By Meteorologist Tom Skilling
The latest squall line to sweep sections of the Chicago area---arguably the most damaging to date of 2011---raced across the Chicago area at up to 60 mph early Monday. The storms had first flared in western Nebraska, northwest Kansas and eastern Colorado Sunday afternoon. They wasted little time in coalescing into a damaging cluster which began night-long rampage across Iowa into western Illinois responsible for a swath of damage which prompted a string of warnings and severe weather watches.
Any semblance of daylight visible in the Chicago area as Monday dawned proved fleeting, snuffed out by the menacingly dark skies produced by opaque thunderheads which towered more than 10 miles above the Midwest---a scene rendered especially frightening here Monday as near hurricane-strength gusts rushing out of the fast-moving storms generated an deafening roar and sent debris airborne as gusts up to 80 mph began toppling trees, taking down power lines and even ripping the roofs off some buildings.
Sheets of wind-whipped rain accompanying the squall line became blinding, delivering sections of the parched metro area its first significant rain in three weeks. The speed with which the squall line swept through would end up limiting rainfall to levels far below those needed in sections of the area. In the end, while 0.83 inches of rain was measured by a Weather Bug rain gauge in Downers Grove, 0.80 inches at Palatine, 0.78 at Waukegan and 0.74 at Union Illinois, only 0.39 inches fell at O'Hare and 0.32 was recorded at Midway-hardly enough to offset the 1.50 to 1.80 inches lost each week to evaporation in the strong summer sunlight.
Expansive mass of blistering heat sets the stage for the storms; advisories for extreme heat hoisted in 25 states
A dome of blisteringly hot air draped across much of the nation's mid-section to the south of Chicago was key in aiding storm development early Monday. Powerful jet stream winds topping 100 mph flourished on the northern periphery of the mammoth pool of hot, humid air. At the same time, more than 2 inches of evaporated moisture stored within the heated air produced a 100-degree high and a 114-degree heat index at St. Louis, where Amazon River Valley-level 75-degree or greater downstate dew points combined with high 90 and triple digit thermometer readings to produce dangerous heat indices.
The abundance of moisture fueled thunderstorm development. The fact these storms bubbled high into the atmosphere where they encountered powerful upper winds allowed the storms to move quickly and to transfer wind energy down to the surface where damage soon followed.
Winds gusts hit 80 mph at Rockford---a hurricane force speed---while roaring at 75 mph at Midway Airport and at 85 mph at Michigan City, IN. Other area gusts topped 70 mph at the water intake crib offshore, as well as at Mundelein, Evergreen Park, Gurnee and DeKalb.
Monday's high speed, bow-shaped squall line ---known as a "derecho"---produces a a 1,400 mile damage path as it rampages across sections of 17 states in 30 hours
Long-lived, especially fast-moving squall lines have been dubbed "derechoes" since the term was first proposed in 1880 in the American Meteorological Journal by the Danish born University of Iowa scientist Dr. Gustavas Hinrichs. Derechoes---Monday's included---have been known to travel a 1,000 or more miles producing straight-line wind damage all the way.
Monday's squall line, which formed in western Nebraska around 3:30 pm Sunday afternoon, reached Chicago just after 8am Monday then proceeded to the Nation's Capital Washington D.C. around sunset Monday evening, traveled an astounding 1,400 miles across sections of 17 states in 30 hours, producing a path of damage and destruction much of the way. By late Monday, the derecho was responsible for more than 770 reports of severe weather to the Storm Prediction Center ---most of them involving damaging winds.
Storm's rapid movement out onto Lake Michigan sets up a seiche; Chicago lake levels oscillate up to 2 feet over three hours Monday
Fast-moving squall lines produce special problems when racing out over Lake Michigan. The outflowing air at the front of these storm complexes produces a dome of water on the lake which is pushed up against the Michigan and Indiana shorelines. Reflected waves return to the Chicago and Illinois shoreline an hour to an hour and a half later and produce measurable oscillations in lake levels---a phenomenon known as a "seiche". Monitoring equipment operated by NOAA at Calumet Harbor, indicated lake levels varied there nearly two feet over a three hour period.
Disturbance could bring some showers late Tuesday; "full-fetch" northeast flow to bring 18-day string of 80-degree highs to an end Wednesday
The Chicago area has moved into a period of cooling and drying which is to persist into Thursday. Tuesday's predicted 84-degree high represents a modest pullback from Monday's 88 at O'Hare, 90 and Midway and 92 at the lakefront. (Midway's 90-degree high marks the 11th time this year a 90-degree or higher reading has occurred at the South Side observation site, two more than the long term 84-year average there and the most 90s to have occurred at Midway in 6 years).
A southeast-bound disturbance responsible for showers and thunderstorms in the Dakotas late Monday may bring a few showers and possibly a thunderstorm to portions of the Chicago area Tuesday evening and night. The strengthening northeast winds which follow arrive in the city after a trek down the length of Lake Michigan Wednesday---a development likely to break an 18-consecutive day string of 80s here. Highs Wednesday are likely to remain in the 70s as a result and may only reach the upper 70s on Thursday as well.
Next shot at t-storms later this week as heat expands toward Chicago; one of summers hotter air masses threatens stifling heat/humidity combo Sunday/Monday
Computer models have been predicting a huge dome of intensely hot air is to come together over the Midwest this weekend. Scattered t-storms may precede the most intense heat Friday and possibly into early Saturday. Heating will then "cap" the atmosphere shutting down the rain formation process and allowing temperatures to surge.
