Hazy, hot and humid air has settled into the region this afternoon as southerly winds pump in the warmth thanks to mid level ridging. Back to our west-northwest, an energetic disturbance is ejecting northeastward through the Ohio Valley. Increased lift and forcing is helping to force the development of thunderstorms within an unstable environment, leading to the threat of widespread severe weather over Pennsylvania and New York State. The Storm Prediction Center has issued a Moderate Risk for severe weather in those areas through this evening.
Why is this threat developing?
There are many pieces at play, but the main culprits are most definitely the Western Atlantic Ridge (actually centered near Bermuda at this point) and the incoming energetic trough. The combination of the two is leading to increased instability, strong forcing for thunderstorms, and sufficient wind shear for storm organization. Figure 1 (below) shows a four panel model image from this afternoons 12z NAM model. On the top left, the model is producing precipitation along elongated height falls from the system to our northwest. On the top right, very warm air at 850mb has advanced into the region with temperatures over 18 C. Finally, the bottom two panels show the energetic pattern at 500 and 300 mb.
What is the latest from the SPC and NOAA?
As mentioned above, the Storm Prediction Center has placed parts of the Northeast US in a rare “Moderate Risk” for severe thunderstorms. This includes much of Western and North/Northwestern Pennsylvania as well as parts of Southwestern New York State. The main threat with these storms is damaging wind gusts. In fact, the SPC suggests the potential for widespread strong to severe winds in thunderstorms in the aforementioned areas. Probabilities for severe-level wind gusts within 25 miles of any point are 45% (Fig 2).
The local National Weather Service offices in the aforementioned areas have already issued several severe thunderstorm and tornado warnings this afternoon. In addition, a large Severe Thunderstorm Watch is in effect for much of those areas, with high probabilities for severe wind gusts and wind damage.
This threat is expected to traverse eastward over time through this evening. The Storm Prediction Center anticipates some weakening of the convective line as it moves through New Jersey. Almost all short term forecast models show this scenario, with the line of strong to severe thunderstorms moving eastward through New Jersey toward the NYC Metro. Although the storms may be weakening as they do so, we still anticipate the threat for thunderstorms capable of producing dangerous lightning and heavy rain. Figure 3 shows the HRRR model’s idea — with the line of storms moving through NJ tonight.
What’s the deal, and why will these storms weaken?
As we mentioned earlier, the bottom line in this entire setup is that the main threat for widespread severe weather will remain to our west-northwest. This is where the Moderate Risk was issued — over PA and NY State. That being said, the storms will eventually traverse east/southeast through PA towards NY and SE NY including the NYC Metro Area. As they do so, we do anticipate that they will weaken considerably enough to mitigate any widespread severe weather in our area. If there were to be an area near us to pinpoint the highest threat for severe storms, it would be West/Northwest NJ and the Hudson Valley.
The storms are expected to weaken to due a capped atmosphere, initially negative buoyancy and strongest forcing in relation to the atmospheric disturbance staying well to our north and west. Still, if the storms can become organized (and possibly even develop a cold pool, although not likely), they could traverse east/southeastward towards our area with some ferocity. We do anticipate heavy rain and frequent lightning in any storms that move toward our area later tonight.
Stay tuned throughout the remainder of the afternoon and into the evening for further information.