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PDS: Particularly Dangerous Situation Tornado Watch Midwest – Severe Flooding Chicago Area – Plowable Snow Twin Cities

18 Apr 2013, 12:18 pm

 

Severe Flooding In Chicago Area. Parts of Chicagoland have picked up 5-7″ rain in the last 2 days. Extensive flash flooding is being reported, impacting travel and facilities in low-lying flood-prone areas. Details below.

Here is the most time-sensitive information:

 

PDS Tornado Watch Until 5 pm. PDS means Particularly Dangerous Situation, which is NWS short-hand for “there is now a high probability of a few large, violent tornadoes in this specific region“. A developing squall line will produce straight-line and tornadic winds; the greatest risk from Terre Haute and Indianapolis to Lafayette, Sound Bend and far southern Lower Michigan.  People in these areas should be on high alert, review disaster readiness plans, and be prepared to move staff and customers to safety. This is the area we are most concerned about, short-term.  This is the first PDS watch of the year.  Greg Carbin, Warning Coordination Meteorologist with the Storm Prediction Center says there are typically about 20 of these issued over an entire year. The most recent was Christmas Day, 2012.

 

Tornado Watch Lower Michigan. SPC has expanded the tornado threat area northward into Lower Michigan, including Grand Rapids, Ann Arbor, Flint and the metro Detroit area – valid until 6 pm.

 

Latest Tornado Watch. It promises to be a very active day for severe weather and tornado potential. SPC has extended the Tornado Watch southward, from southern Illinois and Indiana (Evansville area) into southeast Missouri, the western suburbs of Memphis to Little Rock and Hot Springs – valid until 5 pm.

 

A Witch’s Brew Of Meteorological Ingredients. A combination of severe instability, ample low-level moisture (dew points near 70) and a highly sheared environment (winds changing direction/speed with altitude) will create supercell thunderstorms capable of spinning up tornadoes. The numbers above represent the probability of a tornado within 25 miles of any point. The hatched area (yellow) indicates a 10% probability of a large tornado (EF-2 or stronger) within 25 miles of any point.

 

 

Isolating The Threat. Our in-house EHI model shows the greatest potential for tornadic storms pushing across Indiana by midday; conditions ripe for a few isolated (large) tornadoes from Ann Arbor to Louisville, Little Rock and Memphis later today.

 

 

Mudslides. Heavy (5-6″+) rains falling on saturated soil have resulted in extreme run-off in the Iowa City area, near Des Moines, with reports of mudslides. Much of the Midwest has transitioned from drought to flood in the span of less than a week. Another case of weather-whiplash.

 

 

Flooding Underway. The purple dots above show major flooding currently being reported: on the East Branch of the DuPage River at Bolingbrook, and the North Branch of the Chicago River in Chicago. Flash flooding will impact travel and threaten low-lying facilities with urban and stream/river flooding into at least Saturday morning before waters begin to recede. The latest information for the Chicago area is here.

 

 

Near-Blizzard Conditions Possible. Our in-house BPI (Blizzard Potential Index) shows visibilities under 1/4 mile in moderate snow (with winds in excess of 30 mph) over much of central Minnesota and northwest Wisconsin tonight. Travel will be a mess after dark as temperatures quickly fall below freezing and snow begins to rapidly accumulate.

 

 

What Spring? Residents of the Upper Midwest are losing their stoic sense of weather-humor. The same storm spinning up tornadoes and flooding out parts of the Midwest will dump another plowable snowfall from northwest Iowa into much of Minnesota and the northwest third of Wisconsin. The Twin Cities is expecting some 3-6″ amounts tonight and early Friday, as much as 10-12″ from Duluth into the U.P. of Michigan. Perfectly normal…for late February.

Summary: I’m most concerned about the risk of a few large, violent, long-track tornadoes capable of considerable damage and injury from southern Lower Michigan across Indiana and east central Illinois. It’s impossible to determine, hours in advance, where these tornadoes will spin up, but people should be on high alert, and be prepared to move people to safety later today as required.  This may be one of the 3 or 4 most violent days of spring. The risk appears to be greatest for Indiana, but this could shift in the hours ahead. Severe flooding is being reported from eastern Iowa into the Chicagoland and Milwaukee area – rivers will continue to rise before some improvement over the weekend. This is a volatile, dangerous weather pattern– one that shows no sign of easing anytime soon over the central USA.

 

Paul Douglas, Senior MeteorologistAlerts Broadcaster– Minneapolis

 

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