Looking at the 500mb pattern in place Friday morning we see substantial ridging across the central and southern portion of the United States placing the northern-tier of the United States and a good chunk of the northeast on the crest of the ridge allowing for a northwesterly flow in the mid and upper-levels of the troposphere:
While not all northwesterly flow events produce severe weather or widespread severe weather across New England, some of our biggest events gave occurred with northwesterly flows in the mid and upper levels of the troposphere.
Embedded within this northwesterly flow will be two features which will be very big potential focal points towards the development of thunderstorms along with the potential for some severe thunderstorms. These two features are; a shortwave trough which is expected to push into northern New England along with a subsequent piece of shortwave energy and attendant cold front and the other feature being a potential elevated mixed-layer plume. The potential for an elevated mixed-layer (EML) plume to traverse over the region would vastly increase the potential for not only thunderstorms and a few thunderstorms but would increase the potential for a severe weather outbreak. Too save time to further read and understand EML's please refer to my blog post devoted to them:
http://weatherwiz.blogspot.com/2015/06/what-is-elevated-mixed-layer.html
For Friday we are tracking two potential pieces of shortwave energy. One is expected to push through northern New England during the morning hours. With increasing dewpoints, low-level moisture, and shear, this feature could be associated with showers and thunderstorms which move through the Great Lakes the night before. It is this potential which yields questions with how much heating we see later on in the day. If this feature is associated with a quite a bit of cloud debris and shower/thunderstorm activity, it could take quite a while to clear things out meaning we don't achieve maximum heating. This would yield lower temperatures and subsequently lower instability values. However, this could be more important for northern New England and areas north of the MA Pike as the timing of the second shortwave would favor thunderstorm activity during the daylight hours. Further south across southern New England, the timing may favor thunderstorms after dark, thus the degree of heating is not as important. While typically, especially across this region, thunderstorms lose their strength as the sun begins to set and instability begins to fade, steepening lapse rates (faster decrease of temperature with height through the troposphere) could help to keep the airmass quite unstable well into the overnight and this would yield the threat for thunderstorms (both strong to severe) to persist well into the overnight for southern New England.
The second shortwave feature is the main player and would be the focal point for afternoon/evening thunderstorms. This is expected to approach northern New England during the late afternoon hours and be accompanied by modest height falls.
The next feature is the potential plume of EML air which may traverse over New England. This feature would be key into yielding the moderately to perhaps extremely unstable airmass and would provide sufficient fuel for thunderstorms to not only become quite strong but reach severe limits:
As far as wind shear is concerned, winds throughout the troposphere should be more than adequate to not only help generate thunderstorms but keep updrafts maintained, and also perhaps help with updraft rotation. The best wind shear appears to be located across northern New England where they will be closest to the stronger 500mb and 250mb winds:
What can we expect for Friday? Well in the very least, we won't really have a full idea until Friday progresses as as are then able to pin-down the mesoscale (small features which play major roles in these type of events) aspects. However, right now this is some of the potential that exists...
Some morning shower and thunderstorm activity is expected to push through portions of New England Friday morning. After this activity passes the question becomes how quickly do we clear out? Multiple computer forecast guidance indicates we will clear and do so quickly allowing for temperatures to soar into the 90's with dewpoints near 70F. This combination along with steep lapse rates would yield a strong to extremely unstable airmass for much of New England. The focus now would be across northern New England where they will be closer to the approaching second shortwave and strongest shear. Here we will be on the lookout for the development of more showers and thunderstorms and on a widespread level. Given the potential for strong to extreme instability and strong wind shear several of these thunderstorms could become strong to severe and produce strong to damaging winds, large hail, and given the degree of shear, supercells would be possible yielding the potential for an isolated tornado. The extent of this potential all depends on degree of instability and timing of the features (does the shortwave time perfectly with the arrivial of steep lapse rates and strong shear?). If everything times together we could be looking at a major severe weather event with numerous reports of several weather. If the timing is not perfect than we would be looking at widespread thunderstorms with only a few becoming strong to severe and isolated pockets of damage.
For southern New England...the watch will be on the activity to the north as this activity could come together to form what is known as a mesoscale comvective complex (MCS) and push through portions of southern New England overnight. If the airmass remains quite unstable, this complex could produce a swatch of strong to damaging winds and even large hail. Again this depends on the degree of instability and how the activity to the north evolves.
Too summarize some of the things to watch;
1) EML/lapse rates...There is not 100% confidence or agreement that we do see an EML-plume/associated steep lapse rates traverse into New England. Some computer guidance suggests this while others do not. If we don't see this feature occur the threat for a severe weather outbreak would vastly diminish and the overnight severe threat for southern New England would be very, very slim.
2) Morning cloud debris associated with shower/t'storm activity...again, this is more of a concern for northern New England but if it takes a while to clear out and surface temperatures do not realize their potential, the degree of instability will be vastly less and the threat for a widespread severe weather outbreak would be lessened.
3) Surface dewpoints...This is something else to watch. Some forecast soundings indicate surface and boundary layer winds may become more westerly in nature. If the degree of low-level moisture isn't very high, or if the high dewpoints only exist up to a couple thousand feet or so, westerly winds would work to mix down some of this drier air aloft and that would drop the surface dewpoints and vastly decrease instability and subsequently severe weather potential.
All in all Friday has the potential to be quite an active day for a good chunk of New England and we are just coming off of the region's biggest severe weather event this summer (Monday, July 18th, 2016). While this potential exists, there are certainly alot of questions still to be answered with regards to how the key features evolve and we should get a better idea as new data continues to roll in. As Friday morning progresses, we will develop a very good idea of what to expect as the mesoscale features are pinned down.
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