Severe weather, heavy rain likely Monday

As an expansive, powerful upper level low will shift eastward from the Great Lakes toward parts of the Ohio Valley late this weekend into early next week. Immediately, the southerly flow ahead of it — and the nature of the upper level low shifting so far south this time of year — caught forecasters eye for severe weather potential in the Northeast US.  The combination of warm, southerly flow ahead of the approaching upper level low will at least support the potential for heavy rain, while the severe weather setup has become more complicated.

Sunday evening, the first “threat” for thunderstorms will develop over Pennsylvania and Western New York state, where lift for thunderstorm development and instability will support severe storm potential. These storms will move eastward toward parts of our area later tonight, but are expected to weaken over time as they lose support for organization. On Monday, however, things will change — as southerly flow strengthens in our area, shear aloft increases, and lift develops during the afternoon to aid in the development of thunderstorms. Figure 1 shows the approaching upper level system.

Figure 1 - NAM model showing the upper level system shifting toward the area on Monday.

Figure 1 – NAM model showing the upper level system shifting toward the area on Monday.

The severe weather potential becomes a little more difficult to forecast in our area once Monday and Tuesday come around. As a low level jet strengthens on Monday, numerous individual shortwaves and perturbations in the mid level flow will act as mechanisms to aid in the development of thunderstorms. Increasing instability will juxtapose with locally enhanced shear in the mid and low levels, and may support storms capable of strong winds and isolated tornadoes.

The strengthening low level jet and increased moisture will also lead to the potential for heavy rain in any thunderstorms — and instability will support frequent lightning as well. So while confidence in severe storms is high, the exact nature of the threat will depend on the degree of instability and where the juxtaposition of that instability and shear occurs. Current modeling suggests this may develop from Northern NJ into parts of Southeast NJ and Connecticut on Monday afternoon and evening. CIPS analog guidance (based on modeled conditions, best analogs from the past) — shows high probabilities of severe weather in NJ toward NYC/CT on Monday afternoon and evening. (Figure 2).

CIPS analog guidance showing potential for severe weather in NJ/NY/CT on Monday.

CIPS analog guidance showing potential for severe weather in NJ/NY/CT on Monday.

As the pre-frontal trough shifts eastward on Tuesday, the southerly flow will be negated some. But eventually, additional storms are expected to develop by Tuesday evening. The exact nature of this threat will depend greatly on timing — but so long as the front doesn’t slide too far east, increased forcing and supportive wind shear could allow for another threat of organized severe storms, especially nearer to the coast.

We encourage you to stay tuned over the next few days — especially with the potential for multiple weather hazards. While a widespread severe weather event, or outbreak, is not currently expected, severe storms are becoming likely in the area. The exact location of the greatest threat is yet to be determined — and likely won’t come into focus until Monday morning.