Heavy snow, rain and thunderstorms are poised to return to the flood-weary Central US late this week as a Rocky Mountain-based storm roars across the Central Plains and Mississippi River Valley. The Missouri and Mississippi Rivers are still above flood stage weeks after heavy rain, rapid snow melt, and impenetrable ice jams prompted the rivers to swell to record extents earlier this month. It will therefore not take much more rain to inundate many communities still recovering from the catastrophic flooding.
The storm’s upper air support will actually originate over the Pacific Ocean. But the upper-area of low pressure will quickly run out of juice after dumping heavy rain and delivering yet another heavy snowfall to the Sierra-Nevada Mountains in California on Wednesday. The system will lose the influx of moisture from the subtropical Pacific as it progresses over the Great Basin Thursday. Its surface component will fizzle with a new surface low not developing until the upper low crosses the Rocky Mountains Friday with open access to moisture from the Gulf of Mexico.
Preparing for the upper-low’s arrival, thunderstorms will make for a splash and a bang over portions of the Midwest between Thursday afternoon and Thursday evening. A cold front pushed southward by an intense upper-level low over the Hudson Bay will collide with a warm Gulf-fed airmass under a ridge of high pressure situated over the Central US. The front will help generate isolated to scattered PM thunderstorms. A combination of low-level Gulf moisture and mid-level Rocky Mountain dryness will provide an environment just ripe enough for some of the resulting thunderstorms to become severe, namely over Kansas and eastern Oklahoma. Damaging straight-line winds will be the primary threat from the storms, some of which may become supercellular, in which case hail and isolated tornadoes cannot be ruled out. More numerous thunderstorms are expected Friday, with the same threats as Thursday possible. Fortunately, Friday’s heavy rain and snow event will target other areas of the Midwest for its worst impacts.
The Pacific upper-level trough of low pressure will finally reach the Rockies by Friday morning. Low pressure will intensify just east of the mountains throughout the day along the cold front responsible for Thursday’s thunderstorms. An abundant supply of Gulf of Mexio moisture will ensure a swift intensification process.
Heavy snow initially confined to the Rockies of Wyoming will spread eastward across Nebraska, northeastern Colorado, and northwestern Kansas through early Friday evening. Winds will intensify alongside the snow throughout the day. The heavy wind-whipped snow will result in whiteout conditions. Travel across I-80 from Lincoln, NE to Laramie, WY will be treacherous, if not impossible during the blizzard. Up to a foot of snow is possible over portions of the central Plains, but it is still too early to determine the specific distribution of snow.
While heavy wind-driven snow falls over the central Plains, rain will spread to and intensify over the still-flooded Missouri and Mississippi Rivers from eastern Nebraska and western Missouri to western Illinois and southern Wisconsin by sunset Friday. Residents of cities like Omaha, Des Moines, Cedar Rapids, and La Crosse should stay on high alert Friday night as the heavy rain pounds on saturated soils. There is the potential for up to 2″ of rain to fall before precipitation either ends or transitions to snow. Regional creeks and streams may not be able to support the quick influx of runoff, resulting in an elevated risk for flooding to return to communities still recovering from the recent devastation.
Heavy rain will transition to heavy snow over most of eastern Nebraska, northern Kansas, Iowa, and southern Wisconsin Friday night as the storm ejects into the Ohio Valley. Several inches of accumulation are possible before snow ends early Saturday morning. By this point, the intensifying storm will have begun producing heavy snow over Michigan, enhanced by blustery northwest winds blowing over the largely ice-free upper Great Lakes.
The powerful storm will trek towards Ontario and Québec Saturday. Depending on the storm’s ultimate track, the snow may reach the northern Ohio valley and western portions of New York and Pennsylvania by Saturday night, but the details are still too uncertain to make reasonable judgement.