Stuck jet stream pattern to spawn dangerous 'Maya Express' deluge
Blocked weather patterns tend to be a clue that something extreme is about to happen, and that is the case this week.
To weather forecasters, it has been clear for more than a week now that a series of extreme events will play out across the western and south-central U.S., along with parts of Mexico, during the next several days: everything from snow in Mexico City to dangerous flooding in several states.
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The National Weather Service went so far as to hold a rare Friday afternoon press conference last week to highlight the threat, which Louis Uccellini, the NWS director, said was unusually clear from virtually all the major computer models that forecasters use to help guide their predictions.
Uccellini said the storms are the type of events expected during strong El Niño events, when milder than average ocean waters in the tropical Pacific alter weather patterns worldwide, favoring a strong southern branch of the jet stream and storm after storm rolling into California.
"These storms and their intensities are consistent with the El Niño pattern,” he said on Friday.
The jet stream, which is a river of fast-moving air at the altitude of cruising jetliners, is now more like a raging rapids as it blasts moisture at nearly 200 miles per hour along an unusual path all the way from the Philippines to Hawaii, then into California, down into Mexico, and eventually straight north-northeast into the south-central U.S. from Tuesday through late this week.
The end result will be feet of snow in the California mountains, flooding rains in lower elevations of central and southern California, and a potentially deadly flood event in at least four states: Louisiana, Arkansas, Missouri and Texas. In fact, the flooding in the south-central U.S. may end up being a bigger story than the arrival of El Niño rains in California.
Already, unusual things are happening, with a lot more to come. Los Angeles saw severe thunderstorms on Monday morning, with lightning sparking fires in palm trees and high winds causing airport delays.
Snow levels have been dropping in California's mountains, leading to a desperately needed boost in the state's snow pack during the worst drought in the state's history.
By midweek, the highly contorted shape of the jet stream in particular will resemble a butterfly, as a the jet traces a curving, winglike outline on a weather map. It is rare that such a deep dip in the jet stream -- extending all the way to Puerto Vallarta, Mexico -- takes place in March.
Freak cold, snow in Mexico
The major driver of the weather pattern will be an upper-level low pressure area that will set up shop over eastern Texas and stubbornly drift slowly northeastward during the middle to late part of this week. Before it crosses the border with Texas, however, it is going to take a highly unusual path that will come as a shock to many vacationers along Mexico's western coast, as well as the millions of residents of Mexico City.
The upper-level low will bring a rare bout of cold air to Puerto Vallarta, along with several inches of snow to the mountains located between Puerto Vallarta and Mexico City. In Puerto Vallarta, the high temperature forecast from weather.com on Wednesday is just 70 degrees Fahrenheit, which is 11 degrees Fahrenheit below average for the date.
In the country's capital, which is located at more than 7,000 feet in elevation, snow is expected to fall on Wednesday and Wednesday night in the mountains surrounding the bustling metropolis.
In the city, thunderstorms and windy conditions are forecast for Wednesday and Wednesday night as the upper low moves overhead, with a high of just 60 degrees Fahrenheit, and a low of 38 degrees, which is almost cold enough for snow.
The typical high temperature in Mexico City at this time of year is 77 degrees Fahrenheit, with an overnight low of 48 degrees.
The cold snap could imperil some of the Monarch butterflies that flock to parts of Mexico to spend winter before migrating north into the U.S. and Canada during the summer.
Behold: The Maya Express
The storm systems now affecting the U.S. are working with a surfeit of moisture, with the source region for the California rain and snow located more than 7,000 miles to the west, in the Philippines.
The moisture plume is now flowing from the Pacific into California, and soon will start creeping north from the Gulf of Mexico into the south-central states.
This firehose of moisture is a phenomenon known as an atmospheric river, and the particular atmospheric river pattern that is about to set up is known as the Maya Express.
Think of such rivers as atmospheric hoses that spray flooding rains onto narrow corridors.
Such atmospheric river events are known to favor extreme flooding events in parts of the Midwest, including severe floods that occurred in 1993 and 2008. More worrisome is the fact that this weather system appears to be directed at a region that already saw record, deadly flooding in December 2015.
To get a sense of how much moisture such air currents transport, consider that a strong atmospheric river can carry an amount of atmospheric water vapor about equal to 7.5 to 15 times the average flow of liquid water at the mouth of the Mississippi River, scientists have found.
To improve their understanding of atmospheric rivers, the National Oceanic and Atmospheric Administration (NOAA) and NASA, along with academic research institutions, are flying "hurricane hunter" aircraft into the atmospheric river aimed at California.
In advance of the heavy rainfall expected, the Weather Service has issued flash flood watches across portions of Oklahoma, Arkansas, Texas and Louisiana, with more watches likely to be issued as the week goes on.
The showers and thunderstorms will erupt from Tuesday through at least Friday, with the axis of heaviest rain only slowly shifting eastward with time.
The heaviest rain will likely focus on what is known as the "ArkLaTex" region, as well as parts of Missouri and Oklahoma. At least a foot, and possibly up to 20 inches, of rain is expected in some spots by the end of the week.
“I want to emphasize the uniqueness of the pattern potential in the central U.S.,” said the NWS' Uccellini, referring to the overall weather pattern and its likelihood of producing rainfall totals in excess of 10 to 15 inches over several days.
The storms this week will also spark multiple outbreaks of severe thunderstorms, along with the risk for strong tornadoes, from Texas northward into the Plains states, and then rolling eastward as the week goes on.
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Andrew Freedman is Mashable's Senior Editor for Science and Special Projects. Prior to working at Mashable, Freedman was a Senior Science writer for Climate Central. He has also worked as a reporter for Congressional Quarterly and Greenwire/E&E Daily. His writing has also appeared in the Washington Post, online at The Weather Channel, and washingtonpost.com, where he wrote a weekly climate science column for the "Capital Weather Gang" blog. He has provided commentary on climate science and policy for Sky News, CBC Radio, NPR, Al Jazeera, Sirius XM Radio, PBS NewsHour, and other national and international outlets. He holds a Masters in Climate and Society from Columbia University, and a Masters in Law and Diplomacy from The Fletcher School at Tufts University.
