Threat For Thunderstorms To Increase Across Southern New England Over The Next Few Days. Potential For A Few Strong To Severe Thunderstorms As Well

A rather quiet and stagnant weather pattern which has been in place for quite some time across southern New England is about to come to and end as we near the 4th of July holiday as an increase in heat/humidity and an approaching cold front will set the stage for the possibility of thunderstorms each day from Wednesday through Friday.  Tropical Storm Arthur, which is currently located just east of Florida will also play a factor into our weather as well.  Lot's to cover here so we'll break everything down on a day-to-day basis.

Wednesday

On Wednesday much of the thunderstorm activity is expected to be west of southern New England, however, we can't rule out some thunderstorms working into western areas of MA and CT late in the afternoon/evening or even having a few thunderstorms develop.  The atmosphere across southern New England will be be more than favorable for thunderstorms as the combination of surface temperatures (away from the immediate coast) nearing or even exceeding the 90F mark, surface dewpoints into the lower 70's, mid-level lapse rates around 6.5 C/KM, and cooling mid-level temperatures will lead to a highly unstable airmass across southern New England.  Shear values across southern New England are expected to be rather modest with 850mb winds (~5000 ft AGL) around 20-25 knots and 500mb winds (~18,000 ft ASL) around 25-30 knots.  These values are not very impressive, however, are adequate given presence of a highly unstable atmosphere.  Computer forecast models do indicate that towards evening these shear values may begin to increase across western portions of MA and CT.

(18z/01 GFS graphic displaying projected surface-based cape values around 5:00 PM tomorrow.  Reds and Pinks indicate highly unstable atmosphere)

While the atmosphere across southern New England is certainly supportive of thunderstorms there are a few limiting factors which could prevent t'storms from developing.  1) An atmospheric cap.  700mb temperatures (~10,000 ft AGL) are expected to be around 8-10C and may even warm to as much as 11-12C.  This pocket of rather warm air may be enough to prevent thunderstorms from developing.  2) Lack of a lifting mechanism.  All the best lift and forcing will be back west across New York as they will be closer to the cold front/trough sliding east and will be closer to the stronger winds aloft.  There are some indications, however, that a weak pre-frontal trough will develop and slide across New York and approach southern New England late tomorrow afternoon or early evening.  This will have to be watched and this feature could provide enough of a forcing mechanism to break the cap and allow for some thunderstorms to develop.  The best chance for thunderstorms both during the day tomorrow and tomorrow night will be across western and central southern New England.  Given the presence of high instability and modest shear any thunderstorm's that develop will have the potential to be strong to severe posing a threat for strong to damaging winds along with large hail.  Torrential downpours and lots of lightning are to be expected with any storm and with torrential downpours the threat for localized flash flooding may exist.    

Thursday

Thursday looks to be the more active day across southern New England in terms of thunderstorm activity which is expected to be more widespread as the cold front/trough which will be the focus for thunderstorms tomorrow to our west approaches.  There are some questions though on Thursday with regards to how unstable the atmosphere will be as cloud debris from thunderstorms tomorrow to our west could cut down on the degree of surface heating.  With this said, the latest data from the computer forecast models do indicate that we will see enough surface heating, and combined with dewpoints once again in the lower 70's will yield a fairly unstable atmosphere, although perhaps not to the degree we will see tomorrow given questions regarding how much surface heating we see and mid-level lapse rates may only be around 6 C/KM as opposed to 6.5-7 C/KM.  While instability may be a bit less, winds aloft will be stronger across the region.  The presence of stronger forcing and lift from the approaching cold front/trough and the presence of a fairly unstable airmass will set the stage for numerous showers and thunderstorms across southern New England.  Like tomorrow, the potential on Thursday will exist for some of these thunderstorms to become strong to severe posing a threat for strong to damaging winds and perhaps even some hail.  Torrential downpours and lots of lightning are to be expected with any storm along with the threat for localized flash flooding.

(18z/01 GFS graphic displaying projected surface-based cape values around 5:00 PM Thursday.  Reds and Pinks indicate highly unstable atmosphere)

Friday

Moving to the 4th of July this is where things become much more complicated.  At this time, what is now Tropical Storm Arthur, will be working up the east coast and may even potentially be a weak Hurricane.  There is a great deal of uncertainty as to the exact track Arthur will take as this will really all depend on how quickly the cold front moves through and where the front is situated as Arthur continues working up the coast.  Indications right now are that Arthur will not make it up the coast in time to have direct impacts on much of southern New England, with perhaps the exception of portions of RI and far SE MA.  The indications are the front will slide far enough east through southern New England to where it captures Arthur and pushes Arthur off to the east.  

Regardless, there could be a period late Thursday through Friday that there could be a period of extremely heavy rainfall along with some thunderstorms.  Where this axis of rain sets up or traverses over is very difficult at this time as it will all depend on the placement of certain features such as the cold front as we just mentioned.  Given Friday is the 4th of July, and many people have tons of outdoor plans it is a bit inconvenient to be so uncertain but that's typically the case when dealing with such delicate situations where the exact placement and timing of key features is extremely critical and typically when dealing with such cases it can be nearly impossible to know until as little as 24-hours out.  As new data rolls out the situation should begin to become a bit more clear.