9:38 PM

A preview of the a potentially very active afternoon was seen this afternoon over much of southern Monmouth and Ocean counties of New Jersey this afternoon.  A weak disturbance, a remnant from the MCS that impacted much of the Mid West early this morning, sparked off a cluster of showers and thunderstorms.  These thunderstorms were located south of the warm front and within the very humid and unstable air mass that is building into the entire forecast area this evening.  The thunderstorms produced frequent lightning and very heavy downpours.  Over a half inch of rain fell over northern Burlington, Ocean, and southern Monmouth County. 

As I said though, the thunderstorms this afternoon was only a preview of what I think may end up being an explosive combination of severe dynamics.  Let’s take a look at some of the ingredients coming together.  First, lets take a look at the warm air mass that is driving into the forecast area.  The warm front is currently positioned over southern New Jersey through southern Pennsylvania and pushing north and eastward this evening.  Note the very warm, and moist air that is over the Mid Atlantic and how the warm air is “bulging” towards the forecast area.  The air mass at the surface is producing low level lapse rates of 6 to 7 degrees C, which is an indication of how significantly unstable this air mass is.  Basically the lapse rate is a measure of temperature change from the surface to higher levels.  The higher the lapse rate, the faster a “parcel” in the atmosphere can rise, which supports thunderstorm development.  

Now, let’s move up to another level up, namely 850 MB.  I want to focus on this level to show why the temperatures and dew points are bulging northward towards eastern Pennsylvania and and the Delaware River Valley.  Note that the winds at 850 MB is starting to increase as a low level jet stream develops over the Ohio Valley, western Pennsylvania, West Virginia, and western Virginia.  This low level jet stream will continue to strengthen through the night and force the warm front at the surface through the region.  Note that low level jet stream is also advecting deep low level moisture into the forecast area this evening and will continue to due so through tonight.  

Now, these factors alone would support some interesting ingredients for strong thunderstorms along with the environment favorable for very heavy downpours, however the developments seen on the water vapor images really caught my eye.

There are several features to point out.  The first feature I want to bring to your attention is the deep tropical moisture sitting off the Southeast coast.  The majority of this moisture will remain over the Atlantic, however some influence at 850 and 950 MB will be felt from this plume of moisture.  For the most part, the tropical infusion of this moisture will enhance the deep moisture layer over the region and further support an environment ripe for heavy downpours.  The second and more important feature is the trough over the Mid West this evening.  Note the black colors around Chicago.  This is an area of sinking, dry, cold upper level air, roughly at 500 MB.  The important aspect is note how this area of dry air is bulging eastward towards the Great Lakes and Ohio Valley.  This is a sign of a strong cold front and strong upper level support.  While I still think the best upper level dynamics in the form of divergence and PVA will remain over New England, there will be some influence from the upper levels.  When the dry, cold air at 500 MB begins to interact with the very warm, moist and unstable air mass at the surface and mid levels; I believe rapid development of strong to severe thunderstorms will ensue.  I also expect increasing vertical directional wind shear to be more of an issue than I thought this morning.  The development of said wind shear over the southern Ohio Valley is forcing me to reevaluate the potential for at least an isolated mesoscale low pressure development in these thunderstorms and even a weak tornado.  However, I stress that this is an isolated threat at worst.  Given the dry, cool sinking air at 500 MB, the very moist air mass at 850 MB, and the unstable nature of the atmosphere at the boundary layer and lower levels up to 900 MB; I think the most significant threats will be very strong wind gusts over 55 mph, very heavy downpours, significantly reduced visibility, large hail, and frequent ground to cloud lightning strikes.  There will be potential for flash flooding in some of these thunderstorms.  Also the threat for a down burst wind gusts (where rain cooled air rapidly sinks towards the surface) will be a concern.  Locations over eastern Pennsylvania and much of New Jersey will be under the greatest threats for these storms, but I would keep a special eye over the Delaware River Valley; that would be you in Trenton, NJ and around Philadelphia, PA.

As for this evening, a few showers and elevated thunderstorms will race through the forecast area overnight and into early tomorrow morning.  These showers and thunderstorms are the result of isentropic lifting from the advancing warm front and strengthening 850 MB low level jet stream, which is discussed above.  

Expect temperatures to level out and stop falling by around 3 AM as the warmer air mass builds into the forecast area.