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UPDATE 2 201PM CT:  New PARTICULARLY DANGEROUS SITUATION Tornado Watch issued by the SPC. It looks like the outbreak is going to extend west to include parts of Iowa and Wisconsin.

* EFFECTIVE THIS WEDNESDAY AFTERNOON AND EVENING FROM 150 PM
     UNTIL 900 PM CDT.

   ...THIS IS A PARTICULARLY DANGEROUS SITUATION...

   * PRIMARY THREATS INCLUDE...
     SEVERAL INTENSE TORNADOES LIKELY
     SEVERAL SIGNIFICANT DAMAGING WIND GUSTS TO 80 MPH POSSIBLE
     SEVERAL VERY LARGE HAIL EVENTS TO 3 INCHES IN DIAMETER POSSIBLE

UPDATE 139PM CT: A tornado watch is about to be issued for northeastern Iowa. I will post the image once they issue the watch.

Original diary starts below.


As I wrote about yesterday afternoon, the Storm Prediction Center has gone ahead and called for the potential for a major derecho -- a long-lived, powerful line of storms that produces constant and widespread wind damage over a 240 mile path -- to impact parts of the Great Lakes region.

Please see my diary from yesterday for a comprehensive overview of derechos.

The timing begins around 3-4PM Central Time when the individual storms develop and produce potentially strong tornadoes, and continues through the night with the formation of a line that races eastward. The farther east you go, the later the storms will arrive. I will post another liveblog diary once the storms start up, given the gap between this diary's publication and the formation of the storms.

The SPC just took the unusual step of issuing a high risk -- the highest threat for severe weather -- across parts of Illinois, Indiana, and Ohio in anticipation of this very dangerous severe weather outbreak.

The storms will come in two batches. The first will occur before the potential derecho, sometime after 3PM and before 7PM (likely closer to 3-4PM). Individual storms could produce strong tornadoes across northern Illinois before they merge into a line and begin to race eastward. The line itself will occur a few hours after the initial storms, depending on how long it takes the individual storms to develop and organize. Once a line (potential derecho) forms, it's going to haul ass towards the east at over 50 MPH.

This is from this afternoon's discussion, directly from the SPC's website (emphasis mine):

POTENTIALLY STRONG TORNADO THREAT MAY BE MAXIMIZED DURING THIS EARLY DEVELOPMENT PHASE 21Z-00Z NEAR THE LOW AND FRONT WHILE STORMS REMAIN DISCRETE AMIDST HIGH INSTABILITY AND EFFECTIVE SRH IN EXCESS OF 200 M2/S2.

ORGANIZING INFLUENCE OF THE DEEPENING SURFACE CYCLONE RIPPLING EAST ALONG THE WARM/QUASI-STATIONARY FRONT...IN CONCERT WITH 50-60KT MID-LEVEL JET STREAK DIRECTED PREFERENTIALLY INTO/ACROSS THE DEVELOPING MASS OF CONVECTION SUGGESTS UPSCALE PROGRESSIVE MCS POSSIBLE DERECHO EVOLUTION THROUGH THE EVENING HOURS. GIVEN ANTECEDENT AIRMASS CHARACTERISTICS...STRENGTH OF LARGE SCALE FORCING...AND DEPICTION OF CURRENT CONDITIONS...RELATIVELY HIGH CONFIDENCE EXISTS IN A NUMBER OF STORM-SCALE MODEL SIMULATIONS SHOWING MCS/DERECHO EVOLUTION WITHIN THE HIGH RISK AREA THROUGH THE EVENING HOURS. WRF-ARW SIMULATION FROM 00Z TAKES THE APEX OF THE PROGRESSIVE MCS FROM CHICAGO TO DETROIT IN UNDER 6 HOURS WITH A FORWARD SPEED IN EXCESS OF 40KT. SHEAR AND INSTABILITY WITHIN AND AHEAD OF THIS CONVECTION WILL REMAIN SUPPORTIVE OF BOTH HIGH WINDS POSSIBLY WELL IN EXCESS OF 60KT...AS WELL A FEW TORNADOES POSSIBLY ASSOCIATED WITH SUPERCELL STRUCTURES EMBEDDED WITHIN THE SQUALL LINE/QLCS.

