July to Open with Threat for Showers and Thunderstorms.

As we move into the first day of July, a shortwave trough will be digging into the upper mid-West/Great Lakes region and sliding eastward towards New England pushing a cold front with it.  As the cold front/shortwave trough push towards New England, the system will be encountering a fairly juicy and unstable airmass.  This will set the stage for the development of some afternoon showers and thunderstorms, some of which could be strong to locally severe.  While the greatest likelihood for any stronger to severe thunderstorms will be confined to NY/PA, during the late afternoon, evening and overnight hours some of this activity will work into New England.

During the morning hours a warm front will be pushing northward and will be accompanied by clouds and a few showers.  As the warm front pushes through dewpoints will begin to climb well into the lower to middle 60's making it feel a tad uncomfortable.  With cloud cover expected to break as the morning progresses, ample surface heating is expected with temperatures climbing well into the lower to perhaps mid-80's away from the coastal plain.  While mid-level lapse rates (change of temperature with height through the troposphere) aren't expect to be very steep (on order of 5.5 C/KM to 6 C/KM) and will result in the presence of stronger instability, the combination of dewpoints in the 60's and temperatures into the 80's will still yield as much as 1500-2000 J/KG of surface-based cape, 1000-1500 J/KG of mixed-layer cape, and lifted index values between -2C and -4C.  All these values are suggestive of a fairly unstable airmass and this will provide the fuel for showers and thunderstorms to develop in the afternoon back across PA/NY closer to the cold front.  

In addition to a fairly unstable airmass, winds aloft will increase in response to the approaching shortwave trough.  Computer forecast models indicate as much as 30-40 knots of wind at 500mb (~18,000ft ASL) with an area of 50-60 knots across western PA by early afternoon:

With 700mb winds (~10,000ft ASL) expected to be around 30-35 knots as well these values will provide sufficient shear for sustained updrafts and storm organization.  Not only will winds aloft be on the stronger side tomorrow, but some computer forecast guidance is indicating the possibility of some directional wind shear in the lowest several thousand feet of the troposphere tomorrow.  If we look at a forecast sounding for near Poughkeepsie, NY we can see this (encircled in white):

Notice the wind barbs and how they change direction (from SE to SW in only a matter of a few thousand feet).  This is something to keep an eye on tomorrow across NY/PA and perhaps western New England as if any thunderstorms that develop are able to become strong enough, they could utilize this shear and we could see the possibility of some rotating (supercell) thunderstorms.

As for tomorrow, the greatest potential for showers and thunderstorms (including any threat for strong to severe thunderstorms) will mainly be confined to NY/PA and VT during the afternoon hours.  If any thunderstorms were to become strong to severe they could pose a risk for strong to damaging wind gusts, some large hail (especially if any thunderstorms take on supercell characteristics), and any storm rotation would have to be closely watched.  While the majority of the activity during the afternoon should be west of New England, people across western MA and western CT should keep an eye on the sky.  Given sufficient height falls associated with the shortwave through, we can expect scattered to widespread shower/thunderstorm activity to occur as the afternoon progresses with the storm mode being mainly clusters/line segments, however, a few discrete supercells will be possible.  

During the evening and overnight this activity is expected to work into New England.  While the loss of daytime heating will tend to stabilize the atmosphere, mid-level lapse rates are expected to increase on order of 6.5 C/KM to 7 C/KM thanks to cold air advection in the mid-levels of the atmosphere.  This will help to maintain some instability into the overnight so a strong to perhaps marginally severe thunderstorm will be possible through the overnight.

Not everyone will see showers or thunderstorms tomorrow but for the people that do, some of the storms could certainly pack a punch.  In addition to the threat for winds/hail, frequent cloud-to-ground lightning will be possible along with torrential downpours.