Saturday, July 07, 2012

Worst Of Heat Today

Today is the hottest day of the year in all likelihood -- temperatures starting out in the lower 80's in the city, 70's everywhere else...with a bevy of humidity outside to boost, the resulting heat index values are already around or above 80 in many spots this morning -- as high as 85 in Philadelphia at 7 AM.   The 10th day of this heat wave will bring the best chance of hitting 100 in the region -- we're predicting 101 for the city this afternoon, which is not a record (103 in 2010 is) but some spots could approach 103 by the time the day is out this afternoon.  Skies, which feature a thin cirrus veil overhead, will generally be sunny or at least, sun through high clouds for much of the day.

Both heat and humidity will be around but the final high temperature will depend a little bit on the level of atmospheric moisture in place and how much of that cirrus deck hangs out.  With a dry atmosphere in place over the region for much of today, the potential to mix out the higher dew points that are in place early this morning is pretty good...this will result in temperatures getting that needed push to get over the 100 degree mark and possibly a few degrees above that.  Heat index values will range from 100 (north) to 108 (south) this afternoon...with some isolated "corn effect" spots getting to 110 (Lancaster, Coatesville, Dover have the best shot at highest heat index values since their reporting stations are near farm fields).  Regarding the clouds, cirrus debris from an old complex of storms is tracking nearby...if those blow through in enough quantity we might be able to shave a degree or two off of the high.   If there's a way to avoid 100 in the city, that might be the way we get there.

A cool front is currently over Ontario and will slip south through the day today.  We will be storm and rain free until this evening when some scattered thunderstorms will develop after 5 PM to our north with the slowly sliding south cool front.  Some of those storms may be severe -- we'll chat about that later on today as we get some updated info on timing and placement.  That severe threat would carry over into Sunday for those along and south of the PA Turnpike and I-195 as the front lingers across the region.