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The 00Z (8 am EDT) soundings are in.  The 00Z models have run.  

This could be the "hottest day in living memory" (NWS Fort Wayne/South Bend) if you live in Indiana south of I-80 or Illinois east of I-57.  

The NWS predicts temperatures of 100-106 in Indiana.  Chicago may only hit 98.  St. Louis is looking at 108 F.

More comments below the squiggle:

I've never seen temperatures at 850 hPa (mb) so high in the Midwest.  During the homicidal July 1995 heatwave, Davenport, IA and Lincoln, IL saw temperatures around 28 C.  Today, Lincoln reports 31 C and Davenport reports 29 C.  

The intense ridge overhead is not as intense as it was in 1995.  During the killer heat wave of 1995, these heights were 597-598 dekameters.  This morning, they were 595-596 dekameters.  

An explanation:  Above the surface, meteorologists look at "heights", that is, how high above the ground the barometric pressure is, say, 500 mb (hPa).  In Arctic winters, this level may be on the order of 500 dekameters (5.00 km).  On extremely rare occasions, this level may be 600 dekameters.  

Both 850 hPa temperature and 500 hPa heights contribute to temperature predictions.  The 850 hPa temperature is useful in low-altitude areas like the Midwest because it does not vary much between night and day.  The 500 hPa heights indicate the presence of sinking air:  as air sinks it compresses and gets warmer.  Today, these factors will militate toward highs near 110.  The morning sounding from Lincoln predict 112 F, those from Davenport predict 110 F.
(Unisys Weather link)  If these predictions verify, much of Illinois and Indiana will see their highest temperatures ever.  

Looking upstream, 110+ highs were confined to Kansas and eastern Colorado yesterday.  Other upstream areas saw merely intense heat between 97 F and 104 F.  This would argue for a conservative forecast near or a bit above 100 F.

Unlike the old Midwestern norm, this is a fairly dry heat.  Being a dry heat will probably save many lives since low levels of moisture in the air let temperatures fall freely through the night.  People without air conditioning can open windows to let in cool air well after sunset.  People can do what they like in the first hours after dawn in pleasant temperatures.  

Dry heat has a less benign aspect:  fire danger.  Unlike the Desert Southwest, there are plants growing on just about every surface that is not paved.  The drought has dried them out;  between low humidity, high winds and high temperatures, they will be dessicated, turning into so much tinder.  Almost all of Indiana is under a Red Flag warning today, and most of it is under municipal or countywide burn bans.

(An aside:  Indiana law states that you can start shooting fireworks tomorrow.  Miami County says that its burn ban overrides state law.  Tippecanoe County is not sure.  Cities and towns can regulate fireworks pretty much as they please;  they are tending toward bans.)

This heat will moderate Friday north of the Ohio.  It will not moderate in Kentucky and Tennessee, and will be advected eastward toward the coast.  Last night The Weather Channel was predicting 108 F for Nashville on Friday and 107 F on Saturday.


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