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Dust and Deluge: Heavy Rain in Southeast, High Winds and Grit in Rockies, Plains

By: Bob Henson 3:36 PM GMT on April 15, 2015

A series of drenching rains across much of the South this week is being fed by some of the richest atmospheric moisture on record for April. Flash flood watches and flood warnings are plastered across much of the central Gulf Coast, where pockets of 3+” of rain on Tuesday followed widespread 1 – 2” amounts on Monday. CoCoRaHS reports for the 24-hour period ending Wednesday morning included 5.08” in Hancock County, Mississippi, and 4.94” in St. Tammany Parish, Louisiana. Flooding has been minimal so far across the Gulf Coast, with soil moisture lower than average going into the week’s rains. That’s not the case in already-sodden Kentucky, where yet more heavy rain on Tuesday sent a number of streams and rivers into flood stage. Lexington and Frankfurt have already secured their second wettest April on record, with more than two weeks left to go. Both cities need less than 2” to vault into top-five status for their all-time wettest months.

Figure 1. Departure from average rainfall for the period April 1 – 14. A large part of Kentucky is 5” to 8” above average for the first two weeks of the month. Image credit: National Weather Service/Louisville.

The heavy showers and thunderstorms across the South are being goosed by a strong subtropical jet stream that’s very prototypical of El Niño. NOAA’s spring outlook, issued on March 15, highlighted the potential for unusually heavy rains in the Southeast. The jet dynamics will be reinforced late this week into the weekend ahead of a strong upper-level low that will be sinking into the southern Rockies and moving slowly east across Texas. The 5-day precipitation forecast from NOAA’s National Centers for Environmental Prediction shows widespread 3 – 5” amounts from Texas to the Carolinas.

Figure 2. Projected 5-day precipitation amounts for the period from 1200 GMT Wednesday, April 15, to Monday, April 20. Image credit: NOAA/NCEP/Weather Prediction Center.

There’s plenty of juice on hand for the atmosphere to work with, as vertical profiles show very high amounts of precipitable water (PW) for this time of year. (Precipitable water is the amount of rain that would fall at a given point if all of the moisture in the air above it were squeezed out.) At New Orleans, the PW value measured by radiosonde at 1200 GMT Monday hit 1.96”, not far from the April record of 2.09”. Morehead City, NC, reported 1.86” of PW at 0000 GMT Tuesday, just short of the April record of 1.92”.

Big snow possible in Colorado
Heavy precipitation of a different sort may plaster higher elevations of the Colorado Front Range late this week. After several days of stark disagreement among major computer models, there’s now more consensus that the upper low mentioned above will move very slowly through the southern Rockies on Thursday and Friday, producing a long period of favorable easterly winds blowing upslope against the Rockies. Temperatures in most of the Denver/Boulder area may end up a shade too high for major snow, but foothills and higher peaks just to the west (where a winter storm watch is now in effect) could easily see a foot or more of wet spring snowfall. Some of the heaviest snows in the Denver/Boulder area occur during spring and fall when El Niño conditions are in place, as slow-moving upper lows are especially prone to develop then. UCAR’s Matt Kelsch has just updated a blog that I posted on this topic in 2009, examining the heaviest Boulder snows since 1950. The new stats show that of the 14 storms that have produced at least 20” of snow in Boulder, eight occurred during El Niño periods, but just two during La Niña events, with the other six during neutral conditions. Wunderground weather historian Christopher C. Burt has a more general look at Record April Snowstorms in a post from 2013.

Figure 3. A wildfire looms just beyond the campus of the University of Mary in Bismarck, North Dakota, on Monday evening, April 13. The fire flared up again on Tuesday, moving even closer to campus. Image credit: Kyle W. Martin.

Wind, dust, and fire out West
Strong westerly flow pushed mild, dry air across the northern Rockies and Plains into the Midwest on Tuesday. Chicago tied its all-time lowest relative humidity of 13% on Tuesday afternoon, with temperatures in the 60s and dew points in the teens. (Thanks go to Dave Schwartz of The Weather Channel for this tidbit.) The parched air sent fire weather conditions to extreme levels over the Dakotas, with similar conditions expected over the Southwest and southern High Plains on Wednesday. At Bismarck, ND, the University of Mary was evacuated Tuesday afternoon when a grass fire came perilously close to campus (see Figure 3 above).

Figure 4. Officials inspect the scene of an accident on Interstate 80 west of Salt Lake City on Tuesday, April 14. Howling winds halted flights at Salt Lake City's airport and toppled semitrailers on I-80, causing authorities to close a 100-mile stretch of northern Utah's main east-west highway. Image credit: Scott G. Winterton/The Deseret News via AP.

A powerful cold front and strong surface flow just ahead of it pushed dust across a large swath of the West. Widespread 60 – 80 mph wind gusts were reported by Utah mesonet stations, and power was knocked out for thousands of Utahns. Several vehicles were blown off Interstate 80, and a major pile-up amid the dust killed at least one person and injuring 25. In Salt Lake City, the front pushed temperatures from a springlike 75°F at 2:30 pm to a raw 35°F with light snow by 6:30 pm. Visibility was briefly down to a half mile just after the front hit. An 2012 analysis led by James Steenburgh (University of Utah) shows that Salt Lake City averages 4 -5 dust storms per year that reduce visibility to 10 miles or less. Only about 2% of these storms push visibility below 0.5 mile, suggesting that this was a roughly once-in-a-decade event. GOES-13 satellite images captured the frontal passage and the associated dust over California and Nevada.

Figure 5. A man tries to save his house from fires spreading from neighboring homes in the Siberian village of Smolenka, near Chita, on Monday, April 13. Russian authorities say out-of-control agricultural fires have killed or injured hundreds and destroyed or damaged more than 1,000 homes. The fires were started by farmers burning the grass in their fields, but spread quickly because of strong winds. Image credit: AP Photo/Evgeny Yepanchintsev.

April is off to a dusty, smoky start in parts of Asia, Europe
Dust and fire have made their mark over an unusually large swath of Eurasia over the last few days. A massive, eerie dust storm, apparently one of the most intense in years for Dubai, swept from the Arabian Peninsula to India, as chronicled in Tom Yulsman’s ImaGeo blog on April 8. To the north, the normal seasonal burning of fields by farmers in the Khakassia province of far south Siberia turned disastrous early this week amid high winds and dry, record-warm air. At least 23 people were killed, with close to 1000 seeking medical help and an estimated 5000 left homeless. Intense dust and sand storms have also been reported this week in Ukraine and Belarus, closing several airports, and strong southerly winds pushed Saharan dust as far as southern England, adding to poor air quality in London.

Temperatures this winter (December – February) ran far above average throughout most of Europe and Asia. Jeff Masters will have a full report on national and global climate for March at the end of this week.

Bob Henson

Figure 6. Conditions in Salt Lake City at the height of Tuesday’s dust storm.
Image credit: wunderphotographer hurricanes2018.

Dust Storm Flood

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