Year’s widest coverage hot air mass generates 90s in 47 of the Lower 48 states; heat advisories out across sections of 13 states

It’s a mammoth hot air mass—Summer 2013’s hottest to date—and it’s draped across virtually the entire Lower 48, stretching nearly coast to coast.



90-degree or higher temperatures were recorded Tuesday across sections of every one of the 48 contiguous United States—except New Mexico.



Heat has 3 more days to run here before a potentially stormy late week shift to a cooler, less humid weekend weather



The heat’s impact on Chicago’s weather isn’t over yet—far from it! The area’s to spend the next 3 days within its grasp—a development which all but assures widespread low and mid 90-degree temperatures which are to permit some scattered thunderstorms to “bubble up”, especially in the warmer hours of the day.



The depth of the current hot air mass, now draped across a huge swath of the U.S., means any “cooling” which occurs in the wake of showers or thunderstorms over coming days isn’t likely to be as significant as many would like.



This is an air mass with origins in the tropics. Thunderstorms which erupt in such an environment don’t have the “cooling power” of storms which typically roam these parts. But, the showers predicted to take form as each of the next three days proceed will, at least, help mix the air here, reducing prospects of serious air-quality issues.



Tuesday’s official 92 was 2013’s hottest and Chicago’s highest temp since last August 31—nearly 11 months ago!



Temperatures surged to 92 at O’Hare Tuesday with a peak heat index of 100-degrees. It was the official site’s highest temperature of 2013—a reading 8-degree above normal, but quite a way from the 97 recorded July 17 a year ago.



Other area highs included 93-degrees at Midway and 91 at Chicago’s lakefront.



Temperatures haven’t been as warm here since the 92 degree high on August 31 last summer.



O’Hare 90-degree tally up to 3 for the year; far cry from the 13 last July alone!



With Tuesday’s 92 now on the books, July 2013 boasts a single 90-degree high—quite different from the 13 such readings on the books for July a year ago!  The year’s tally of official 90s has moved to 3—well behind the long term average of 11 by this date.



Heat’s been SO limited this year that air conditioner usage is way down. Temperature data here suggests Chicagoans have used 51% less air conditioning since the start of meteorological summer on June 1 than was the case a year ago.  July air conditioning has been running 48% below last year’s levels.  However, coming days will see local air conditioner usage surge!



Badger State temps sizzle in the 90s while heat indices reach triple-digits at a number of locations



The heat extended far beyond Chicago and was on full display across Wisconsin. Heat indices were actually higher at some reporting stations in the Badger State than here because of higher dew points (i.e. atmospheric moisture levels).



At the Sullivan, Wisconsin National Weather Service Forecast Office, the 92-degree high temperature Tuesday was accompanied by a dew point of 79-degrees—a combo which produced a 108-degree heat index! Other 100-degree heat indices in Wisconsin occurred at Kenosha (101-degrees), Madison (100) and Green Bay (100).



Weather balloon launched from the Dayton, Ohio National Weather Service office details atmospheric expansion because of the heat



The developing hot air dome behind the heat wave, currently increasing its grip on the Midwest, is now in the record books for having produced the highest 500 mb. height on record at the National Weather Service’s Dayton, Ohio upper air observation station. Let me explain what that means.



When weather balloons are launched—as they are at Dayton and many other sites across the U.S. and around the world—they measure the height at which certain barometric pressure readings occur. As the atmosphere heats, these heights increase. That’s because the atmosphere literally expands when temperatures rise.



The current hot dome has expanded to the point that the atmosphere, measured by the balloon launch from Dayton Monday evening, found the 500 mb. barometric pressure reading higher in the atmosphere than at any time over the term of searchable records at the Dayton site going back to 1958.



The degree to which the atmosphere expands is one way meteorologists gauge the intensity of a hot air mass. With the Dayton, Ohio 500 mb height reading now in the record books, this indicates the core of this hot air dome ranks in the top tier of hot air masses which have occurred there.



Heat could end with a bang here; squall line possibilities to soar ahead of Friday night cold frontal passage



Heat, humidity and a cold front combine forces with a southward sagging jet stream later Friday to set the stage for active thunderstorms which may grow severe in the Chicago area.  The storms are expected to be fueled by the day’s intense heat and high humidity.

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