The same upper-level low that generated severe weather in the Plains and the Mid-South this week will bombard the Northeast US Friday afternoon and evening before dissipating over the weekend. A series of cold fronts will nudge hot, soupy air upward along the Atlantic Seaboard, sparking widespread thunderstorms. Several environmental factors will contribute toward some of these storms becoming severe, producing powerful wind gusts, large hail, and perhaps a tornado. Areas from New York City northward to Saratoga Springs and northeastward to Worcester, MA are most at risk for these storms.
A warm front passed through the Northeast Friday morning, bringing wide swaths of downpours to Pennsylvania, New York, and northern New England. Behind it, dew points surged into the mid 70s, humidity levels more suitable for the Caribbean than the Northeast. This first batch of rain is crucial, and may either hinder or enhance severe weather later in the day.
The trough will weaken as it approaches the I-95 corridor. The air mass beneath the trough has weakened substantially since emerging from Canada earlier this week. Against the smoldering tropical heat and humidity along the Atlantic Seaboard, the cool, relatively dry air mass will still act as a sufficient trigger for violent storms. The cold frontal boundary leading the charge will propagate east-southeastward with the help of an eager surface cyclone straddling the Saint Lawrence River. The surface low will escape hundreds of miles northeast from its dissipating upper-level low situated, as of Friday morning, over the Ohio River Valley.
Additional forcing for storms Friday afternoon will round the base of the trough toward the Northeast by means of tiny shortwave impulses. These kinks in the jet stream vertically stretch the atmosphere to their east to conserve angular momentum. Rising motion, strong enough for robust thunderstorm updrafts, is the means through which these kinks stretch the atmosphere.
A final source of forcing will arrive with an intensifying low-level jet streak. Areas caught by the rain in the inland Northeast Friday morning will warm slower than the coastal plain. A boundary will develop between areas seeing clouds and rain and areas that started the day with sunshine. This will enhance the difference in temperature behind and ahead of the main cold front. Enhanced winds above the region, where temperature differences are most stark, is the result of this. These enhanced winds will act to force air upwards in a similar way to kinks in the jet stream. Hot, moist air must rise, cool, and precipitate to restore balance between the two air masses, a process that occurs along the axis of these strong winds. The most intense precipitation will occur near the right entrance and left exit regions.
Extreme instability will develop in the soupy airmass ahead of the cold front, serving as abundant fuel for thunderstorms. Shear will be just strong enough north of I-80 to support the development of a few sporadic supercell thunderstorms. Wind shear is essential, the change in wind direction and/or speed with height is needed to separate storm updrafts from downdrafts. Where the storms arrive before dusk, west of I-395 in New England, there may be enough low level shear to generate brief tornadoes. Even where storms do not become severe, the instability will encourage storms to become water-logged, capable of dumping vast quantities of rain in brief periods of time. It has been several days since the flash flooding in parts of the Northeast, so floodwaters have had time to recede. Despite this, these storms still pose a flash-flooding risk, with wet antecedent soils.
Scattered storms will develop well ahead of the cold front, from inland Mid-Atlantic and New England as well as from northern Virginia to Vermont by mid afternoon. A few supercell structures are likely to develop within this region, especially along and west of I-87. Strong straight-line winds from microbursts and outflow boundaries from the heavy rain and cold downdrafts are the main threats, but before merging into clusters some of the storms could produce hail of at least 1″ in diameter as well as perhaps a few tornadoes. The storms will be swift moving, but with a stale upper-level pattern, storms could redevelop over the same areas and pose a flash flooding risk. A zone encompassing Scranton, Birmingham, and Albany is most at-risk for these storms.
Simultaneously, a few storms could pop-up late afternoon along the I-95 corridor, but the main axis of storms will not approach I-95 until just before sunset. By then, discrete storms will have combined into linear systems, at which point the primary threat will be damaging winds. A region connecting Newark, New York City, Poughkeepsie, Hartford, and Springfield is the most likely to experience severe wind gusts from the linear systems late in the evening. A few of these storm segments could develop further south toward Philadelphia, South Jersey, and Delmarva, but with lesser shear the storms are less likely to cause damage.
Overnight, the inland northeast from the Great Lakes to I-81 will be impacted by another line of storms from the main cold frontal passage. They will not be severe due to the loss of daytime heating. Residual clusters will likely persist further east as the upper-level low slowly creeps eastward with its associated lift.
As the dissipating upper-level low inches closer, lift will intensify. If morning showers and cloud debris clear in time for afternoon heating, scattered thunderstorms are likely to develop again, mainly south of I-90, but severe chances will be minimal.