Record-Strength Upper Low Brings Extreme Rains to South U.S., Thundersnow to Mexico

A remarkably rare atmospheric event is unfolding over Mexico and the Southern U.S., where an upper-level low pressure system of unprecedented strength in the historical record for that location has stalled out, bringing multiple days of torrential rain to the Southern U.S. and snow to the mountains of Mexico. The upper low tapped into an atmospheric river of moisture from both the Western Caribbean and the Eastern Pacific, bringing rainfall amounts one would expect to occur only once every 200 years (a 0.5% chance of occurrence in a given year) over portions of northern Louisiana. According to the latest NOAA Storm Summary, as of 9 am EST Thursday, the city of Monroe, Louisiana had received 17.25" of rain since Monday, and Shreveport had picked up 16.70" at Barksdale Air Force Base. The heavy rains led to numerous high water rescues, evacuation of at least 3,500 homes, and closures of hundreds of roads. Portions of two interstate highways in northern Louisiana--I-20 and I-49--were closed on Thursday morning due to flooding, according to KSLA.com. Three drownings have been reported since Monday from the storm system--one each in Texas, Oklahoma and Louisiana.


Figure 1. Flood damage in Haughton, Louisiana, on March 9, 2016. Thirty homes near Haughton were inundated by flood waters on Tuesday night, forcing evacuations. Image credit: Michael Dean Newman.


Figure 2. A webcam from Zacatecas, Mexico, catches snow falling on Wednesday, March 9, 2016. Thundersnow was reported at the city’s official reporting site. Image credit: webcamsdemexico.com, courtesy Eric Blake.


Figure 3. Observed 48-hour precipitation for the period ending at 10 am EST Thursday, March 10, 2016. Portions of northern Louisiana received over 16" of rain, and a large area of 8+" fell over portions of Eastern Texas, Northern Louisiana, Southeast Arkansas and Northwest Mississippi. Image credit: NOAA/NWS.


Figure 4. Average recurrence interval in years for the 24-hour rainfall amounts that fell ending at 7 am EST Wednesday, March 9, 2016. Rainfall amounts one would expect to occur only once every 200 years (a 0.5% chance of occurrence in a given year) fell over some portions of northern Louisiana just east of Shreveport. MetStat computed the recurrence interval statistics based on gauge-adjusted radar precipitation and frequency estimates from NOAA Atlas 14 Volume 8, published in 2013 (http://dipper.nws.noaa.gov/hdsc/pfds/.) The real-time analysis (observed) can be monitored for free at: http://metstat.com/solutions/extreme-precipitation-index-analysis/ or on their Facebook page.  MetStat also offers a subscription for precipitation interval forecasts and analyses at http://metstat.com/solutions/extreme-precipitation-index-forecasts/


Figure 5. The Bayou Dorcheat at Lake Bistineau, Louisiana reached its highest water level on record Thursday morning. The extreme rains in northern Louisiana have poured into local lakes and rivers, sending a few close to or in excess of their highest water levels on record. On Friday, the Sabine River near Burkeville is predicted to exceed its highest crest since 1884. Image credit: NOAA.

Weather weirding par excellence: Strongest upper low ever observed over central Mexico?
This upper low originated from energy that moved across southern California late in the weekend, producing heavy thunderstorms. Rather than barreling across the southeast U.S., the powerful subtropical jet stream carved out a progressively deeper trough into Mexico that cut off from the jet stream, forming a slow-moving closed low. At 00Z (7:00 pm EST) Thursday, this cold-cored upper low was centered in central Mexico, roughly in the vicinity of Guadalajara. Thundersnow was reported on Wednesday in Mexico at Zacatecas (altitude 8010 feet), about 200 miles north of Guadalajara. However, weather records researcher Maximiliano Herrera told us in an email that snow in the mountains of northern Mexico has occurred in April, May and even early June, so is not that unusual. As shown in Figure 7 below, this upper low featured a large 558-decameter contour (the 558 dm, which is 5580 meters, refers to the height at which the atmospheric pressure is 500 mb, or about half of the typical surface pressure). Such a large, strong upper low appears to be an unprecedented event in modern weather observations for Mexico; upper-air analyses dating back to 1948 from the NCEP/NCAR reanalysis project suggest that no previous upper low in this region has been strong enough to generate a 558-dm contour. During the last “super” El Niño, in mid-December 1997, a powerful upper trough extended south from the United States, producing what was reportedly the first snow observed at Guadalajara since February 1881. Low-level temperatures have not been quite as cold this time around, given that it’s now early March rather than mid-December, but the thundersnow reported at Zacatecas indicates the strong instability being produced by the deep cold at upper levels. The gradient between this upper low and a strong upper ridge over the eastern U.S. has also intensified the southerly flow pumping moisture into the south-central states.


