A very active day across the Northeast back into the Midwest with damaging winds being the main concern. Latest model guidance has backed off on the forcing and the instability and are not as convincing for a long tracked MCS from NY/PA to Southern New England. This is the result of a few things:
1) Pre frontal activity is looking more likely and will be capable of convective overturning ahead of any line that would develop. Meaning the line would eventually run into some cold inflow weakening it.
2) Weaker linear forcing showing up which will favor a window for supercell development, but the main idea was an MCS developing and as the trough pulls away it would veer the low level winds becoming more uni-directional and feed into a fast moving MCS across NY state into PA.
Another issue showing up on models is the cloud debris. This has a few impacts as well:
1) Lowering instability. The models are overdoing the cloud debris associated with activity today into tonight to the west. I do think cloud debris will be an issue and might limit severe potential over a few areas highlighted above.
2) But I also think this will contribute to a few differential heating boundaries and many areas warming up more so then models project. This is important because those who do see sufficient heating will reach Convective Temperatures. Reaching convective temperatures means the atmosphere is ready to go and the very slightest lift from the sfc will kick things off. A differential heating boundary could contribute to this. Along with the sea breeze front and pre frontal troughiness (hence the aggressiveness in our forecast to the east and south).
3) Cloud debris or not CAPE values will not be too high on Sunday since mid level temperatures are warm limiting the potential for higher CAPE. But as noted above in number 2 it will enhance severe threat in other ways.
W NY/NW PA
Right now this area is in the area for supercells/isolated tornadoes. The reason we highlighted this area in red is because the threat is increasing slightly for supercells. In our last blog we highlighted the edge of the mid level winds contributing to an area of 25-30Kts of vertical shear over this area with a very strong LLJ beneath it. Strong speed shear and directional shear in the lower levels allow for some nice clockwise curved hodographs. And as we pointed out in our last blog this environment will favor right moving supercells enhancing SR flow in the anvil allowing more ventilation for the updraft. Thus, more updraft momentum to achieve supercell status before system becomes outflow dominant with the water loading. In addition to supercells we expect this area to see small scale organized line segments along lake breeze boundary/pre frontal confluence and perhaps some bowing segments as we head into afternoon with veering flow.
Rest of New England/Northeast
The reason why we highlighted this area in isolated to scattered severe is because of the uncertainty of convective evolution. We cannot pinpoint a few areas since we expect the radar to light up along the pre frontal trough/higher terrain/pre frontal confluence/remnant outflow boundaries. We are confident in destabilization with enough diurnal heating along the coastal areas to add them to the mix. The sea breeze could be another player in the convective initiation near the coastal areas with convective temperatures being reached with low LFC heights. In all area the dry air aloft will enhance the severe threat with damaging winds. The nice thing is the weak vertical shear in these areas aloft preventing too much negative energy to updraft momentum. Flash flooding threat is high along with the potential for damaging winds. Hail will not be a factor for the most part but a few marginally severe reports are possible in stronger storms. The severity of the threat depends on the organization or lack of organization of the multicells that develop well ahead of the cold front. We think enough storms will fire off to allow for the opportunity of converging cold pools to allow for some semi-organization. Shear orientation and speed in the lower levels will favor upscale development along these cold pools, another contributing factor to make us add scattered severe to this area.
Stronger heating combined with the sfc convergence along the front will lead to severe multicells in the afternoon. Main threat will be for damaging winds and hail along with flash flooding. Hail threat will be highest over this region as SBCAPE values will exceed 3500J/Kg in portions of the area. Weak winds aloft will prevent a more widespread severe threat as storms should pulse out quickly after development.