Last week temperatures in the upper 80s and low 90s kept air conditioners humming across the Miami Valley but this week, whether your a fan of summer or not, temperatures will break.  The reason?  an upper level low… but it isn’t all peaches and cream.  Every once in a while a set up like this can breed strong to even severe storms.

Here is why.  An upper level low is pressing the jetstream south of our area.  While that will bring in cooler temperatures and drier air it also lowers the temperature in which a parcel (imaginary balloon of air) needs to rise too in order to go up (thats how clouds form, rising air). 

With plenty of cold air around in the upper levels of the atmosphere temperatures in the mid to upper 70s will be all that we need to get those afternoon clouds going and as the low moves further south on Wednesday the chances for those clouds, showers and thunderstorms will dramatically rise.  Since the air is cooler then normal, hail can form in much weaker showers/thunderstorms.  So perhaps in a normal situation this time of your you would need a 30,000 foot tall storm to create some 1 inch hail, with so much cool air around a 20,000 foot tall storm can do the same.  Also with winds at the surface and aloft generally out of the same direction, straight line winds could also be a concern.

Another important factor to watch will be weak little boundaries circulating around the upper level low.  These can also enchance rotation leading to stronger storms and in some cases tornadoes.  So while temperatures may be cooler then average if we see enough sun in the morning conditions may be right for some pop-up severe.

This temperature profile of the United States also give you a good idea of whats going on. (by the way these temperatures are in Celsius).  Last week  850mb temperatures varried from 18 to 21C.  This week come Tuesday and Wednesday they will be close to 8-10C.

Cooler… yes.  Severe weather free…  possibly not.  One way or another enjoy the Summer time version of the January thaw….  or the June cool down!