Summary of April 15-20 Storm
Weather system ‘Yogi’ (as designated by The Weather Channel) had something for everyone this past week: flooding rain, tornadoes, record snowfalls, and record temperatures. Historically it was not that significant, but that mattered little to those affected. Here is a brief summary of the storm’s most extreme manifestations.Flooding Rainfall
The most significant aspect of the storm was the heavy rainfall in portions of the upper Midwest that fell on April 17-18, especially in Illinois and Michigan, resulting in major flash and river flooding. A very large portion of the region received 4”+ of rainfall. On April 17th an atmospheric river of moist air surged into the Midwest ahead of a strong cold front and low-pressure system centered over Iowa.Water vapor image for 8 p.m. CST on April 17th when heavy rain began to move into the Chicago area.
Peak 24-hour rainfall amounts by state included 7.93” at Augusta, Illinois, 6.45” at North Liberty, Iowa and 4.81” at Cuttlerville, Michigan. Rainfall in the Chicago area topped out at 7.34” at Naperville. Chicago O’Hare (the city’s official weather site) totaled 5.55”, a record 24-hour total for April and one of the top ten greatest rainstorms on record (the all-time record for 24 hours remains the 9.35” that fell on August 13-14, 1987).A flooded underpass on Belmont St. at the Metra train station in the Chicago suburb of Downers Grove on the morning of April 18th.
Photo uploaded by ‘aerojad’ to wunderground.comThe final precipitation totals for the storm in the Chicago metro area.
From NWS Chicago.
The wide extent of the heavy rainfall caused at least 12 gauge sites on six rivers in northern Illinois to crest at their all-time record high levels. Later this weekend the outflow from all these rivers will cause the Mississippi to crest at or above flood stage. See Jeff Master’s recent blog on this subject.River sites in northern Illinois that have reported their highest flood stages on record.
Amazingly, only two (perhaps three) deaths have so far been reported as a result of the flooding. This is particularly fortunate since the worst flash flooding occurred in the wee hours of the morning on April 18th when most residents were asleep and thus not driving about on flooded roads.Heavy Snowfall
The other notable aspect of the storm was the very heavy late-season snowfall that affected a broad area from the Rocky Mountains to northern Michigan. Blizzard conditions occurred over Wyoming, Nebraska, Minnesota, and the Upper Peninsula of Michigan. The top storm total accumulations reported from each state were as follows:COLORADO:
(April 15-17) 46.2” at Elliot Ridge. At lower elevations Fort Collins measured 27.0”, Boulder 16-20”, and Denver 9.0” (although parts of the city received up to 14”).WYOMING:
(April 15-17) 39.0” at Douglas 27S. Cheyenne picked up a 3-day total of 17.9”, one of its greatest snows on record (all-time record being 25.2” on November 20-21, 1979). The city also suffered through four consecutive days of coldest daily maximum temperature on record during the April 15-18 period (ranging from 24° to 27°).NEBRASKA:
(April 15-17) 16.0” at Harrison 4 NWMINNESOTA:
(April 17-19) 23.0” at Babbitt. Duluth recorded 17.7”, bringing its April total (so far) to an all-time April record of 41.7” (former record was 31.6” in April 1950). For the season, Duluth is now in 5th place with 120.3” in regard to its snowiest season on record (snowiest season being 135.4” in 1995-1996).WISCONSIN:
(April 17-19) 22.0” at Cornucopia, Bayfield County.MICHIGAN:
(April 18-20) 21.4” at Twin Lakes 2SW. The snow depth at Marquette NWS office at the airport stood at 35” on April 20th. This is the deepest snow ever measured at the site for so late in the season (since the site was established in 1963). However, it should be noted that the NWS site at the airport sits inland on a hill and thus is not indicative of conditions in the city of Marquette itself, (which is located on the shore of Lake Superior), where the snow depth is a modest 6”. Hoist Basin in Marquette County now has a snow depth of 50”.A charming image of ‘spring’ in Ewen, Michigan (Ontonagon County in the western portion of the Upper Peninsula) during the recent blizzard.
Photo by Jeffery Tormala, posted on the NWS Marquette web site.Severe Storms/Tornadoes
Although there was plenty of moisture, a sharp cold front, and deep low pressure both at the surface (down to 980 mb/ 29.00” over Lake Superior on April 19th) and aloft, the ingredients were fortunately not all there (lack of a powerful low level jet) for a widespread tornado outbreak to happen. Many heavy thunderstorms did develop to the east of the front from Texas to New York, and everywhere in between, resulting in numerous high wind and severe thunderstorm reports and warnings, but it appears that only about 7-10 tornadoes have been confirmed so far. This is not impressive for such an intense storm system in April. The worst was an EF-2 that struck Mansfield, Georgia on April 19th injuring one and destroying several residences. Far to the north, an EF-0 tornado was reported east of Binghamton, New York near the town of Bainbridge during the evening of April 20th. No structural damage was reported. Large hail in Kansas was said to be responsible for an automobile accident that injured six high school students and their teacher when the teacher lost control of his SUV on I-70.Sharp Temperature Contrasts
This April has seen a never-ending series of intense cyclones and cold fronts march across the eastern two-thirds of the Nation. If you thought the recent storm was familiar you are correct. Compare the two surface analysis plots below, one from April 9th and the other from April 16th. Just one week apart and remarkably similar.
We can also see (below) that, once again, an amazing temperature gradient formed over northern Texas, Oklahoma, New Mexico, and Kansas:A close up of surface conditions that existed midday on April 9th (top map) and on the afternoon of April 17th (bottom map). Note the amazing temperature spreads over northern Texas in both instances.
Maps from UCAR.
It looks like another significant storm system will be impacting all the same places again this coming week.
Christopher C. Burt