The Storm Prediction Center has placed the area in a rare “Moderate Risk” on the Day 2 Outlook, valid for Thursday July 26 2012, mentioning the
potential for widespread and significant damaging winds with strong and severe thunderstorms. The Moderate Risk line runs through North-Central New Jersey from southwest to northeast, including the NYC Metro Area. Points to the south of this line are included in a “Slight Risk” (more typical for our area), with the risk of severe thunderstorms dropping off somewhat as one heads into Central and Southern New Jersey. However, to the north over Northern New Jersey, Southeast New York (including NYC Metro) and Connecticut, the potential exists for a more significant severe weather episode, characterized by the potential for strong and damaging wind gusts as well as isolated tornadoes. The Storm Prediction Center also mentions the potential for a “Derecho”, which produces damaging winds in excess of 60 miles per hour over a large area The next few paragraphs will dive into some more advanced meteorology, so if you’re not looking to hear the down and dirty details, just skip below.
The potential for such a widespread severe weather episode in the Northeast is being driven by an approaching cold front over the Ohio Valley, but the exact processes behind the severe thunderstorms remain incredibly complex. To start, forecast models agree on a decaying MCS (system of thunderstorms) moving east across New England along a warm frontal boundary early Thursday morning. The models are struggling with the exact track of this feature, and where it ends up will ultimately iron out a ton of details in regards to the second round of storms Thursday PM. Several models keep the morning activity to the north, while a few also bring it into our area. Wherever it tracks, the potential exists for some isolated severe weather reports (mainly some hail in elevated convection). Regardless, a plume of extremely impressive kinematics will be involved…with forecast models indicative of effective shear over 40 knots by Thursday afternoon. A surface low developing near Albany will also bring enhanced low level shear along a gradient near NYC.
By afternoon, forcing along the mid level height gradient will develop storms over North-Central PA and Central New York, that will begin to slide eastward. Well back to our west, activity will also fire along the cold front. But along the gradient, which runs from North-Central PA to near NYC and Northern NJ, is the best juxtaposition of instability and shear parameters. Here, the potential exists for a developing thunderstorm complex which could eventually mature into a Derecho .. and produce strong damaging winds over a large area. Where this gradient sets up will be critical to storm development.
One potential caveat to this severe weather potential will be, as mentioned above, the exact positioning of the gradient and baroclinic zone. Forecast models have been ticking this farther north during the day today — which could eventually place the best gradient for the MCS just to the north of the NYC area. If this is the case, the widespread damaging winds could develop and track from Northern PA through Southeast New York and possibly into Western New England.
In summary, the potential for a high-end severe weather event is definitely there. The Day 2 Moderate Risk from the Storm Prediction Center certainly reflects that potential. However, the threat is not very clear cut at all. In fact, forecast uncertainty is extremely high at this point, with the event expected to transpire beginning with the activity moving through New England late Wednesday Night into Early Thursday morning. So, we urge you to stay tuned both tonight and into Thursday. We’ll be activating our Live updates page, and will continue to update via Facebook and Twitter. We’ll also have a new blog post out around 2am with details on the new Day 1 Outlook from the Storm Prediction Center.