The type of cold front sliding through tomorrow is known as what is called a backdoor cold front. Typically for our area, we see cold fronts mainly come from the west, northwest, or north but every once in a while we see cold fronts work in from the Atlantic ocean from either the northeast or the east. These cold fronts are more driven (or are developed) due to the temperature differences between the cooler ocean waters and the warmer land while cold front which arrive from the west, northwest, or the north are driven (or developed) due to different airmass which exist over land. With this, backdoor cold fronts are typically very shallow in nature and have a lesser slope than regular cold fronts. So I don't get any more technical here the jest of this is...the less shallow the slope, the less intense the upward vertical motion is associated with the front. Think of it like cold fronts vs. warm fronts. This is one reason why warm fronts are usually associated with steady rain as opposed to big time thunderstorms like cold fronts can be. This is because warm fronts have a much more shallow slope than cold fronts...hence weaker upward vertical motion.
As for tomorrow, the backdoor cold front will be sliding in from the northeast and we can see this by looking at and analyzing a 2-meter surface projection for tomorrow afternoon from one of the computer forecast models. The northeast to southwest kink in the 2-meter isohytes indicate the movement of the front will be from northeast to southwest:
There are several ingredients that will be in place tomorrow (some of which was described in the opening paragraph) which do favor some isolated thunderstorms but there are also several red flags (one being the backdoor nature of the front) which indicate the activity may not be all that widespread and the storms will struggle to get very strong.
As stated in the opening paragraph, some computer forecast models are actually hinting that tomorrow will be a rather active afternoon and evening in terms of thunderstorm activity, however, I think this is overdone. In order to save time and keep this post from being any longer than needed I will use one example to illustrate each flag. If you look at the GFS computer forecast model's projection of surfaced-based CAPE tomorrow (a measure of how unstable the airmass is. The larger the cape, the more unstable the atmosphere is said to be) you will see some rather hefty values of 3,000-5,000 J/KG...those values are nearly unheard of here across southern New England...especially anything near 5,000 J/KG:
What could be the cause for this? The answer...dewpoints...really, really high dewpoints. Whenever I see CAPE values being forecasted that high around here the first two things I check to see are 1) Are the surface dewpoints being projected to be too high? 2) If not, how steep are the mid-level lapse rates going to be? The answer here is the dewpoints by the GFS are being forecasted to be way too high. In fact, the GFS has dewpoints well into the mid and upper 70's across a good chunk of southern New England tomorrow:
umm yeah this is not happening. There are a few reasons for this but the one reason we really need to look at are the projected direction of winds within the lower portion of the troposphere. Winds are forecasted to be a more northwesterly direction which typically allows the atmosphere to mix much higher (and the result is mixing down drier air from the upper portion of the lower troposphere and lower portion of the middle troposphere) and is also a downsloping flow for some off of the Berkshire mountains. We can see this from a point-and-click forecast sounding taken from northern CT:
Severe other computer forecast models also suggest dewpoints reaching the 70F mark or even exceeding it, however, there is just no way that happens given projected low-level wind direction. The result here...computer forecast models are overdoing CAPE by ALOT and this results in them forecasting widespread thunderstorms with a greater risk for some strong to severe thunderstorms. This low-level wind flow direction also tends to limit low-level convergence (which leads to rising motion) which is also not good when looking for widespread thunderstorms.
The next flag we will look at is the lack of much stronger wind shear. Using the same sounding as an example, winds aloft aren't projected to be rather strong at all, however, winds from 700-500mb are forecasted to increase to >30 knots as the day progresses from north to south and this will work to increase deep-layer shear a tad. Deep-layer shear is important for both aerial coverage of thunderstorms and how long storms can live as well as enhancing how strong they can become. When you have weak wind fields, this works to lessen the degree of upward vertical motion and it also prevents any sort of updraft and downdraft separation. For storms to have longevity you want updraft/downdraft separation so the storm can continue feeding off warm/moist air, otherwise if there is little to no separation, the storm's updraft is dominated by downdrafts/rain cooled air and the storm rapidly weakens. Given how we won't have really strong shear tomorrow and only modest deep-layer shear, I think this will lead to storms really struggling to get very strong and also mean they are 'pulse-type' meaning they weaken rather quickly:
Lack of a more defined and stronger piece of shortwave energy rotating through the middle troposphere and lack of stronger height falls will also preclude activity from becoming more widespread and prevent activity from becoming too strong. A stronger piece of shortwave energy really helps to accelerate upward vertical motion as does stronger height falls. For the height falls, if this map were to animate you wouldn't see much of a southward progression of these isohytes which indicates heights remain rather steady:
The end result..I think tomorrow we will see a few thunderstorms pop up as the boundary progresses southward through the region and the greatest chance may be confined to southern CT, southern RI, and far SE MA, especially if a sea-breeze boundary were to develop...this interaction between this boundary and the backdoor cold front could really locally enhance convergence and allow for a few really decent thunderstorms to develop. All in all I think most towns will stay dry but for those towns who do see some action, there is a possibility the thunderstorms could briefly become strong/severe and pose a threat for strong winds or even some hail but I think this potential is rather limited/isolated at this time.
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