CAP vs CAPE

To most people, the only difference in the two is the "E".  To weather geeks, the CAP defeats the CAPE.  Please allow me to explain.  A CAP is a layer of relatively warm air aloft which suppresses the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, and the rising air stabilizes. As a result,  thunderstorm development is inhibited even with extreme CAPE.  Which leads me to CAPE (Convective Available Potential Energy). This is a measure of the amount of energy available for convection.  CAPE is often attained through heating of the earth's surface and high humidity during the warm seasons.  Higher CAPE guidance values indicate greater potential for severe weather. The "magic" number I look for in the modeling is ~1,000 joules per kilogram for thunderstorm development.  However, as with ALL other numerical guidance, many other parameters must be analyzed, and there are no real threshold values above which feisty wx becomes likely. 
So why this academic discussion?  Because the persistent upper level cold air is being replaced by substantially much warmer air.  I like to use the 500 mb (18,000 ft) and the 850 mb (5,000 ft) maps for this.  The map below is the forecast for the 500 mb level for Sunday night.  As you can see, there is a nice 588 isobar over PA.  That little yellow blob over northern PA is the proverbial "fly in the ointment"!  This is a little spin in the upper atmosphere that could churn out a brief area of showers or thunderstorms as is moves in the flow to the SE.  That is basically our only chance of rain now through Wednesday afternoon!

Notice how by Decoration Day evening, the upper level high pressure has strengthened to 594 mb.  This further supports increased CAP and prevents convection from developing even with surface temps very hot and dewpoints quite humid!

This dynamic atmosphere in which we live will change however by Thursday as the air aloft cools thanks to the large upper level low over Quebec.  This will push cooler air towards PA both at the surface and aloft.  Thus, a cold front will develop and spread thunderstorms towards our area by Wednesday afternoon.  But not until we hit 90ºF for our anticipated "heat wave" of at least 3 consecutive days of 90ºF or higher temps! 

The map below shows another favorite of mine and that is the "thickness" map.  No, that does not describe my skull!  This is just a snapshot of again another ever changing parameter.  As heights lower, colder air aloft is often responsible, which leads to instability, and potentially precipitation.  Height falls are more exciting for me in January when snow possibilities can be determined from these thickness maps.  You can clearly see the front and its modeled precip from the Canadian maritimes back to Arkansas.  (Did you know that Arkansas is the only state mentioned in the Bible?  I believe it is in Genesis where it is stated that "Noah looked down from his ark and saw water everywhere....!........sorry I digress!)

 By next weekend, a stretch of very nice wx should be in store for us as a large high pressure system with lower humidity and cooler initial temps will settle in right over the mid-Atlantic region.  This should set the weather stage for our BEAUTIFUL WEATHER WATCH which by the next posting might very well be upgrades to a BEAUTIFUL WEATHER WARNING for next Friday-Saturday time frame!

On a side note, I would be remiss to not thank each and every one of you for actually taking the time to click on this "blog" and read my opinions about the weather, sports, and occasionally even some politics.  It is my goal to inform, educate, and even entertain through this electronic medium.  The counter to the right indicates over 1,000 hits and yes, I do notice it.  And I sincerely thank you all for "hitting" this site.  In fact, since we did go over 1,000 hits, all of you will continue to enjoy this site free of any subscription fees!  And speaking of hitting, the only thing that stopped those suddenly hot BoSox from hitting was of course what else....the weather (14-2 over Cleveland Wednesday and 14-1 over Detroit Thursday...wow!).  While I was writing this, CCR came on XM and asked in their 1970 song "Who'll Stop the Rain?".  Well, by reading this, y'all should now be able to tell John Fogerty that the CAP will stop the rain!  The video below is this 2.5 minute classic!

Happy Decoration Day!

Smitty

AA:  Heat wave this week followed by nice wx for the end of the week.  A cold front will pass through Wednesday evening with some storms, but they do not look too severe from this point!  Enjoy your holiday my friend.