Review of the Active Start to April 2010

Severe weather season had gotten off to a slow start, and while March went out like a lamb, April came in like a lion. Record setting heat on April Fool’s Day spurred the first severe thunderstorm warning of the year for the Quad Cities county warning area (CWA). It was issued in the early afternoon of April 2nd, as a squall line raced through parts of Benton, Linn and Buchanan Counties. The event was short lived, and was followed by more above normal temperatures rather than seeing things return towards normal. A large trough over the western U.S. put the Midwest under southerly flow for the first few days of the month. Not only did this keep temperatures as much as 20 to 30 degrees above normal, but it allowed moisture from the Gulf of Mexico to stream northward, into the Mississippi and Missouri Valleys. As the upper level trough began to eject to the east, and 120 knot jet stream winds rounded the base, all the ingredients were in place for an outbreak of severe weather.

For more information on any terms in this summary, please visit our National Weather Service glossary.
April 4th:

On Easter Sunday a developing low pressure in Kansas began to push a warm front northward through Missouri. Right along the boundary instability was high, and shear was more than adequate for supercell thunderstorms. The warm front was forecast to lift north throughout the day, and as a result a tornado watch was issued around 3:30 in the afternoon, for northern Missouri, southeast Iowa and west central Illinois. Clouds to the north of the front prevented warming, and helped to stall the front from northeast Kansas, through Kansas City, to just north of St. Louis. As a result the tornadic threat was confined to northern Missouri, but severe storms did develop well to the north of the boundary too. Warm, moist air aloft, and a 50 knot low level jet at 850mb, provided the elevated instability and forcing for those severe storms. Because these storms were on top of a near surface stable layer, the main threat was large hail, with an isolated threat for stronger storms to force damaging winds to the surface.

Our office issued 10 severe thunderstorm warnings during the day. We received over 30 reports from spotters, including half up to the size of golf balls (1.75”) in Cordova and Morrison, Illinois, and a 65mph wind gust at Alexandria, Missouri. These storms continued into the late evening, where hail dented cars near Muscatine, Iowa. As the evening advanced, the tornado watch expire, and a new severe thunderstorm watch was issued to the south and east, across central Missouri.

Radar image of 72 dBZ hail core (purple colors) after leaving Muscatine with half dollar sized hail (1.25").

At the end of the day the Storm Prediction Center (SPC) logged 4 reports of tornadoes, 15 damaging wind reports (>58mph), and 30 large hail reports (>1”).

 
April 5th:

On Monday morning the warm front continued its slow journey northward, as the low pressure system moved into central Kansas. The strong low level jet continued to help fire severe thunderstorms, this time near Kansas City, where the first severe thunderstorm watch of the day was issued. As those storms lifted north and east a second severe thunderstorm watch was issued at 1:30PM, for large hail and isolated damaging winds as the warm front continued to advance north. This watch was the first for our CWA, and extended as far east as a Cedar Rapids to Keokuk, Iowa line.The eastern portion of the warm front draped southeast through south central Illinois and Indiana, thanks to the influence of the cold Great Lakes. Along that part of the boundary strong heating at the surface contributed to instability, and coupled with the high shear, the threat for tornadoes increased. As a result the SPC issued two tornado watches, one at 3:00PM for Illinois, and another at 4:30PM for Indiana, with one tornado being reported in central Illinois.

Thanks to the elevated storms in Iowa producing cold outflow, the warm front stalled along the Iowa/Missouri border. But continued heating south of the boundary was creating a volatile situation, with a high tornado potential should any thunderstorms develop. The SPC issued a third tornado watch around 7:30PM, for northeast Kansas, northwest Missouri, and south central Iowa. However, warm air aloft capped the atmosphere, not allowing the warm surface air to rise and create the updrafts necessary for thunderstorms. It wasn’t until after sunset that the cap broke, and thunderstorms began to fire along the front. The tornado threat lasted a few hours, especially in northeast Kansas, before the near surface air stabilized enough to hinder tornado development. By about 10:30PM a new severe thunderstorm watch was issued along and north of the warm front, for northwest Missouri, southeast Nebraska, and southwest to south central Iowa. Again the primary threat was for large hail, with isolated damaging wind gusts possible as the warm front continued its march north.

At 11:00PM the seventh and final watch of the day was issued for severe thunderstorms capable of large hail and isolated damaging wind gusts. This was the second watch of the day for our CWA, and including parts of eastern Iowa and northwest Illinois, and extended east all the way to Detroit, Michigan. Over the course of the two watches, our office issued 14 severe thunderstorm warnings, and received over 90 reports from spotters. The largest hail reported was ping pong ball (1.5”) 6 miles WSW of Erie, Illinois. As the warm front continued north, eventually reaching the Highway 30 corridor, the elevated supercell storms began to tap back into the boundary layer and winds began to reach the surface. At Atkins, Iowa the fire department measured a 74mph wind gust along with scattered wind damaged throughout parts of Benton County.

Supercell thunderstorm that produced half dollar sized hail (1.25") in Coggon and a 68mph wind gust at Monticello Regional Airport.

