Over the past few weeks, strong ridging has dominated much of the United States thanks to a very strong Bermuda High. However, over the past several days, a trough has been developing and amplifying as it moves eastward and this is allowing for the ridge to slowly break down. As this trough continues to amplify and the area of surface low pressure continues to deepen, the increased pressure gradient will allow for the winds aloft in the lower, mid, and upper levels of the atmosphere all to increase. Throughout the day on Saturday we will see the low-level jet increase to 25-35 knots, with the mid-level jet increasing to 35-45 knots, even upwards of 50-60 knots across central/northern New England, and the region will be in the right entrance region of a 90-100 knot upper-level jet max which will really help to increase divergence and lift across much of New England. With the increase in the mid-level wind fields, 0-6km shear values should increase to 30-40 knots, which is more than sufficient for organized t'storms.
While we may have more in the way of cloud cover on Saturday, temperatures are expected to reach anywhere from the mid-80's along the coast to as high as the low to mid 90's inland. This coupled with dewpoints in the upper 60's to lower 70's will once again make for very uncomfortable conditions outside. During the day on Friday steeper mid-level lapse rates (change of temperature with height) have advected into the region, and there is even a hint at a weak elevated mixed-layer (EML) across much of southern New England as mid-level lapse rates are approaching 7 C/KM and some forecast soundings do have an EML look. As we move into Saturday, mid-level lapse rates are expected to remain quite favorable, in the 6.5-7 C/KM. These stepper lapse rates coupled with surface temperatures ranging from the mid-80's to the lower to mid-90's and dewpoints in the upper 60's to lower 70's should yield to a moderate to extremely unstable atmosphere. We could be looking at surfaced-based cape values ranging from 2500-4000 J/KG, mixed-layer cape values in the 1500-2500 J/KG range, MUcape values in the 2000-3000 J/KG range, and lifted index values as low as -6C to -9C...these numbers are indicative of an extremely unstable atmosphere and will be the fuel for the t'storms.
Out ahead of the cold front, a pre-frontal trough is expected to develop across the Hudson Valley and then slide east during/just past prime heating and this will be the focus for lift. The combination of increasing lift along with adequate wind shear and a moderately to extremely unstable atmosphere will lead to the development of widespread showers and t'storms tomorrow afternoon and lasting through much of the evening and into the early overnight hours. Winds aloft will be fairly unidirectional which suggests that the main storm mode tomorrow will be a linear squall line with bowing segments. If we are to achieve the higher end of instability, then some supercells will be possible as well. Forecast soundings also show an inverted-v signature. Given all this, the main severe threat with thunderstorms tomorrow will be damaging winds in excess of 50-60 mph. If we do see supercells, then the threat for large hail, possibly as large as golf balls would also be a possibility. All storms will contain vivid cloud-to-ground lightning along with torrential downpours, perhaps leading to poor drainage flooding.
Below is a map indicting the risk area along with hazards:
***NOTE*** Not everyone in the shaded area may receive rain or even t'storms. Not everyone will experience a strong to severe t'storm either. The highlighted area outlines the area where severe weather may occur***
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