Assessing Today/Tonight's Thunderstorm/Severe Weather Potential

The potential exists for numerous shower and thunderstorm activity to develop region wide later this afternoon with the activity lasting well into the overnight hours.  Some ingredients will be in place for a few of these thunderstorms to become strong to severe and have the potential to produce strong to damaging wind gusts and perhaps even some large hail, especially in any rotating thunderstorms.  The next big question is, do these ingredients (which we will look at below) all come together at the right time to produce a bigger severe weather event?  This is a question in which we will slowly begin to answer as the afternoon progresses.  The reason for this is due to the mesoscale (small-scale) factor of the setup.  Often times with convective events, the severity and extent of them all comes down to very small-scale features and it is very, very difficult to forecast/pinpoint these features more than several hours in advance.  

Instability

When dealing with the potential for convection, one of the questions we always face is how unstable will the atmosphere be.  For the past several days, computer forecast guidance has hinted that with the combination of heat/humidity the airmass could become quite unstable...the question was whether or not cloud debris from previous day's convection over the Great Lakes would allow for less in the way of surface heating.  Given the satellite trends this morning, this is not the case.  While we do have some cloud debris moving over the region, strong surface heating is occurring across many places and away from the coast temperatures should soar well into the lower to middle 90's.  With dewpoints in the mid to upper 60's this will yield quite an unstable airmass.  

While typically we see the airmass become more stable as the sun begins to set (with the sun setting and surface temperatures lowering this allows for a stabilizing atmosphere), there are certain ingredients which could keep us quite unstable well into the overnight hours.  These ingredients include; the potential steepening of mid-level (700-500mb lapse rates) thanks to cooling temperatures in the mid-levels of the atmosphere and/or the advection of an elevated mixed-layer plume, and a further moistening of the low-level airmass.  The latest NAM computer forecast models show mixed-layer cape values as high as 1500-2000+ J/KG indicating the airmass will remain quite unstable well into the evening and into the overnight hours:

Wind Shear

As the shortwave energy approaches winds throughout the troposphere are expected to increase and this would help any thunderstorms that develop organize and also enhance the potential for strong to damaging wind gusts.  By early evening we see the nose of a rather modest 35-45 knot 500mb jet punching into southern New England.  While the 500mb winds over southern New England alone aren't that strong, the core of the stronger winds just off to the west can actually help to intensify convection if the thunderstorms are just to the east of this axis.  

We also see an increase in 700mb which should approach/exceed 30-35 knots.  This will help yield vertical shear values of around 35 knots which is supportive for thunderstorms to become organized and also may help with the enhancement of storm updraft rotation.  

850mb winds are expected to approach 30 knots as well which will help with stronger low-level shear values and also enhance some potential for updraft rotation.  The combination of these strong winds aloft will yield a threat for strong to damaging wind gusts with the strongest thunderstorms:

While the shear still looks to remain adequate, latest trends have decreased the degree of wind shear over southern New England for later on.  If this trend does continue the threat for severe weather would be diminished and we would only be looking at a few isolated reports of wind damage and maybe some hail.  Something to keep an eye on as the afternoon continues to progress.

Dry Mid-Levels

While some dry air is good to have in the middle levels of the troposphere, too much dry air is no good as this eats away at storms updraft strength...at the same time it can enhance the potential for stronger winds to mix  down.  Forecast soundings do indicate a quite a bit of dry air in the mid-levels, however, some indications are they could moisten up somewhat.  This is something to keep a close eye on.  If the degree of dry air is too much, this would limit thunderstorm coverage and could decrease the potential for severe weather.  This is a rather difficult signal, however, as one model shows a great deal of dry air and the other model not nearly as much.  Too illustrate we will look at a bufkit sounding for Windsor Locks, CT for around 5:00 PM this evening:

Forcing

Perhaps one of the biggest questions is the degree of forcing.  Latest data has continued to slow down with the eastward progression of the shortwave energy moving through Quebec Provence in Canada.  Forcing is huge as it provides the lift necessary for parcels to rise and generate showers and thunderstorms.  While there are hints at some localized areas of enhanced forcing across the region, it's unclear as to whether that would be enough to spark any activity.  Looking below we see the main piece of energy still well west at 8:00 PM this evening.  in fact, it is entirely possible we see this energy push through tomorrow sparking a threat for thunderstorms, including a few strong to severe thunderstorms:

  

What to Expect:

At the least it looks like we'll see some shower and thunderstorm activity develop later on, however, the coverage and extent of this activity is still not all that clear.  As the afternoon unfolds the situation will become more clear.  At this time it appears the potential foe widespread severe weather may be lowering, however, if a cluster of thunderstorms does develop it could produce a swath of damaging winds and perhaps even some hail.  

Given the slowing down of the shortwave energy, this may be opening up a window for the possibility of thunderstorms tomorrow.  The atmosphere does look to be unstable and with steep lapse rates and strong wind shear, thunderstorms could become strong to severe and pose the threat for strong to damaging wind gusts and large hail.  

There is lots to work out over the next several hours but if today doesn't work out that could just mean increased potential tomorrow.