After an active day of severe storms from the Southern Plains through the Ohio Valley and into the Mid-Atlantic, there will be a higher risk beginning on Tuesday across much of the same region as well as New England. Not only are damaging winds as high as 70 mph possible, there is also a risk (although lower) for large hail and a couple tornadoes. Flash flooding is also possible, especially in the Mid-Atlantic where Flood Watches are in effect.
AWatch has been issued for parts of CT, MA, NY, PA, VT until 8 PM EDT and for parts of Connecticut and Massachusetts until 9 PM EDT.
A Severe Thunderstorm Watch has been issued for parts of New York and Pennsylvania until 8 PM EDT, for parts of MA, NH, RI until 9 PM EDT, and for parts of CT, DE, MD, NJ, NY, PA until 11 PM EDT.
The setup will be very conducive for the development of severe thunderstorms in the afternoon and evening across the Northeast on Tuesday. There are four main ingredients needed for severe weather: shear, lift, instability, and moisture. The risk for wind shear will be moderate to high. The highest wind shear will be to the north of the warm front, which is where cooler temperatures and drier air will be located. This environment will not be favorable for severe storms. The best wind shear will be located closer to the south of the front. There will also be some directional shear, which is a change in the wind direction by height. The greatest directional wind shear will be in eastern Pennsylvania, much of New Jersey, southern New York, and western Connecticut. Therefore, a couple isolated tornadoes cannot be ruled out. Speaking of low pressure, lift is the second ingredient. This low pressure will be weak, so lift will be moderate, but a stationary front will sag to the south, becoming one of the main instigators for a line of storms that will form. Instability is the third factor, and will be plentiful. Instability is measured in J/kg by looking at CAPE, or the convective available potential energy. Widespread values of 2000 to 4000 J/kg is forecast by various models. The highest of CAPE will be several miles to the south of the warm front across most of the Mid-Atlantic. Some of these higher CAPE values will also extend into southwestern New England. Of course, moisture is needed for thunderstorms, and it will be widespread as well. Typically, dew points of at least 65 degrees are needed for severe storms; in this setup, dew points will be in the upper 60s to low 70s. It will also be a hot day, with widespread highs in the 80s. All of theses factors coming together point to some severe weather in the Northeast.
The Storm Prediction Center is also indicating that there is the potential for a derecho, following the likely derecho that hit the Mid-Atlantic on Monday. There is a large area of the Northeast is at risk for wind gusts of up to 75 mph from the line of storms that develops, prompting them to issue a rare ‘Moderate Risk’ for this part of the country: “impacts could be of [derecho] caliber in the area affected… the potential for a well-organized swath of damaging wind — including a few gusts around hurricane force.”
Now let’s talk timing: There will be a large area of rain moving through Upstate New York and much of northern New England during the morning hours. That rain will then become a bit more scattered as all the atmospheric dynamics come to produce the powerful line of severe storms beginning at around noon. This line will develop over western New York and Pennsylvania and will track to the south and east, reaching the I-95 corridor between 4 and 6pm in southern New England and 7 and 8pm in the Mid-Atlantic down through Philadelphia. These storms will then weaken as they reach the coast. Washington, DC may get clipped by the southern extent of this storm. Back toward the west, thunderstorms over the Southern Plains and western Ohio Valley will mainly dissipate by the afternoon hours. That will allow for new thunderstorms to develop in the Ohio Valley during the afternoon, but these storms are not expected to be severe.
These are the current maximum chances for the following threats, according to the Storm Prediction Center:
Tornadoes: Up to 10% chance
Damaging winds: Up to 60% chance
Large hail: Up to 30% chance
There will also be a few strong to severe thunderstorms that develop in the Southern Plains. Storms will fire up by the evening across West Texas. These storms will then become more widespread as they track east into central Oklahoma and north-central Texas, before weakening and mostly dissipating early Wednesday morning. Stronger storms may move into cities like Dallas, Fort Worth, and Oklahoma City. Steep mid-level lapse rates and strong surface heating will encourage isolated pockets of strong/damaging winds and perhaps large hail, with the most intense cores during the afternoon and evening.