The latest squall line to sweep sections of the Chicago area---arguably the most damaging to date of 2011---raced across the Chicago area at up to 60 mph early Monday. The storms had first flared in western Nebraska, northwest Kansas and eastern Colorado Sunday afternoon. They wasted little time in coalescing into a damaging cluster which began night-long rampage across Iowa into western Illinois responsible for a swath of damage which prompted a string of warnings and severe weather watches.
Any semblance of daylight visible in the Chicago area as Monday dawned proved fleeting, snuffed out by the menacingly dark skies produced by opaque thunderheads which towered more than 10 miles above the Midwest---a scene rendered especially frightening here Monday as near hurricane-strength gusts rushing out of the fast-moving storms generated an deafening roar and sent debris airborne as gusts up to 80 mph began toppling trees, taking down power lines and even ripping the roofs off some buildings.
Sheets of wind-whipped rain accompanying the squall line became blinding, delivering sections of the parched metro area its first significant rain in three weeks. The speed with which the squall line swept through would end up limiting rainfall to levels far below those needed in sections of the area. In the end, while 0.83 inches of rain was measured by a Weather Bug rain gauge in Downers Grove, 0.80 inches at Palatine, 0.78 at Waukegan and 0.74 at Union Illinois, only 0.39 inches fell at O'Hare and 0.32 was recorded at Midway-hardly enough to offset the 1.50 to 1.80 inches lost each week to evaporation in the strong summer sunlight.
Expansive mass of blistering heat sets the stage for the storms; advisories for extreme heat hoisted in 25 states
A dome of blisteringly hot air draped across much of the nation's mid-section to the south of Chicago was key in aiding storm development early Monday. Powerful jet stream winds topping 100 mph flourished on the northern periphery of the mammoth pool of hot, humid air. At the same time, more than 2 inches of evaporated moisture stored within the heated air produced a 100-degree high and a 114-degree heat index at St. Louis, where Amazon River Valley-level 75-degree or greater downstate dew points combined with high 90 and triple digit thermometer readings to produce dangerous heat indices.
The abundance of moisture fueled thunderstorm development. The fact these storms bubbled high into the atmosphere where they encountered powerful upper winds allowed the storms to move quickly and to transfer wind energy down to the surface where damage soon followed.
Winds gusts hit 80 mph at Rockford---a hurricane force speed---while roaring at 75 mph at Midway Airport and at 85 mph at Michigan City, IN. Other area gusts topped 70 mph at the water intake crib offshore, as well as at Mundelein, Evergreen Park, Gurnee and DeKalb.
Monday's high speed, bow-shaped squall line ---known as a "derecho"---produces a a 1,400 mile damage path as it rampages across sections of 17 states in 30 hours
Long-lived, especially fast-moving squall lines have been dubbed "derechoes" since the term was first proposed in 1880 in the American Meteorological Journal by the Danish born University of Iowa scientist Dr. Gustavas Hinrichs. Derechoes---Monday's included---have been known to travel a 1,000 or more miles producing straight-line wind damage all the way.
Monday's squall line, which formed in western Nebraska around 3:30 pm Sunday afternoon, reached Chicago just after 8am Monday then proceeded to the Nation's Capital Washington D.C. around sunset Monday evening, traveled an astounding 1,400 miles across sections of 17 states in 30 hours, producing a path of damage and destruction much of the way. By late Monday, the derecho was responsible for more than 770 reports of severe weather to the Storm Prediction Center ---most of them involving damaging winds.
Storm's rapid movement out onto Lake Michigan sets up a seiche; Chicago lake levels oscillate up to 2 feet over three hours Monday
Fast-moving squall lines produce special problems when racing out over Lake Michigan. The outflowing air at the front of these storm complexes produces a dome of water on the lake which is pushed up against the Michigan and Indiana shorelines. Reflected waves return to the Chicago and Illinois shoreline an hour to an hour and a half later and produce measurable oscillations in lake levels---a phenomenon known as a "seiche". Monitoring equipment operated by NOAA at Calumet Harbor, indicated lake levels varied there nearly two feet over a three hour period.
Disturbance could bring some showers late Tuesday; "full-fetch" northeast flow to bring 18-day string of 80-degree highs to an end Wednesday
The Chicago area has moved into a period of cooling and drying which is to persist into Thursday. Tuesday's predicted 84-degree high represents a modest pullback from Monday's 88 at O'Hare, 90 and Midway and 92 at the lakefront. (Midway's 90-degree high marks the 11th time this year a 90-degree or higher reading has occurred at the South Side observation site, two more than the long term 84-year average there and the most 90s to have occurred at Midway in 6 years).
A southeast-bound disturbance responsible for showers and thunderstorms in the Dakotas late Monday may bring a few showers and possibly a thunderstorm to portions of the Chicago area Tuesday evening and night. The strengthening northeast winds which follow arrive in the city after a trek down the length of Lake Michigan Wednesday---a development likely to break an 18-consecutive day string of 80s here. Highs Wednesday are likely to remain in the 70s as a result and may only reach the upper 70s on Thursday as well.
Next shot at t-storms later this week as heat expands toward Chicago; one of summers hotter air masses threatens stifling heat/humidity combo Sunday/Monday
Computer models have been predicting a huge dome of intensely hot air is to come together over the Midwest this weekend. Scattered t-storms may precede the most intense heat Friday and possibly into early Saturday. Heating will then "cap" the atmosphere shutting down the rain formation process and allowing temperatures to surge.
