Thursday, July 7th, 2016 high heat/humidity induced thunderstorm possibilities.

As we push through the first week of July real sultry summer weather has bestowed upon us with temperatures climbing well into the 90's with dewpoint temperatures well into the 60's to even lower 70's.  While the duration of the high heat/humidity will be on the brief side (only a few days), its presence will certainly be felt.  Also, as typical with this time of year when we run into bouts of high heat/humidity, we also run into the potential for afternoon showers and thunderstorms and as we move through the next couple of days we will deal with that potential.  For right now, however, we will take a look at Thursday.

While widespread showers and thunderstorms and widespread severe weather is not expected tomorrow, a few isolated thunderstorms are expected to develop.  Not every town is going to experience a thunderstorms (in fact only a handful of towns may be so lucky) but for the towns that do the storms could be on the strong side.

For Thursday, we are expecting to see numerous locations (especially away from the immediate coastline) experience their second consecutive day of 90+ degree temperatures.  This, combined with dewpoint temperatures into the 60' to perhaps lower 70's will yield a moderately unstable airmass and this will provide the fuel necessary for the development of thunderstorms.  In fact computer forecast models are indicating as much as 2000-3000 J/KG of surface-based cape across the region with as much as 200-2500 J/KG of mixed-layer cape.  Typically it is difficult to get mixed-layer cape values these high without the presence of 6.5 C/KM or higher 700-500mb lapse rates (they will only be on order of about 5.5 C/KM tomorrow), however, given the presence of upper 60's to lower 70's dewpoints this will help compensate a bit for weaker lapse rates:

   The question is will we have any source of lift to generate showers or thunderstorms..the answer to this question is it appears so:

Now with such high instability values and a lifting mechanism moving overhead, you would perhaps expect more in the way of widespread thunderstorms or even a bigger threat for severe weather, however, we are lacking a few key ingredients.  Below is a forecast sounding for Windsor Locks, CT tomorrow afternoon.  I have highlighted a few different features:

white:  weak wind shear:  The winds throughout the troposphere tomorrow will be very, very weak.  In fact, up through 20,000+ feet winds will only be around 5-15 knots.  That is very weak.  Winds this weak will make for storm organization quite difficult and also prevent sustained updrafts.  This will mean anything that gets going will quickly collapse and weaken.

blue:  drier air:  Notice the spread between the dewpoint temperature (solid green line) and the temperature (red solid line).  This indicates the presence of some very dry air.  Now, dry air in this level of the atmosphere is a good thing, so long it doesn't mix to the levels below it.  In tomorrow's case, conditions will allow for some of this drier air to mix down into the lower levels which could also end up dropping dew points a little bit, thus lowering instability values.

Purple:  warming:  There are two areas here, the solid purple line next to the temperature line and then circled below on the bottom left with "700-500mb lapse rates".  This warming in this level yields a temperature change between about 10,000ft and 18,000ft of only 5.5 C/KM.  This is very weak.  What this means is as rising air parcels encounter this level, this will rise rather slowly and with weak shear storms will struggle to really penetrate this level and when they do it will only last very briefly which also indicates short-lived (or pulse storms).

Putting all this together what can we expect?  Most towns stay dry tomorrow, however, given the presence of high heat/humidity and an approaching system, atmospheric ingredients will be in place for the development of a few thunderstorms, especially along the east sides of the Berkshire/Worcester Hills.  With weak shear in place and high instability, any storm could quickly become quite strong, however, with the weak shear will be short-lived.  The presence of the drier air aloft and dewpoint depression at the surface (surface temperatures in the 90's minus the dewpoints in the upper 60's to lower 70's yielding a depression ~20F) will result in the potential for some strong wind gusts.  With high moisture content this could yield a wet microburst (heavy rain/strong winds) which could result in some wind damage.  Not everyone sees a storm tomorrow but whoever does could see a nasty thunderstorm.  Thunderstorms will also be slow movers so they could drop a lot of rain in a short timeframe resulting in some flash flooding, even despite the dry ground.