NY NJ PA Weather Forecasts

Strong thunderstorms on the move, more to come later

7:18 AM

There’s a lot to discuss this morning, so lets dive right into it!

An impressive line of thunderstorms has developed early this morning in response to the strong low level jet stream at 950 and 850 MB this morning.  The warm front continues to lift north and eastward, but is having some difficulty doing so along the coast.  The thunderstorms have already reached severe levels over many locations across eastern Pennsylvania.  As these thunderstorms progress eastward, I expect the thunderstorms to become elevated.  This means the thunderstorms will be forced to move over the relatively cooler air mass along the New Jersey coast and much of the New York City metro including Connecticut.  The thunderstorms as a result will weaken slightly but will continue to produce heavy downpours, strong wind gusts exceeding 45 mph, and frequent lightning.  However, this round of showers and thunderstorms is only the first round of potential severe weather for the region.  

There are several important features to note on this morning’s surface map, provided by the SPC.  The warm front, which is still east of the forecast area basically is situated from eastern Virginia through central Maryland, and east-central Pennsylvania this morning.  There is a push of warm, moist air directly behind the warm front which is driving the thunderstorms this morning.  An easterly fetch ahead of the warm front along the New Jersey coast is supporting an environment for dense fog and low clouds, which is also stabilizing the atmosphere.  This is an important to key to the forecast going forward!  Meanwhile, the cold front is still located over central Ohio.  

The question for the forecasting of severe weather this afternoon is just how much clearing will develop this afternoon ahead of the cold front .  Also, there is now a question of just how far north and east the warm front will get before an occlusion develops and limits severe thunderstorm potential.  Currently, my thoughts is that the warm front will have a difficult time getting east of the immediate New Jersey coast, New York City, Long Island, and much of Connecticut.  These locations will have a period of heavy rainfall and thunderstorms through the early afternoon hours, followed by a break, and another round of showers and possibly thunderstorms in the very early evening before drying out completely.  The stable nature of the low level atmosphere will limit the development of thunderstorms and weaken any convective precipitation that enters the region.  However, a much more complicated forecast can be found over eastern Pennsylvania and much of the Delaware River Valley.  This location includes much of the Philadelphia metro and interior central, southern New Jersey.  The warm front will move north and east of these locations later this morning and some clearing will take place, which will allow the June sun to do some dirty work.  Temperatures will spike into the upper 70’s to lower 80’s, which will significantly destabilize the atmosphere.  The low level lapse rate will likely jump into the the 6 to 7.5 C/km range, which will support the development of strong to severe thunderstorms later this afternoon as the cold front approaches.  Winds shear will be a factor to deal with as well as I expect weak directional wind shear between 850 and 500 MB, which is a mid level wind shear component and will support the development of weak mesoscale lows within the thunderstorms.  Meanwhile, the strong 850 MB low level jet stream will interact with the stronger upper level winds at 500 MB to produce some speed shear as well.  Combine these factors with plenty of ample moisture and strong PVA from the approaching trough, and the ingredients do come together nicely for the potential for severe weather.  

One additional aspect to keep an eye on is seen on the water vapor image this morning.  Note the black colors over the Great Lakes and Ohio Valley this morning.  This is an area of cool, dry, sinking air that is right behind this cold front.  This area of cooler air can be found at 500 MB and is actually extending further east over the developing convection over the Ohio Valley.  This cooler air with support a significant destabilization of the air mass over much of Pennsylvania and could extend into western New Jersey.  If this does happen, then the potential for large hail and very strong downburst wind gusts will increase over the forecast area.  Sometime to keep an eye on through this afternoon.

The cold front will stall over southern New Jersey on by Wednesday morning with weak waves of low pressure passing to the south of the region, which will bring a threat for a few isolated showers.  However, most locations will remain dry with partly cloudy skies.  Another low pressure system currently organizing over the central Plains will race towards the eastern Great Lakes on Thursday.  The low pressure system will force the stationary front north and eastward on Wednesday night and early Thursday morning.  The cold front will follow on Thursday afternoon with a near similar set up to what is being seen this morning.  Another round of strong to severe thunderstorms will be possible through early Thursday afternoon before the cold front exits off the coast.

High pressure from the Great Lakes will briefly build into the region on Friday and Saturday with clear skies and near normal temperatures.  However, another slow moving low pressure system will impact the forecast area on Sunday afternoon through Monday morning with showers and thunderstorms once again.  The active pattern is expected to continue through next week as well with temperatures averaging near normal.

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