Figure 6. GOES satellite imagery from 0245Z Thursday, March 10, 2016 (9:30 pm EST Wednesday), shows the immense swirl of an upper-level low centered over central Mexico, as well as the stream of moisture extending from the waters of the eastern tropical Pacific (still warmed by El Niño) across the western Gulf of Mexico into Texas and Louisiana. Image credit: NASA Earth Science Office and NOAA.


Figure 7. 500-mb map for 00Z (7:00 pm EST) Thursday, March 10, 2016, as initialized in the GFS model. Image credit: NOAA/NCEP.


Figure 8. 500-mb map for 12Z (7:00 am EST) Friday, December 12, 1997, about the time that the upper trough had reached its maximum extent into Mexico. Image credit: NOAA Daily Weather Map.

Premature spring warmth swaddles Northeast
Dozens of temperature records melted like so much gelato beneath sun-filled skies across much of the Northeast on Wednesday. Boston basked in temperatures that topped out at 77°F--not just a daily record, but the city’s warmest official reading on any day in astronomical winter since records began there in 1872. New York City’s Central Park also had its earliest 77°F in records that, likewise, go back to 1872. The uncannily mild air served as a fitting curtain call after New England’s warmest meteorological winter on record (Dec-Feb). The lack of persistent snow cover across New York and New England helped give this week’s warmth an extra boost. Some of the records set on Wednesday, March 9, 2016 included:

Earliest 80°F on record
Albany, NY:  81°F (previous record March 16, 1990)
Hartford, CT: 81°F (previous record March 20, 1945)
Newark, NJ: 82°F (previous record March 13, 1990)Poughkeepsie, NY: 82°F (previous record March 13, 1990)

Earliest 75°F on record
Boston, MA: 77°F (previous record March 14, 1946)
Concord, NH: 77°F (previous record March 18, 2012)

Earliest 70°F on record
Glens Falls, NY 77°F (previous record March 13, 1990)

Earliest 65°F on record
Montpelier, VT: 66°F (previous record March 15, 1990)
St. Johnsbury, VT: 65°F (previous record March 16, 1990)


Figure 9. Spring fever sweeps across the campus of Columbia University in New York City on Wednesday, March 9. Image credit: Bob Henson.

The forecast: more flooding in the South, more warmth for the East
Looking ahead, the atmospheric river of moisture responsible for this week's heavy rains has shifted slightly eastwards, and was still at near-record levels in excess of 200% of normal on Thursday morning. The 12Z Thursday morning balloon sounding at Lake Charles, Louisiana showed an astonishing 2.15" of precipitable water in the atmosphere--the second highest value on record for the months of December - April (thanks go to Peter Mullinax, ‏@wxmvpete, for this stat.) This moisture will continue to feed torrential rains over Louisiana, Arkansas and Mississippi on Thursday and Friday.


Figure 10. Predicted precipitation for the 3-day period ending 7 am EST Sunday, March 13, 2016. Heavy rains in excess of 7" (brown colors) are expected over eastern Louisiana, including New Orleans. Image credit: National Weather Service.

A solid week of unusually mild air lies in store for most of the nation east of the Rockies. Though we can expect dozens if not hundreds of daily record highs to be set between now and then, it looks highly unlikely this mild spell will dislodge the Great Warm Wave of March 2012 from its place of pride in our late-winter/early-spring climate annals. That phenomenal stretch of warmth brought close to a week of temperatures topping 80°F from the Midwest to the Northeast. The town of Lapeer, Michigan, hit 90°F on March 21 (setting a state record for March). Readings that would have broken records for April, much less March, extended into the Canadian Maritimes.



Video 1. Drone footage of flooding in Bossier City in Northern Louisiana on March 9, 2016. Thanks go to wunderground member Skyepony for posting this video in the blog comments.

Bob Henson and Jeff Masters

Flood

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