A powerful low pressure system with a violent history will sweep a dangerous line of thunderstorms across the Southeast, Ohio Valley, and Mid-Atlantic Sunday into early Monday morning as it treks northeast towards Québec, putting nearly 100 million Americans at risk for damaging and life-threatening severe weather. Those in the path of the storms ought to take warnings seriously. Saturday, the storm system was responsible for spawning at least 15 tornadoes across east Texas and the lower Mississippi Valley. The tornadoes took at least one life and caused at least 22 injuries. Tree-toppling straight-line winds will be the primary threat with Sunday’s storms, but tornadoes and large hail will also be possible across vast portions of the Eastern US.
The storms directly related to Saturday’s severe weather outbreak survived the night, and as of early Sunday afternoon were impacting the Tennessee Valley, Georgia, the Florida Panhandle, the Carolinas, and western portions of the Mid-Atlantic. These storms have gradually outpaced the favorable forcing mechanisms and instability needed to develop new storms and sustain ongoing storms. The severe threat with this first ongoing round of storms is therefore low except in Georgia and Florida, where low-level winds and moisture transport from the Gulf of Mexico are still quite strong.
The first round of thunderstorms will fizzle over the Appalachian mountains from Pennsylvania to the Carolinas by mid-afternoon, leaving plenty of time for sunshine to break through the lid on convection. The air mass behind the storms will be warm and incredibly humid. Temperatures in the 70s and low 80s matched with dew points in the 60s and low 70s will make for an environment capable of supporting severe weather across an expansive area encompassing most of the Southeast, Ohio Valley, and Mid-Atlantic. Matched with a 70-90 mph low-level jet stream, the storms will have an abundance of dynamic support from speed shear and from enhanced lift for becoming and remaining severe.
The storms of most concern will first fire over the Ohio and Tennessee Valleys early Sunday afternoon with help from the low-pressure system’s trailing cold front. The cold front will separate air masses more than 30°F apart in temperature across the span of just a few miles. The potency of the cold front will quickly organize the scattered storms into a series of linear systems by late afternoon over Ohio, Kentucky, eastern Tennessee, and northern Georgia.
Some of the storms that eventually merge into linear structures could develop rotating updrafts by late afternoon. These supercells will be capable of producing damaging wind gusts, golf-ball sized hail, and isolated tornadoes. The greatest chances for tornadoes will be in eastern Ohio and northern Kentucky, where the most favorable severe thunderstorm dynamics will overlap peak heating.
The linear storm complexes themselves will have a much lower risk for hail than the individual storms that develop before them. However, they will still pack quite a punch. The linear complexes will carry a more widespread risk for straight-line winds strong enough to topple trees into buildings, knock down power-lines, and lift roofs from certain structures. Chances for wind damage will be especially strong in linear complexes that become “bow echos,” in which strong thunderstorm-generated low-level winds push the storms in the middle of a segment outward like an archer’s bow. Tornadoes may also be generated by these linear complexes, although they will generally be weak. These types of storms will be most favorable from western Pennsylvania, western West Virginia, eastern Ohio, and northeastern Kentucky.
Eventually, linear systems will merge into one intense squall line of thunderstorms. The squall line will form over the western periphery of the Appalachian mountains by early evening over western Pennsylvania and New York southwestward to upland portions of the Carolinas. The upper-level energy maintaining the storms will also act to force instability northward ahead of them despite the setting sun such that the squall line will maintain its intensity long into the night. The squall line will be so intense that its outflow will help generate new scattered thunderstorm complexes over the Mid-Atlantic during the evening hours well ahead of its arrival, namely in Virginia, Maryland, eastern Pennsylvania, and northern New Jersey.
The squall line will be most intense through the late evening hours. It will reach the I-81 corridor by midnight and the I-95 corridor by 2am. The sun will have long set yet damaging winds and isolated tornadoes will still be a threat for unsuspected, sleeping residents of cities like Binghamton, Scranton, Harrisburg, Newark, Philadelphia, Baltimore, and Washington, D.C.
Coastal areas away from south-facing shore lines like New York City, Atlantic City, Norfolk, and the Outer Banks will be last to undergo the nasty impacts of the squall line before it decays over southern New England, Long Island, and the Atlantic Ocean early Monday morning. The stability of the cool ocean usually serves as a graveyard for thunderstorms. Enough instability will remain aloft over Long Island to slow the storms’ demise and support isolated damaging wind gusts, but the instability will fall flat over southern New England, where dynamics alone cannot sustain the storms’ intensity.
A widespread severe weather outbreak will threaten millions of Americans across the Eastern US Sunday and Sunday night. All modes of severe weather are possible, although damaging wind gusts and weak tornadoes are the most probable. Those in the line of the storms are urged to heed warnings from the National Weather Service and to take precautions ahead of the storms to prevent loss of life and property. Secure or bring-inside loose outdoor plants, furniture and other belongings before the storms strike. Be sure to have batteries, warm blankets, a radio, a flashlight, a first-aid kit, wrench to shut-off utilities, medications, and at least three days of water and non-perishable food in the event of a power outage or strong tornado.