An approaching mid/upper level trough which is not all that well defined will be sliding eastward. Associated with this trough is a surface area of low pressure and a pretty strong piece of shortwave energy. As the trough approaches weak height falls will begin to occur and the surface area of low pressure will initially be further deepening. In response to the weak height falls and some continued deepening of the surface low early on, a warm front will be lifting northward and finally stalling out somewhere around the VT/NH/MA border. South of the warm front a very warm and very moist airmass will work into place. Dewpoints will be climbing through the mid to upper 60's...possibly even getting to 70F or a few ticks higher! With 925mb temps climbing between +22C and +25C and 850mb temps climbing to around +20C full sunshine and full mixing would yield surface temperatures in the mid to upper 90's. It appears likely there will be some cloud debris to begin the day and b/c there are uncertainties to how quickly we will get rid of these clouds we will likely not achieve maximum heating. However, we should yield enough sunshine for temperatures to get into the mid to upper 80's...perhaps even some lower 90's across NE PA/SE NY/N NJ where cloud cover initially may be much less of an issue.
Some computer forecast models also are hinting at an elevated mixed-layer plume to advect into the region from the upper Mid-west and some forecast soundings to verify this. It is very interesting to note, however, this elevated mixed-layer plume is actually located a bit more shallow in the atmosphere compared to where we normally see them located. Normally with elevated mixed-layers you will find them from about 800mb up through 550mb or so. This elevated mixed-layer plume looks to exist from about 850mb up to only 700mb or so. Since elevated mixed-layers are associated with some pretty dry air it's preferred to have them located a bit higher aloft b/c this keeps the dry air toward the mid-levels of the atmosphere and keeps the low-levels of the atmosphere very moist. Having the elevated mixed-layer plume this shallow could allow for drier air to end up mixing into the lower levels of the atmosphere drying things out a bit which could reduce low-level instability. Given how the low-level flow is expected to be from the southwest and remain southwesterly this should keep a belt of strong moisture flowing into the lower levels preventing the drier air from working in. The elevated mixed-layer plume being so shallow may also lead to some fairly poor lapse rates in the 700-500mb level. Normally with elevated mixed-layers you have very steep >7-7.5 C/KM 700-500mb lapse rates as temperatures from 700mb to 500mb drop fairly significantly. With the elevated mixed-layer plume so shallow the difference in 700mb and 500mb temps my not be as good as they normally would be, however, computer models continue to show that lapse rates here should still be on the order of 6.5-7 C/KM.
With the approaching trough/surface area of low pressure progged to be sub 1000mb and strong piece of shortwave energy wind fields aloft will also vastly increase and we will be dealing with dynamics we'd typically in the early spring or fall months. Computer models are forecasting a low-level jet in the 925/850mb levels to be on average about 20-30 knots coming in from the southwest...helping to keep funneling in low-level moisture. The mid-level jet at the 700/500mb levels is also expected to increase to as much as 35-40 knots at 700mb to 45-55 knots at the 500mb level with winds here turning to a more westerly direction. The region will also be in the Right fornt quadrant of a 85-90 knots upper-level jet max with upper level winds over the region of about 50-60 knots coming from a more westerly/west-north-westerly direction. 500mb wind speed values of 45-55 knots should yield to about 35-45 knots of vertical shear. With all this said there will be a great deal of both speed and directional shear present. We will also have to watch surface winds across the region, especially the valley locations where surface winds may back to the south or even south-south-east thus further increasing helicity, especially in the 0-1km and 0-2km level. The presence of speed/directional shear will also yield to large amount of helicity with 0-3m helicity forecasted to be in the 200-400 m2s2 range and 0-1km helicity in the 150-300 m2s2 range. This is VERY favorable for supercell t'storms as well as tornadoes.
With surface temperatures expected to climb well into the 80's with dewpoints nearing 70F under the presence of an elevated mixed-layer plume with 700-500mb lapse rates on the verge of 6.5-7 C/KM the potential for moderate instability to develop is there. In fact, several computer models are generating as much as 2500-4000 J/KG of SBcape, 1500-3000 J/KG of MLcape, 2000-3000 J/KG of MUcape, LI values of -6C to -9C and Ncape values as high as 0.25. This is all indicative of a moderately unstable airmass.
During the morning hours as the warm front lifts northward showers/t'storms are expected to develop as isentropic lift increase as well as low-level warmth and moisture leading to an increase in elevated instability. In fact, a mesoscale convective system is possible. Some strong to localized severe storms are possible with this feature as it's moving through. After this complex, if one develops moves through the question then will become how quickly can we get rid of the cloud debris and how quickly can we receive sufficient heating to rapidly destabilize the atmosphere to achieve those instability values just lifted above? It does appear that we will indeed clear out the cloud debris and by early afternoon we see sufficient heating allowing for a rapid jump in temperatures/instability. Once this occurs we should begin seeing the development of t'storms; first across PA/NY and moving towards southern New England with further development expected there.
Several rounds of severe weather are expected tomorrow and lasting well through the evening and overnight hours as the front slowly drags eastward. Early on in the afternoon through the early evening hours numerous supercell t'storms will be possible with the threat for destructive winds, very large hail as well as tornadoes. Once we near evening and near sunset we start losing those stronger helicity values and the severe threat should shift from tornadic supercells to bowing squall lines capable of destructive winds and large hail...an isolated tornado with these can't be ruled out either. Torrential rains will also occur with any storm as well as vivid cloud-to-ground lightning. Flash flooding will also be possible.
Everyone is strongly urged to pay extremely close attention to updates and latest forecasts throughout the day and take each and every warning extremely seriously. This is no run of the mill typical Northeast severe weather event and we have a much higher than normal threat for severe weather with the potential for tornadoes.
Below is a graphic displaying the areas with severe weather potential and areas of heightened potential:
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