a change from the nighttime 30s which occurred in the area just over a week ago.


An excessive heat watch has been posted for Minneapolis/St. Paul commencing Sunday and running through Thursday. Similar advisories are likely to be issued across a number of adjoining states as Midwest temperatures rise in coming days.


Hints of a weak frontal passage and windshift off Lake Michigan mid and late next week are this forecast cycle’s wildcards


While intense heat is all but certain to grip the Midwest in the first days of the coming work week, it’s what happens next which is being carefully monitored by forecasters and about which there’s a degree of uncertainty.


Several computer models suggest passage of a back-door cold front on Wednesday. The “bubble” of high pressure which would follow this front into the Chicago area might well re-introduce easterly winds Wednesday and Thursday—important because such winds could tug cooler, less humid air into the region. Beach and lakeshore areas would be most likely to see the cooling.


It’s not unheard of for models to over-estimate the strength of such a back-door cold front when the overall atmosphere is predicted to remain warm. Our forecast includes a mid- and late-week windshift but is concentrating any cooling which would occur to areas adjoining Lake Michigan.


Any strengthening of the mid-week cold front in subsequent model forecasts could require us to lower predicted late-week temperatures.


Given the bias toward overestimating the strength of such fronts, our approach for now is to handle any cooling with moderation.


Next weekend’s Labor Day holiday looking hot


Even if a modest pullback in readings does occur with the frontal passage later this week, there are growing indications that heat will resurge into the Midwest for the upcoming Labor Day holiday weekend.

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