The intrusion of upper-level low pressure systems into the continental United States from Alberta is always pesky business. Dubbed “clippers” in the winter, they’re notorious for transcontinental swaths of light to moderate snow. But their most vicious impacts generally arise in warm weather, when they can prompt the development of violent thunderstorms. Early Wednesday morning, an upper-level low crossed the border into Montana. Though it will have no special nomenclature as it does in the winter, this system (like many others this spring and summer) will leave a trail of scattered transcontinental thunderstorm damage as it races from the Central Plains Wednesday to the Northeast Friday.

Nebraska and South Dakota will be the first to face the storms. As the upper-level shortwave crosses the Rocky Mountains Wednesday, stretching of the air beneath it will intensify the system both aloft and at the surface. A surface cyclone will drag the boundaries of a cold front east-southeastward along its western flank as well as a warm front along its eastward flank.




In the moist warm sector within the vicinity of the cyclone, an abundance of wind shear will overlap areas of strong instability. The wind shear will be more than sufficient to generate rotating updrafts, which will support long-lived supercell thunderstorms. Wind shear is change in wind direction and/or speed with height and is necessary to separate thunderstorm updrafts from thunderstorm downdrafts. Storms can’t intensify if their updrafts are cutoff by their downdrafts.

All modes of severe weather are possible Wednesday evening in central Nebraska through south-central South Dakota. Isolated thunderstorms will first develop Wednesday afternoon along the Black Hills. Storms will become more numerous and more intense as they spread southeastward throughout the afternoon and evening. Aided by strong wind shear and large scale ascent associated with the shortwave, many of the scattered storms in far south-central South Dakota through central Nebraska will develop rotation and become supercelluar. The storms could leave long trails of wind damage and large hail. Some of the hail could exceed 2″ in diameter, large enough to shatter windows, windshields, and roofs as well as posing a threat to people, pets, and livestock. The strongest supercells could also produce isolated tornadoes, especially over north-central Nebraska, where low level shear will be strongest. Few population centers are expected to be impacted by these storms but travelers along I-80 in Nebraska are encouraged to take extra precautions while driving, as storms are likely to approach the busy corridor Wednesday night.

The Southern Plains will be the next target Thursday afternoon and evening. Storms will gradually die over Nebraska Wednesday night as the upper-level low weakens and exits southeastward. Remnant showers will exist over Nebraska Thursday morning, with their outflow boundaries racing east-southeastward toward Kansas. Well within the warm sector southeast of the surface cyclone, the region between roughly Oklahoma City, OK to Lincoln, NE will be most at risk for severe thunderstorms. The strongest shear and ascent from the approaching shortwave will overlap a region of deep instability that will develop Thursday afternoon. By evening, scattered storms will have developed across the Southern Plains, becoming most widespread from central Kansas to central Oklahoma, where the best dynamics will overlap the strongest instability. Strong storms will also spread into northern Arkansas and southern Missouri, but with weak shear, supercells and are less likely.

All modes of severe weather will also be possible Thursday, but with slightly different ratios.  Storms will be more likely to cluster and organize Thursday evening, such that damaging winds will be the predominant threat. Isolated large hail over 1″ in diameter and a few tornadoes will be possible with the discrete supercell thunderstorms. Cities including Oklahoma City, Topeka, Kansas City, and Lincoln are at risk for the strongest storms Thursday. Storms will be less intense east of this zone, so intense wind will be the only threat in Arkansas and southern Missouri.




With the shortwave trough broadening, the region of lift will expand to the Mid-South, Ohio, and Tennessee River Valleys. Sufficient shear, moisture, and instability will exist over this corridor east of the shortwave. However, with weaker forcing and slower updraft speeds, supercells will be more isolated here. This leaves multi-cellular thunderstorms emerging as the dominant type, with damaging winds and isolated hail the main threats.

The Northeast will confront the shortwave as it broadens and gets absorbed by the jet stream Friday. Still drying out from the record flooding earlier this week, wind damage may accompany additional downpours in the Northeast. Steamy temperatures along the I-95 corridor, with dew points in the low to mid-70s throughout the coastal plain of the Mid-Atlantic and Southern New England, will ensure ripe, antecedent conditions for thunderstorms. Extreme instability will develop under this steamy air mass. The instability is of the “tall and skinny” variety, which means that as air rises it will only be slightly warmer than the environment surrounding it. In other words, the air will not be particularly buoyant despite the deep instability. However, model guidance indicates pockets of impulses riding along the shortwave that could provide enhanced lift along the I-95 corridor.

It won’t take much additional lift to result in towering thunderstorms given the deep instability. The scattered impulses contained within the larger upper-level shortwave should provide an abundant source of extra lift.  In addition to more torrential rain, the storms will be capable of producing strong wind gusts and hail. Modest wind shear will develop late in the afternoon along and east of the I-95 and I-87 corridors. Storms will be longer lived and more organized here than areas further west, resulting in the coastal cities of the Mid-Atlantic and much of southern New England facing the brunt of the severe threat.

Isolated hail of up to 1″ in diameter is possible early in the thunderstorm activity, but strong wind gusts will be the biggest threats. D.C., Philadelphia, New York, Albany, Hartford, and Boston are all at risk for the stronger storms.  Since much of the activity will be driven by diurnal heating, the storms will weaken throughout the evening with the loss of sunlight.

This is an evolving system, with timing and specific impacts likely to change for the Northeast over the next few days. Be sure to check back for updates as the severe weather event draws closer.



Author

Josh is a lifelong nature and weather enthusiast as well as the Head Meteorologist at WeatherOptics. He began regularly forecasting for New Jersey, Long Island and New York City in 2014 on social media, contributing to community pages such as SBU Weather. He holds degrees in Physics and in Atmospheric and Oceanic Sciences from Stony Brook University, from which he graduated in 2018. In the Fall of 2018 Josh will start graduate school for his M.S. in Marine and Atmospheric Sciences at Stony Brook, continuing his research on approaches to non-convective wind gust forecasting.

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