As supercells continued to develop and track east, wind damage continued to be reported from Linn County, east through Dubuque, Jackson and Jo Daviess Counties, and finally into Carroll County. In Carroll County a strong supercell contained an elevated core of rain and hail that descended to the ground and resulted in a downburst with extreme winds in the town of Milledgeville, Illinois.

Supercell thunderstorm nearing Milledgeville, Illinois. Note the high reflectivity (reds and whites) near Milledgeville denoting heavy rain/hail.

Same supercell 5 minutes later. Note that high reflectivites have descended to the surface and the leading edge has now bowed out, indicating strong winds.

The next day a survey team from the office was sent to examine the damage, and it was estimated that winds were at least 90mph, with higher gusts to near 100mph possible near some of the worst damage.

 

Damage to a crib west of Milledgeville, on Dutchtown Road, near the start of the downburst wind damage.

West wall of a church blown down in Milledgeville. Winds lifted up the roof section causing the collapse of the wall.

At the end of the day, the SPC had received 119 reports of large hail, and 65 reports of damaging wind gusts, in addition to the 1 tornado.

 
April 6th:

On Tuesday morning thunderstorms were ongoing in central Iowa, and were rapidly moving east. Near dawn the storms began to intensify, and a severe thunderstorm watch was issued at 4:45AM for parts of eastern Iowa and northern Illinois. The flare up in convection produced large hail, up to the size of golf balls (1.75”) near the I-80 truck stop.

Elevated thunderstorm producing hail up to the size of golf balls (1.75") near the I-80 truck stop.

The complex continued east, and a second severe thunderstorm watch was issued at 9:30AM for southwest Michigan. As this convection moved east, it helped to push the warm front south again, to about the Mississippi River at the Quad Cities. To the north, low clouds and light rain kept temperatures in the upper 40s to lower 50s, while to the south full sunshine quickly boosted readings into the 70s. Eventually strong sunshine and south winds began to push the front back north to about the Highway 20 corridor.

Warm frontal position at 3:00PM April 6th, location highlighted by the white line.

Shortly after 2:00PM a severe thunderstorm watch was issued for eastern Kansas and western Missouri, for the thunderstorms that were forecast to develop along the cold front extending from the low pressure in southeast Nebraska. By this time temperatures were nearing 80 degrees south of the warm front, while still in the 50s to the north. Along and south of the front the strong warming resulted in convective available potential energy (CAPE) approaching 3000 J/kg (high instability), with 0-6km shear values near 80 knots (high shear). In addition, locally backed winds, out of the southeast instead of south, produced high helicity values which indicated the rotational potential of the winds at low levels. Considering all these factors, the SPC issued a tornado watch at 3:00PM for most of southeast Iowa. Thunderstorm development initiated just east of Des Moines, with storms quickly becoming severe, and two supercells in particular becoming dominant. The northern cell produced hail up the size of tennis balls (2.5”) from Waterloo through Cedar Falls, Iowa.

Quarter sized (1") hail from 4W Stanley, produced by the supercell thunderstorm that dropped 2.5" hail near Waterloo.

The southern cell produced hail up the size of golf balls (1.75”) near Monroe, 75 mph wind gusts near Brooklyn, and a funnel cloud to the northeast of Grinnell.

Supercell thunderstorm in Poweshiek County showing a three body scatter spike (TBSS), indicative of very large hail.

That southern cell continued into the our service area capable of large hail and damaging winds. However, in eastern Benton County the supercell cycled and began to take on tornadic signatures again.

Supercell thunderstorm displaying a classic "hook echo" signature.

Storm relative veolicity image of the same supercell, showing motion inside the storm. Green is towards the radar site (bottom right corner of the image) and red is away. This type of signature shows cyclonic rotation that could indicate a developing tornado.

Hail up to golf balls was reported in Delhi, with 70mph winds north of Independence, and numerous reports of wall clouds with brief funnel clouds from Palo, to Alburnett and Central City.

Wall cloud west of Belle Plaine, Iowa, just before the supercell entered the Quad Cities county warning area.

Eventually with the loss of daytime heating the storm began to weaken as it approached Dubuque, Iowa. There continued to be severe storms with 3 severe thunderstorm watches being issued from southern Wisconsin, southwest through the Mississippi Valley and into eastern Oklahoma. At the end of the day our office issued 7 severe thunderstorm warnings, 2 tornado warnings, and received over 20 reports of severe weather, ranging from hail and wind to wall clouds.

Nationally, the SPC logged 37 damaging wind reports, 73 large hail reports, and thankfully no tornadoes on Tuesday.

By Wednesday severe weather moved into the Ohio Valley, and the weather calmed down over the Midwest. It was an active start to severe weather season. Case in point: over three days parts of our service area were in 6 watches (4 severe thunderstorm and 2 tornado), our office issued 33 warnings (31 severe thunderstorm and 2 tornado), and we received over 150 reports of severe weather. 
This provides an opportunity for us to thank all those involved in the warning process, from those who help communicate watch and warning information, to those who spot and report severe weather, to those who heed the warnings and seek safety and shelter.

As we head into the heart of severe weather season, remember as always to be prepared and keep up to date on the latest National Weather Service forecasts when severe weather threatens.


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