Friday, August 18th, 2017 Thunderstorm Potential

A rather vigorous piece of shortwave energy currently across the upper mid-West will continue pushing off to the north and east into Canada over the next 24 hours (although this energy and the system will weaken) and as it does so a warm front will approach southern New England by Friday morning with a trailing cold front pushing through late Friday evening.  As this system approaches we will see the potential for showers and thunderstorms increase.  As the warm front begins to approach early tomorrow morning this will be accompanied by showers and perhaps even a few thunderstorms.  This initial activity will clear out by late-morning.  While the theme will be for extensive cloud cover to persist, there are indications that we could see some breaks in the clouds.  With temperatures and dewpoints increasing with the passage of the warm front, any breaks of sun will be rather crucial towards the afternoon potential for showers and thunderstorms as more heating would mean a more unstable atmosphere and more fuel for storms to work with.  While thunderstorms aren't expected to be too widespread during the afternoon the thunderstorms that develop will have the potential to become strong to perhaps severe.

With the main piece of shortwave energy weakening as it moves northeastward into Canada mid-level wind fields aren't expected to be all that impressive, however, computer forecast models are showing 35-40 knots of wind at 500mb (~18,000' off the ground) and 25-30 knots of wind at 700mb (~10,000' off the ground).  These values aren't overly impressive, however, they are adequate enough for any thunderstorms that do develop to become organized:

One of the biggest questions will be how unstable does the atmosphere become?  With mid-level lapse rates rather poor (on order of 5.5 C/KM to 6 C/KM) this will limit how unstable the atmosphere will become, however, with dewpoints expected to climb through the 60's and perhaps into the lower 70's and surface temperatures warming through the 70's and maybe even lower 80's if enough breaks of sun can occur the atmosphere should become unstable enough to support the potential development of thunderstorms as well as the potential for these thunderstorms to become strong to severe.  There is a quite a bit of uncertainty, however, with exactly how unstable the atmosphere becomes.  A look at a sref plume for various computer forecast models and their output of mixed-layer cape values (a parameter used to access atmospheric stability) for Windsor Locks, CT shows this well.  Each of these "lines" is an output from these different computer forecast models with the dark black line indicating the mean of all the models.  Notice how you have some models forecasting much higher mixed-layer cape values while others offer not much at all.  This indicates that there is a great deal of uncertainty for tomorrow with regards to the actual degree of instability:

There are some indications though that we could see numerous breaks in the cloud cover as the afternoon progresses thanks to the still strong August sun and some drier air working into the mid-levels of the atmosphere.  (We will look at this on the image below the next paragraph).

If we are to see thunderstorms tomorrow afternoon what hazards could we potentially be dealing with?  In addition to torrential downpours and lighting, these thunderstorms could contain strong to damaging wind gusts, perhaps some hail, and we do have to watch for any rotating thunderstorms.  The 18z NAM bufkit for Windsor Locks, CT for late tomorrow afternoon features a rather interesting look.

1) Circled in white are forecasted cape values through different levels of the atmosphere.  This particular computer forecast model indicates that by 5:00 PM tomorrow afternoon there could be as much as 350-400 J/KG of 0-6km cape.  While not overly impressive this is certainly enough to help fuel any thunderstorm which develops and enhance the potential for these storms to become strong to perhaps severe.  With just over 300 J/KG of hail cape being forecasted as well, this indicates the potential for hail within the stronger thunderstorms.

2) Circled in red is what is called a hodograph.  This provides a visual of how the winds are behaving as you ascend in the troposphere.  This asses both wind speed and direction with height.  Just above the origin notice how the line begins sort of straight but eventually begins to curve off to the right?  This is indicating that not only are winds changing direction with height in a portion of the  troposphere but you have a portion of the atmosphere where winds are strengthening with height.  This indicates that if any thunderstorms do become strong enough they could develop some rotation.
3) The value output for cape just over 1100 J/KG shows that this computer forecast model indicates we could see greater than 1000 J/KG of mixed-layer cape develop which is certainly supportive for any thunderstorms to become strong to severe.  The value output of just over 200 helicity (a value which measures that change of wind speed with height and change with wind direction with height) indicates there is at least enough of a change of wind speed with height and change of wind direction with height that could warrant the potential for stronger thunderstorms to begin rotating.  The value output for shear of 28 m/s indicates that there is enough wind shear present (speed/change in speed with height) that storms could utilize this wind energy which could further enhance the potential for any thunderstorm to become strong to severe, especially if there is enough instability present.

4) The red and green lines circled on the thermodynamic diagram are showing temperature (red line) and dewpoint (green line).  When these two lines are very close together this indicates a nearly saturated atmosphere and when they are further apart this indicates a much drier atmosphere.  From about 925mb on up see how these lines becomes further apart?  This indicates the presence of drier air which indicates we could see sufficient breaks in the clouds.  This would yield more surface heating and greater instability and a higher threat for thunderstorms and strong to severe thunderstorms.

All in all, as of now its a bit difficult to fully assess the potential tomorrow given the uncertainties with how unstable the atmosphere can become.  With the main shortwave weakening, only modest height falls, poor lapse rates, and potential for a good deal of clouds, we may not see widespread thunderstorms tomorrow but for any storms which develop or move into the region they could certainly become strong to severe.  This potential will last well through tomorrow evening.