The active severe weather pattern will continue to bring more thunderstorms to parts of the South and the Plains through the end of the week. The most active days are expected to be Wednesday in the Appalachians and Thursday in the northern Plains. Storms capable of producing tornadoes Wednesday will be confined to central Pennsylvania. Elsewhere, strong wind gusts and golf ball-sized hail will be the main threats. On Thursday we’ll see violent thunderstorms develop over an even wider swath. Storms capable of producing colossal hail and tornadoes will threaten eastern Montana, North Dakota, and northern South Dakota.
As early morning and afternoon showers clear in Pennsylvania southward through Georgia, a soupy environment left behind by a warm frontal passage will destabilize ahead of a shortwave trough of low pressure. This same shortwave has a history of violent weather. It brought widespread severe storms to the Midwest Tuesday, including 12 tornadoes across five states. Due to weakening of the trough and lower instability, the severity of the storms is not expected to be as widespread today as it was yesterday, but some may be just as intense.
Pennsylvania and Maryland will see the highest risk for violent thunderstorms, although over only a relatively small area. Very strong wind shear ahead of a cold front will support scattered thunderstorm development over Pennsylvania southward through northern Virginia. Residing along the left exit region of the mid-level jet streak, some of the thunderstorms will become supercellular over central Pennsylvania and western Maryland. The left exit region of a jet streak, where winds slow down, is associated with enhanced lift, which could accelerate thunderstorm growth.
Some of the supercells in Pennsylvania and Maryland could produce tornadoes, but cloud debris and timing around sunset will prevent these storms from becoming widespread. Otherwise, damaging wind gusts and hail up to 1″ in diameter are the most likely threats, with only isolated severe weather expected further south in the Mid-Atlantic and Appalachians.
Severe weather is not expected throughout the rest of the Northeast. The storms in Pennsylvania and Maryland are expected to weaken as they race northeast ahead of the favorable instability. They will nonetheless bring heavy downpours to the Northeast through Thursday evening.
More widespread severe weather is expected over northern Georgia, the western Carolinas, Tennessee, and southern Kentucky, but neither supercells nor tornadoes are expected. With clearing occurring earlier and with greater instability, more storms will be able to develop over this region than in the Appalachians of the Mid-Atlantic states. A vigorous moisture feed from the Gulf of Mexico will ensure that storms that do develop will be capable of dumping copious amounts of rain in a very short period of time, constituting a modest flash flooding risk. Additionally, this downward momentum will contribute to a risk for scattered damage from strong wind guts.
Storms will develop from the northwest to the southeast in the South. Convection will initialize in the early afternoon over Tennessee and Kentucky, before weakening as it reaches Atlanta around sunset.
Unrelated to the upper-level low in the East, a frontal boundary will force a broken line of storms in the upper Midwest. Strong wind shear and large quantities of instability will support isolated storms with the capacity to produce damaging winds and up to golf-ball sized hail in South Dakota and Minnesota.
Thursday will be far more violent in the upper Midwest and the lee side of the Rocky Mountains. Tornadoes are likely to form over eastern Montana, North Dakota, and northern South Dakota in addition to colossal hail, which may exceed two inches in isolated spots. North Dakota cities most at risk for these violent storms include Bismarck and Dickson. These violent storms are less likely but still possible elsewhere including Billings (MT), Fargo (ND), and Grand Rapids (ND).
Scattered storms will develop across Montana and the Dakotas Thursday afternoon behind a warm front in the midst of extreme instability and moist flow. As a vigorous jet stream approaches ahead of an upper-level low with an associated cold front, a wide corridor of strong wind shear will develop over the threat area.
The wind shear will be more than sufficient to generate supercells during the evening over eastern Montana, North Dakota, and northwestern South Dakota. The window for tornadogenesis will be short since supercells will not remain discrete for long. Tornadogenesis refers to the formation process of tornadoes. Tornadoes that do form have the potential to be very strong and life-threatening. Given the unpredictable nature of tornadoes, residents of this threat area are highly encouraged to pay attention to weather alerts throughout the day.
The strong shear will also help maintain multicellular systems into the night over eastern North Dakota and northern Minnesota, long after the supercells merge. Coverage of storms will be greater over the latter areas, but the tornado threat will be significantly lower. The hail risk will also be smaller over these areas than further west, and hail that does form is unlikely to exceed the diameter of a golf ball. Wind gusts from the storms will nonetheless continue to be potent.
Chances for severe weather in the Plains and eastern Rocky Mountains will continue into Friday. Storms will target areas mainly southern and eastern from the storms Thursday.