The train of tropical moisture that has inundated parts of the Northeast with historic flooding this week will derail after a cold front collides into the moisture flow Friday. This derailment will result in an explosion of strong thunderstorm clusters Friday afternoon and evening before clearing this weekend. Strong wind gusts, large hail, and even a brief tornado will be possible. Unfortunately, recently inundated cities along the Susquehanna River are expected to be impacted by these thunderstorms.
Throughout last week, a strong Bermuda High over the western Atlantic used the East Coast of the US as a freight line to ship vast quantities of moisture from the Tropical Atlantic. Heat alongside this tropical moisture has caused vast quantities of instability to develop over the Northeast this week. Instability will be even stronger Friday as the train of tropical moisture accelerates with the approach of an upper-level low pressure system. Temperatures will warm to the upper 80s and lower 90s in tandem with the increase in flow from the tropics. The strong instability will add fuel to the development of thunderstorms as cool, dry Canadian air collides with hot, wet Caribbean air.
The upper-level low will further contribute to thunderstorm development Friday. Strong winds rounding the trough will contribute to strong wind shear, the change in wind speed and direction with height. Wind shear is pivotal to the longevity and organization of thunderstorms. Shear separates the storms’ updrafts from the downdrafts, permitting them to grow stronger.
The cold front itself will not be strong enough to bring widespread severe weather. However, the upper-level low will also support a vigorous jet streak resulting from the 5°F – 10°F change in temperature behind the cold front trailing its surface cyclone. Jet streaks are the atmosphere’s way of decreasing the differences between two air masses to restore balance. This is done by forcing air upwards towards the right of where wind enters it and to the left of where wind exits it. Jet streaks enhance precipitation and instigate thunderstorm development. Upward motion from the jet streak and the cold front encourages the formation of weak surface low pressure ahead of the cold front known as a “pre-frontal trough”, which can prompt rounds of thunderstorms ahead of the cold frontal collision.
The robust jet streak, the leading pre-frontal surface trough, the cold front, the strong speed and directional shear, and the intense instability will all contribute to ripe conditions for severe thunderstorms in the Northeast Friday afternoon. Scattered thunderstorms will initially develop inland along a corridor from northern Virginia northeast through southern Quebec. Any of these initial storms could develop rotating updrafts and become supercells, capable of producing strong wind gusts, hail with diameters in excess of one inch, and possibly a brief tornado. Some of these storms will compound flooding woes in Pennsylvania cities along the Susquehanna River like Harrisburg and Lancaster.
The afternoon storms will be most numerous in northeastern Pennsylvania, the eastern Mohawk Valley, the lower Champlain Valley and the Hudson Valley, where the pre-frontal trough will pass during peak daytime heating hours in the first half of the afternoon. Flow up the mountains in southern Pennsylvania could also result in more numerous thunderstorms. Storms in these areas are most likely to become supercells. Weaker storms are expected further west since the atmosphere will not yet have destabilized by the time of the cold frontal passage. Cities that could be impacted by supercells capable of producing strong winds, large hail and possibly a brief tornado include Scranton, Albany, Saratoga Springs, and Pittsfield.
As the derailment of the tropical moisture train tumbles toward I-95 during the height of rush hour, outflow from the thunderstorms and further daytime heating will contribute to the merging of storms into clusters and quasi-linear segments. Diurnal destabilization and outflow boundaries from storms further west before the arrival of the large scale forcing will result in scattered thunderstorm development late in the afternoon along the coastal plain. Consequentially, supercells will be unlikely and only isolated severe weather is expected. Some of these pop-up storms could produce gusty winds and small hail but their main impact will be to prematurely cutoff updrafts from adjacent storms and impede the development and progression of linear segments ahead of the cold front. Linear segments that do progress to the I-95 corridor will still be capable of producing damaging winds just ahead of them. Washington, D.C., Philadelphia, Trenton, and New York City are all susceptible to impacts from these strong storms.
Momentum from the derailment will have slowed by the time the storms reach eastern New England and the immediate Mid-Atlantic coast. The storms will have lost their severe characteristics by the time they reach Atlantic City early in the evening and Providence, Boston, and Portland later in the evening.
The tracks will be cleared of debris this weekend as the Bermuda High that has used the East Coast as a freight line for rain over the last week shifts eastward. Only a few scattered thunderstorms are expected Saturday, mainly over upstate New York and New England. Sunday is expected to be dry before rain returns Monday when the freight line returns to use.