Overnight and into tomorrow morning an area of surface low pressure will be tracking to the northeast well to the west of southern New England. As the surface low pushes through western PA into western NY and into Quebec, Canada a warm front will surge northward through southern New England where several computer forecast models indicate the warm front could stall in the vicinity of the MA Pike. South of the warm front the airmass will become very humid. Surface dewpoints are expected to surge back into the mid 70's with precipitable water values (PWATS) increasing to over 2.3''...indicative of a very moist airmass:
As the warm front approaches we will have to deal with the potential for showers and perhaps even some thunderstorms and we will have to watch this potential rather closely. If the morning warmup happens to be associated with any thunderstorm activity there will be favorable parameters in place not only for some of these thunderstorms to become strong but perhaps severe and carry the risk for a tornado. Below is a bufkit profile for New Haven, CT for 9:00 AM tomorrow morning from the 18z 4km NAM computer forecast model. We see strong moisture advection, good solid cape in the lowest 6km of the atmosphere and very good low-level directional shear and speed shear indicated by nearly 30 m/s of shear. We also see pretty low lifted condensation levels (LCL's) which mean cloud bases will be rather close to the ground:
The potential for morning thunderstorms would likely be in the 6 AM to 10 AM timeframe. This is not a given but is something that needs to be watched given the parameters that will be in place. In addition to the potential for a tornado, storms could contain some gusty winds along with torrential downpours and frequent lightning. After we wrap up with any morning activity our attention will quickly shift towards the afternoon and evening hours where the potential will exist for a much more potent and perhaps bit more widespread severe weather threat.
With the warm front getting hung up somewhere across central MA, this will enhance the likelihood that surface winds across much of CT, RI, and SE MA will remain "backed" and be more out of the south to even southeast. This is significant because as you go up a few thousand feet in the atmosphere the winds will begin to transition to a more southwesterly direction with winds then transitioning to a more westerly direction even higher up. When you have these change of wind directions with height this is what is called directional shear and creates "atmospheric spin" which can allow for a thunderstorms updraft to not only become tilted but begin rotating if the updraft is able to utilize this. Winds aloft are also expected to increase throughout the afternoon hours as the area of surface low pressure begins to strengthen. In fact, some computer forecast models indicate that winds about 5000ft ASL will increase to as much as 30-35 knots with winds even higher up increasing to 40-50 knots. This increase of winds with height is called speed shear. One way to measure direction and speed shear is through helicity values. Looking at 0-1km and 0-3km helicity values for tomorrow they are forecasted to be very impressive. Typically when looking at the possibility for a tornado you want to start seeing 0-1km helicity values approach 125 m2s2 and 0-3km values approach 150 m2s2. These values, especially 0-3km helicity certainly approach or even exceed those thresholds:
While we know the wind shear will be there the next question is with regards to how unstable will the atmosphere become and how much surface heating we see. With such a moist low-level airmass in place it will not take much sun for the atmosphere to rapidly destabilize, even with poor mid-level lapse rates (though this will be a bit of a negative for getting even more potent updrafts to occur). While clouds could be an issue early on, especially if we see lots of morning activity, computer forecast models indicate we will break out into some sun and strong surface heating will commence with temperatures surging through the 80's. Given the degree of wind shear aloft, if the atmosphere can become quite unstable the threat for several strong to severe thunderstorms will exist and posing the threat for a tornado, strong to damaging winds, and even some hail.
Below is a bufkit profile sounding for Windsor Locks, CT from the 18z 4km NAM computer forecast model for 6:00 PM tomorrow evening and what we see is a very favorable thermodynamic/dynamic environment for any thunderstorm to become strong to severe and pose a risk for a tornado, strong to damaging winds, and hail. In fact, this sounding would indicate the potential for a strong tornado would be on the table:
As typical with thunderstorms and severe thunderstorm potential across southern New England, we really won't know what will happen or unfold until we are able to see the details iron out during the morning hours. These are always very complex setups and in a region where severe weather (especially high end severe weather and tornadoes) aren't as common as further west there are typically always red flags. These setups and how they unfold and produce all depend on the timing of all the main features...does the strongest instability coincide with the strongest wind shear and strongest forcing from the approaching cold front? These are some of the questions we will have to iron out tomorrow.
Given how these setups don't produce all that often around here its very tough to say for certain as to how much in the way of severe thunderstorms we'll see or even if we'll see any tornadoes, however, right now that possibility is on the table and we will have to watch how the conditions unfold in the morning.