Here are the severe weather probabilities. Purple/pink indicates a HIGH risk for severe weather. Red indicates a moderate risk. Yellow is slight risk. Green is general, non-severe thunderstorms.

Here are the wind damage probabilities, indicating a xx% chance of wind damage within 25 miles of any point in the shaded area. The black hatching indicates the risk for damaging winds in excess of 75 MPH. The 60% zone is the highest risk for wind the SPC issues, and it's very rare to see.

Here are the tornado probabilities, same as above. Black hatching indicates the possibility of violent, long-track tornadoes.

Here is the hail risk today. Today's risk is mainly wind and tornadoes, but any hail that forms will be driven by extremely strong winds, making it like a barrage of bullets.

The weather models indicate that the outbreak today has the potential to be incredibly bad. As with all weather events, this is a prediction by experts at the National Weather Service based on what the weather models are showing will happen. Most weather enthusiasts and meteorologists are trying to straddle the thin line between hype and conveying the potential danger of today's event. Today is one of those rare top-of-the-scale, sound-the-alarms type of days that requires extreme wording.

I will start a liveblog and keep it going constantly once the storms start going. Please alert your friends and family in the area that they need to keep a very close eye on the weather today, and take immediate precautions to protect themselves from danger if the storm threatens.

Prepare, but don't panic. Secure all loose items outside, as they will easily blow around in winds. Grills, patio furniture, garden gnomes, and whatever else you've got should go inside a shed, come inside, or get tied up against a sturdy object.

Make sure your cell phones are charged and you've got gas in your car.

Wear sneakers/tennis shoes/whatever you call them tomorrow in case you have to walk through broken glass or debris outside.

Make plans in case you lose power for an extended period of time.

Keep a very close eye on the Storm Prediction Center and your local National Weather Service office. Like I said, it'll happen very quickly once the storms form and the line(s) develop. People closer to the Appalachians will have more time to react than people in Illinois or Indiana.

Hopefully a derecho will not happen, but the risk exists, and it's better to be informed than scared by the news stations that can't be bothered to do in-depth reporting.

I'll continuously post updates tonight and tomorrow

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Storm Prediction Center Main Page
Storm Prediction Center -- Current Severe Weather Watches
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Storm Prediction Center -- Mesoscale Discussions
Storm Prediction Center -- Storm Reports
Storm Prediction Center -- Mesoscale Analysis Pages

Wunderground's Detailed Radar (click the + nearest to you to see your local radar)

NOAA Weather Models

iMapWeather Radio App for iPhone/iPod Touch (costs $9.99 but well worth it)

Follow me on Facebook for updates on this severe weather outbreak and other weather news.

10:21 AM PT: There's a big controversy in the weather world right now over people prematurely using the term "derecho" to describe a potential severe weather event. Here's my defense of the term's use...

The term "derecho" is a very specific term that's usually assigned to a damaging line of storms after the fact, once all the wind damage reports come in and one can see that the system met the "240 miles of constant wind damage" requirement, only then can it truly be called a derecho.

However, after the one that tore through Chicago to DC last summer and left millions without power for weeks, the term "derecho" has entered the public vocabulary as any bad line of storms, regardless.

I resisted calling this event a "derecho" as long as possible. The Storm Prediction Center has used the term six times in the last three hours. They're calling it a "possible derecho" based on the fact that the environment is capable of sustaining a severe line of storms for the distance and length of time that would produce significant wind damage over a 240+ mile swath of the Great Lakes region.

Calling it anything but a "derecho" causes the public to tune out. It's a catch 22 when it comes to weather education, but it is what it is. The only thing I and others can do is to drive home the definitions and qualifiers to the forecasts.


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