It’s not just the Southeast that has to confront storminess this holiday weekend. A convoluted synoptic pattern will result in thunderstorms developing over many parts of the U.S. throughout the weekend. Albeit impacts won’t be as formidable as those related to Subtropical Storm Alberto in the Southeast, these thunderstorms will be disruptive nonetheless.
The area of concern will shift further east Sunday, when conditions in most of the Northern Plains and parts of Upstate New York will favor a slight risk of thunderstorms in the afternoon and evening. An Omega Block (the pattern in which the jet stream follows a shape like the capital Greek letter Omega) formed by Saturday’s upper-level low over the Great Basin, a strong region of high pressure over the Southern Plains, and the upper-level low associated with what is now Subtropical Storm Alberto over the Gulf of Mexico will keep the synoptic pattern stagnant. Consequentially, the western upper-level low will broaden, shifting the region of lift towards the Dakotas, eastern Wyoming, and western Nebraska. Temperatures will be in the mid 80s to upper 90s in the western area of concern but unlike Saturday, the boundary layer will be more moist, with dew point temperatures in the upper 50s to mid 60s. With a small area of moderate wind shear, this environment would be more favorable for widespread strong thunderstorms than Saturday if it were not for the weakening of the upper-level low, and therefore the weakening of upward motion associated with it. Therefore, only isolated clusters of the scattered thunderstorms that develop Sunday will be capable of producing severe weather. The primary threat will be damaging straight line winds, but some of the storms may be capable of producing quarter-sized hail.
In New York there will also be a slight risk for thunderstorms to become severe. Robust mid-level disturbance will travel along the northern jet stream in southern Ontario accompanied by a cold front. Meanwhile, a backdoor cold front will approach from the northeast, crossing New England and approaching the Mohawk Valley by late afternoon. Ahead of these two colliding cold fronts, the boundary layer will be warm with temperatures in the low 80s. Along the shores of Lake Erie and Lake Ontario, lake breezes will stabilize the atmosphere, preventing the development of thunderstorms. Further east, with modest instability developing in the afternoon, scattered thunderstorms will develop as the boundary layer destabilizes. Weak values of shear will keep the storms clustered in nature, but there is a chance some supercells may develop that will be capable of producing damaging winds and quarter sized hail.
There is still uncertainty associated with how far west the back-door cold front penetrates. If it is slower to arrive, strong storms may develop as far east as Cooperstown. If it is quicker to arrive or extends further west,. clouds, debris and a sable atmosphere will prohibit storms from reaching further east than Syracuse.
Further south and east in the Mid-Atlantic, scattered showers and thunderstorms will develop along two separate stalled frontal boundaries. Clouds and weak shear should hinder any of these storms from becoming severe.
Storminess will decrease across the northeast, but some lingering showers will remain possible in the North Country of New York and Northern New England, with lingering cloudiness with peaks of sunshine elsewhere in the Northeast. Further west, showers and thunderstorms will develop throuhout the day Monday in the western High Plains, Wyoming, Montana, central Idaho and northern Utah as the upper-level low remains idle. With the further stagnation of the air mass, the showers and storms are not expected to be severe.
The Southeast will remain wet Monday with heavy rain associated with what is now Subtropical Storm Alberto. We have more on that story here. We will keep you updated on any developments related to severe weather and Alberto as warranted throughout the weekend. Have a safe and enjoyable Memorial Day Weekend!