Scenarios for Tuesday’s severe weather.

This remains a very difficult forecast even for 24 hours out. The SPC has gone with an enhanced risk of severe weather for parts of the New England states, and this event certainly has high-end potential. However, there are many things that must all come together for such to be reached.

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WRF-NMM Simulated radar at 8:00 am EDT June 23, 2015. The MCS is decaying over northern New England and the St. Lawrence Valley.


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WRF-NMM Simulated radar at 2:00 pm EDT June 23, 2015. Notice redevelopment over New York State.

Round 1: Remnants of Great Lakes storms

Most models show that a mesoscale convective system (MCS) will likely be decaying over the eastern Great Lakes and St. Lawrence Valley early Tuesday morning. That will likely only affect northern New York and northern New England but could lay down outflow boundaries that may enhance wind shear later, if the air mass recovers. Severe weather from this round is most likely in the far western areas, as the line will likely be weakening before clearing the Great Lakes, due to the loss of daytime heating and less conducive air mass compared to in the central Great Lakes region. Any leftover cloud cover that persists through the morning and early afternoon may prevent atmospheric destabilization and reduce the threat.

Round 2: Redevelopment

Models insist on redevelopment of convection either late Tuesday morning or early Tuesday afternoon, likely in a curved axis from Ottawa to Syracuse to State College to Youngstown to Columbus (or just east of that area). In addition, cells may also form ahead of that axis over the Mid-Atlantic through southeastern New York and southern New England as the afternoon progresses ahead of the front. In terms of threats, large hail and damaging winds are certainly threats no matter what, given the highly unstable air mass, warm temperatures and dynamics aloft. Tornadoes are much more of a question mark, however.

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EHI (energy-helicity index) at 2:00 pm EDT June 23, 2015. Notice the low-level dynamics are greatest farther north, closer to the low. They increase in southern New England later.

The best shear lies closer to the low center over the St. Lawrence Valley and northern New England. However, instability will likely be lacking there, as the cold front may overtake the air mass before the cloud cover can clear and redevelopment can take place. However, should clear breaks occur before the cold front passes and the low-level wind shear remains in place (as suggested by other model runs), that could be a focused area for supercell development including tornadoes due to the high amount of spin in the atmosphere and the outflow boundaries which may be present as a result of the morning MCS.

Farther south, across southeastern New York and southern New England, a clear window of opportunity for development and severe weather exists in the afternoon hours. All parameters for storm development and damaging winds are expected to be in place, with very high instability. The helicity values may be marginal for tornadic development there, however, so large hail and damaging winds are likely the main threats. A few tornadoes are also possible. If helicity is greater than forecast, more significant tornadic activity is not out of the question.

In the mid-Atlantic, wind shear is definitely a limiting factor, which is typical for summertime setups. As a result, while widespread thunderstorm activity is likely there as well, severe weather should be more isolated. Nonetheless, damaging winds and isolated large hail are possible as far south as Virginia.

It will be a complicated weather day that will likely not be resolved until activity develops. For there to be a significant tornado outbreak, the greatest instability and low-level shear (including spin) need to line up, which models don’t think will happen but some earlier runs suggested it was possible. It is advisable regardless to keep an eye to the sky on Tuesday afternoon and evening and keep in touch with the weather, in case watches and warnings are issued. I will post many updates on my Twitter feed at @EternalWeather1 as well.

Forecaster Craig Ceecee

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