Multiple locations across southern New England has experienced mid-summer heat/humidity over the past few days as many areas saw the thermometer soar well into the 90's and even close to 100F. When you coupled dewpoints which have been in the mid 60's to at times even lower 70's the combination of the heat/humidity as made it almost unbearable for some. Good news though for those who don't like the heat or humidity as there is relief in sight. However, we have to go through about one more day of heat/humidity along with the potential for thunderstorms.
A cold front is slowly pushing eastward across New York state and will close in on southern New England later this morning and afternoon hours. Out ahead of this cold front a pre-frontal trough is expected to develop and this will be the focal point in the development of showers and t'storms later this morning and afternoon.
With the cold front still well off to our west this once again means we will be under a southwesterly flow giving us another day of heat/humidity. With computer forecast models trending slower with the approaching and passage of the cold front this will give us yet another day of temperatures in the 90's with dewpoints around 70F. With cloud cover and showers/t'storms expected to develop a little later on this will likely prevent temperatures from reaching their full potential, however, it does appear many areas away form the immediate coastline will once again approach 90F and even get into the lower 90's. Down towards the coast a more southwesterly flow will make for the development of the sea-breeze which will hold back on temperatures here. With 90F expected across many inland locations this afternoon this will give us our 3rd consecutive day of 90F+ temperatures which will give us our first official heat wave of the 2012 summer season!
With the combination of heat/humidity and a cold front/pre-frontal trough approaching from the west this will set the stage for showers and thunderstorms to develop later this morning/afternoon. While it will be very hot and humid the atmosphere will become fairly unstable with Surface-Based Cape (Convective Available Potential Energy) values and Most Unstable Cape values expected to be in the 1500-2500 J/KG range. Lifted Index values are expected to get to -3C to -5C. Given the presence of this moderate instability some of the storms that develop could approach the strong or even severe side and the main threat would be strong winds which could be capable of knocking down large limbs or small trees. Given the presence of high dewpoints and abundant low-level moisture all storms will produce torrential downpours and this could lead to areas of localized flash flooding/poor drainage flooding.
While thunderstorm activity will be widespread today widespread severe weather is not expected this afternoon. The presence of marginal mid-level lapse rates (change of temperature with height) in the 700-500mb (~10,000ft ASL to ~18,000ft ASL) will inhibit much stronger instability from being realized as Mix-Layer Cape values are only expected to be around 1500 J/KG at most. Winds aloft, while they do increase as the afternoon goes on are marginal as well as shear in the 0-6km layer is only expected to be around 20-25 knots though it does increase to around 30 knots late in the afternoon.
All in all showers/t'storms will begin developing as early as late morning and should last through the afternoon hours and well into the evening hours. Multiple rounds of storms are expected to develop given the presence of heat/humidity and should form into multi-cells or multi-cell clusters. While widespread severe weather is not expected a few of the storms will have the potential to become strong to severe posing a risk for gusty winds capable of knocking down large limbs or small trees. While hail isn't expected to be a high threat can't rule out some hail in the strongest of storms. Torrential rains will also occur with all storms.
The front is expected to finally clear the region early tomorrow afternoon so we can't rule out some showers/t'storms tomorrow, however, instability will be much less than today so activity won't be as widespread or as strong. Once the front passes more seasonable temps will work into the region and this oppressive humidity will make an exit.
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