Published: June 9,2016
Severe thunderstorms will become more numerous Friday into the weekend from the Upper Midwest into the Northeast as building heat and humidity interact with an active jet stream.
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Severe weather has been spotty this week. Wednesday, a line of severe thunderstorms produced wind damage in eastern Pennsylvania, including Philadelphia, and across southern New Jersey, including Atlantic City.
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The main severe threats will be damaging winds and large hail. The probability of tornadoes appears to be low, but isolated tornadoes can occur in these situations.
Below is our latest forecast thinking on the timing and magnitude of the severe threats, followed by the severe weather setup.
(INTERACTIVE: Your 7-Day Severe Weather Outlook)
Severe Weather Forecast
Friday- Clusters of morning t-storms are possible from parts of Minnesota into the Great Lakes and possibly the Ohio Valley with some hail and locally heavy rain.
- Scattered strong to severe storms should fire up in the afternoon in parts of the eastern Dakotas, Minnesota, northern Wisconsin and Michigan's Upper Peninsula. This activity should then push southeast Friday night into Wisconsin, Michigan, and northern Illinois.
- Threats: Damaging straight-line winds and hail are the main threats. A tornado or two can't be ruled out with initial discrete cells in Minnesota in the afternoon.
- Cities: Duluth | Minneapolis | Ironwood | Wausau
Friday's Thunderstorm Forecast
- A somewhat more widespread threat for severe thunderstorms from southern Michigan and Ohio, eastward to southern New York and the Mid-Atlantic region.
- Threats: Damaging winds, hail, and perhaps a tornado or two.
- A second area of severe weather is possible in the northern High Plains of eastern Montana, and western North Dakota.
- Threats: Large hail, damaging winds and isolated tornadoes are possible.
- Cities: Detroit | Cleveland | Buffalo | Pittsburgh | New York | Philadelphia | Washington
Saturday's Thunderstorm Forecast
The jet stream pattern will be quite exaggerated by the end of the week. A large ridge (northward bulge in the jet stream) will build into western and central Canada. At the same time a trough (southward dip in the jet stream) will be located just off the East Coast of the U.S.
Mid to upper-level winds from the Great Lakes to the Mid-Atlantic will be racing from the northwest to the southeast (often referred to as a "northwest flow" pattern). This pattern is not uncommon in June when high pressure ridges become stronger, but significant troughs can still be in place farther east.
Occasionally, a strong disturbance (or a series of disturbances) can ride down the eastern side of the ridge toward the trough. The disturbance gains strength as it approaches the trough. At the surface, temperature and moisture contrasts along a frontal boundary become enhanced. This creates a more favorable environment for thunderstorm development.
Thunderstorms that develop can feed on the dynamics (energy in the atmosphere) created by the disturbance gaining strength. Depending on the situation, strong to severe thunderstorms can develop. These storms are capable of producing large hail and damaging winds.
By Friday, we expect that a disturbance riding down the east side of a high pressure ridge aloft will trigger thunderstorms, some severe, in the Upper Midwest and northern Great Lakes regions.
On Saturday, a stronger disturbance aloft and a frontal boundary at the surface will enhance the threat of severe weather from the lower Great Lakes to the Middle-Atlantic region.
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