Simulation of jet stream pattern July 22. (VentuSky.com)
BY MATTHEW CAPPUCCI AND JASON SAMENOW
JULY 25 AT 4:35 PM
Is it July or March? A rare and steep summer dip in the jet stream has brought an incredibly wacky week of weather to the Lower 48.
Usually, this fast-moving air current along which storms track retreats in summer, meandering listlessly in Canada.
Not so in recent days.
“The jet stream has stretched all the way from Canada to the Gulf of Mexico over the eastern U.S., helping to trigger the intense tornado outbreak in Iowa, and the persistent, torrential rains in the Mid-Atlantic,” said Jeff Halverson, Capital Weather Gang’s severe weather expert. “Even veteran meteorologists with decades of experience are astounded by the extreme nature of this jet stream pattern.”
As this jet stream and pool of cold air in its wake have interacted with all of the intense midsummer heat over the Lower 48, it has incited all sorts of unusual and severe weather. Not only has the pattern been extreme but it has also become stuck in place. This has led to day after day of remarkable and frequently disruptive weather.
Three twisters were spotted in Bondurant, Iowa, and another tornado struck Marshalltown, Iowa, on July 19. (Patrick Martin /The Washington Post)
Things kicked off Thursday when the jet stream dived south through the Midwest, triggering at least a dozen confirmed tornadoes in Iowa. Despite a direct hit to Marshalltown, a city of 28,000, and hefty damage on the outskirts of Pella, there were no fatalities — a testament to the high lead time provided by the National Weather Service in warnings issued that day.
Farther south Thursday, a derecho with widespread 60 to 80 mph gusts tracked from southern Nebraska down into northern Arkansas, spelling disaster at Table Rock Lake in Branson, Mo. The storm had a history of producing damaging winds — up to 89 mph in Lecompton, Kan. — along its nearly 500-mile track. The well-forecast storm caught some off guard, killing 17 people when it sank a duck boat trapped in high winds and waves.
The upper-level disturbance wrapped up in the diving jet stream plowed farther south and east Friday, shifting the threat for severe storms over the Ohio and Tennessee valleys. The Storm Prediction Center had a “moderate risk” of severe weather over portions of Indiana, Kentucky and Tennessee.
Jet stream streaming into storms and energizing them pic.twitter.com/iOYS6lbA7b
— Stu Ostro (@StuOstro) July 21, 2018
The biggest threat proved to be damaging winds. The National Weather Service received 395 reports of high wind or wind damage. But that wasn’t all. Softball-size hail — incredibly rare in July, thanks to high freezing levels in the atmosphere — was reported in Kentucky, and this picturesque tornado spun up in Corydon, Ind.:
A tornado appeared in Harrison County, Ind., on July 20. (Gregory A. Linker)
The weird jet stream helped to drag moisture up the Eastern Seaboard, setting the stage for prolific rains. At Reagan National Airport near Washington, four inches came down. Since 1941, only a dozen other calendar days have seen four inches or more.
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To the south, a pair of nighttime supercell storms northeast of Chattanooga, Tenn., merged and formed a line around 2 a.m. Saturday. This line collapsed south into Georgia.
The National Weather Service hoisted a severe thunderstorm watch for the Atlanta metro area at 3 a.m. as the windy storms raced south during the morning rush hour. As soon as they were gone, the sun came out, the atmosphere reloaded, and the stage was set for another round of dangerous storms in the afternoon. For the second time that same day, a severe thunderstorm watch was in effect.
One storm that afternoon even split and formed two new supercells. Thanks to northwest flow and an unusual turning of the winds owing to the bizarre jet stream, the “left split” supercell blew up into a massive storm. The really weird part? This storm was spinning clockwise, or anticyclonic. Less than 2 percent of mature supercells do so. That storm went on to produce tennis-ball-size hail in Dacula, Ga. — the second-largest hailstone on record in the Peach State in July.
Records since 1950 show the tennis ball sized hail (2.5") reported in Dacula, GA Sat 7/21 tied for 2nd largest hailstone in GA in July. Tied with 2.5" on 7/14/2004 in Blue Ridge GA and behind 2.75" on 7/20/1998 in Austell GA. pic.twitter.com/h5rarhgLNx
— NWS Atlanta (@NWSAtlanta) July 23, 2018
Sunday and Monday
Storms continued south into Florida on Sunday, and on Monday, they threw it into reverse — literally. Though the steering flow pushed clouds southeast, the cluster back-built westward along the front at the same rate. This gave the appearance of a stationary line of storms at the mouth of the Mississippi River, producing nearly 200,000 lightning strikes over the course of its 19-hour life span. It was nuts.
What's happening over the Gulf is simply stunning. 😍 A cold front over #Louisiana has sparked a massive Mesoscale Convective System producing 20,000+ #lightning strikes PER HOUR! It's retrograding westwards. More instability in purple = it keeps going through 10 PM. #LAWX #ALWX pic.twitter.com/yGtpssE0gn
— Matthew Cappucci (@MatthewCappucci) July 23, 2018
Tuesday and Wednesday
The unusually juicy atmosphere, with low cloud bases and a hint of spin, also managed to produce a brief EF0 tornado at Thomas Jefferson High School in Northern Virginia.
The stalled storm system has been blamed for widespread flash flooding from the Carolinas up through New England, drawing a fire hose of tropical moisture up the East Coast. Baltimore has picked up more than 15 inches of rain, a July record.
And it’s not over yet — a high risk of excessive rainfall and flash flooding is up Wednesday, including waterlogged portions of Pennsylvania and south central New York.
*HIGH RISK* of excessive rainfall and flash flooding today across portions of central Pennsylvania. Flash flooding will be possible over much of the Mid Atlantic into New York as well. Over half of flood-related fatalities occur in vehicles. Use caution if traveling! pic.twitter.com/lxEiL3xdC0
— NWS WPC (@NWSWPC) July 25, 2018
The long-range pattern suggests that more scattered heavy rainfall and flooding are in store through at least next week, although the intensity of this extreme pattern should ramp down.
While the West Coast has not dealt with storminess, it has endured extreme weather of its own. Like a see-saw, the jet stream has lifted north allowing scorching heat to swell from the Desert Southwest to Seattle.
Death Valley, Calif. soared to a daily record of 127 degrees Tuesday, very close to its highest temperature measured in decades: 129 degrees on June 30, 2013. Seattle is predicted to have its longest streak of hot weather on record with temperatures at least 88 degrees for a week. The Weather Service is also calling for “very dangerous heat” in the Sacramento Valley.
Jason Samenow is The Washington Post’s weather editor and Capital Weather Gang’s chief meteorologist. He earned a master’s degree in atmospheric science and spent 10 years as a climate change science analyst for the U.S. government. He holds the Digital Seal of Approval from the National Weather Association.
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