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Sunday April 10th severe outbreak


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A rather poignant shift of the mod risk boundary to now include northeast Illinois and eastern Wisconsin all the way to Lake Michigan.

I acutally thought it would shift west if anything. This heightens my anticipation even more. Interesting that they say the greatest tornado threat will be in Eastern Iowa, Northern Illinois and Southern Wisconsin near the warm front.....I thought the warm front would be up near Green Bay by that point. In any case, spotters should feel free to peruse the I94 corridor between Madison and Milwaukee.

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I am going to be sitting on top of Blue Mounds (400 feet above the surrounding terrain) Sunday. You are able to see 25-30 miles in all directions and it is located west of Mt Horeb and along hwy 18-151. I will be trying out my new i phone with radar scopesmile.gif.

That sounds like a very good spot to be. Been past Blue Mounds a few times. I'll be doing the same down in Iowa City trying to find a high spot, although things keep shifting eastward it just might blow up on top of us or east of us.

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Regarding the weather brieifing..

NWS LOT (Gino to those of you who know him) was talking about indications that the storms are likely to develop along a "convergence zone" starting in the early afternoon along, or just east of the Mississippi River, with a higher chance of the developing storms being discrete supercells. He did mention the tornado threat as well, stating as these storms move east, the tornado threat will increase, esp. after sunset.

Tomorrow looks positively scary... I now have a 10 year old bouncing off the walls in anticipation....he is both excited about the storms, and very afraid of them at the same time......so tomorrow is going to be interesting. I can't remember the last time we had a severe threat like this, this early in the year.

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Another day, another KLOT live weather briefing. Coming up at 1 p.m. . .

http://www.crh.noaa....on=1&glossary=0

Izzi was on again today. Sorry for the bible but it was something else to listen to.

Three main points he made: things have changed and not for the better - much more concerned about a tornado threat (and long lasting supercells) than 24 hours ago and less concerned about widespread wind damage - things can continue to change and hard to pinpoint exact timing/location of sups.

To recap:

As the warm front currently in central Illinois lifts north overnight, there is some potential for storms over Iowa this evening to clip northern Illinois north of I-88. Those storms could produce large hail and damaging winds, but should stay north of Wisconsin line. Storms clear out by Sunday morning.

Sunday will be fairly sunny by late morning and early afternoon. Clouds should not be a factor. Cirrus should stay to the south. Little in the way of convective debris in the area - storms in early morning to the west, etc. should either weaken or move northeast to the north of the area.

Winds tomorrow during the day forecast at 20-30 mph with gusts to 40. Southerly flow ushers warm ("almost hot") moist, humid air to create volatile conditions in afternoon. Anticipates temps in KLOT in mid-80's with a few locations possibly reaching 90. He said 90 around here at this time of year "is not a good sign."

Contrary to earlier thinking, pretty much all the models are now showing convection along a pre-frontal convergence line, yielding discrete supercells ahead of the cold front. Not as likely to see a rapid transition to a squall line, which would be more typical for this part of the country. As a result, threat of tornadoes is higher / threat of widespread wind damage is lower.

With supercells remaining discrete for longer times, there is a threat for long-track and potentially strong tornadoes across the area.

As far as timing, such warm air during the afternoon may not be conducive for tornadoes - expects temps to drop a couple degrees at sunset and low level winds to increase from 30-40 to 40-60 mph. Increased shear at that time will increase threat for tornadoes. Expects a window of 2-5 hours with supercells and potential "long track supercells with tornadoes." Prime time for tornadoes from 6-10 p.m. and possibly as late as 11. They could fire as early as 3-4 p.m. in north central Illinois, but generally increasing into the evening. However, where the storms will be is still tough to determine at this point.

As far as location, he first mentioned Mississippi River for starting point, but acknowledged they could fire further west and threaten more of eastern IA and western WI, or they could fire further east and make a direct impact to the Chicago forecast area.

He also mentioned that people involved in cleanup efforts as a result of initial storm damage need to be aware of later arriving cold front, posing an additional threat (but much less of a severe threat).

He acknowledged that SPC has mentioned a high risk outlook and that the last in KLOT was nearly four years ago on June 8, 2007.

Have talked about favorable conditions for an outbreak in the past, but need every ingredient to come together just right.

Another live briefing either late morning or early afternoon tomorrow.

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I still think the best chance for significant tornadoes will be starting out in SE MN/NE IA, continuing in a swath directed towards the ENE across much of Wisconsin. I like the directional shear in this area, and supercells should remain discrete for a longer period of time. Further south into northern IL and the Chicago metro, the risk is a bit more conditional depending on whether the flow ends up too unidirectional or not. It's possible that the surface winds there may back more than currently modeled, but right now I'm less confident about that than further north.

A high risk upgrade tomorrow seems entirely justified. I think this ends up being one of those outbreaks that is relatively localized in areal extent, but very intense with 2-3 very prolific tornadic supercells.

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I still think the best chance for significant tornadoes will be starting out in SE MN/NE IA, continuing in a swath directed towards the ENE across much of Wisconsin. I like the directional shear in this area, and supercells should remain discrete for a longer period of time. Further south into northern IL and the Chicago metro, the risk is a bit more conditional depending on whether the flow ends up too unidirectional or not. It's possible that the surface winds there may back more than currently modeled, but right now I'm less confident about that than further north.

A high risk upgrade tomorrow seems entirely justified. I think this ends up being one of those outbreaks that is relatively localized in areal extent, but very intense with 2-3 very prolific tornadic supercells.

So what part of Wisconsin do you foresee seeing the worst of it? It sounds like South Central, Southeast and East Central Wisconsin might be in the line of fire. Earlier that area seemed to be farther northwest.

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So what part of Wisconsin do you foresee seeing the worst of it? It sounds like South Central, Southeast and East Central Wisconsin might be in the line of fire. Earlier that area seemed to be farther northwest.

I would say probably the top half of the current SPC moderate risk. This could change through the day tomorrow of course, depending on where the surface low tracks and how the surface winds respond.

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Saving for posterity...

217

FXUS63 KLOT 091937

AFDLOT

AREA FORECAST DISCUSSION

NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL

237 PM CDT SAT APR 9 2011

...OUTBREAK OF SEVERE THUNDERSTORMS AND TORNADOES POSSIBLE SUNDAY

AFTERNOON/EVENING...

DISCUSSION

236 PM CDT

SHORT TERM (TONIGHT THROUGH SUNDAY NIGHT)...

ATMOSPHERE IS THE THE PROCESS OF PREPPING FOR WHAT IS LOOKING TO BE

A POTENTIALLY MAJOR SEVERE WEATHER/TORNADO OUTBREAK LATE SUNDAY

AFTERNOON INTO SUNDAY EVENING.

UNUSUALLY STRONG COLD CORE DEEP LOW CONTINUES TO DIG SOUTHEASTWARD

OFF THE SOUTHERN CALIFORNIA COAST THIS AFTERNOON HEIGHT FALLS IN

ADVANCE OF THIS BIG TROUGH HAVE ALREADY RESULTED IN CYCLOGENESIS TO

THE LEE OF THE COLORADO ROCKIES WITH A WAVY WARM FRONT EXTENDING

EAST FROM THIS LOW ACROSS THE PLAINS TO CENTRAL IL/SOUTHERN IN. WARM

FRONT LIKELY TO SURGE NORTH ACROSS THE CWA TONIGHT WITH UNSEASONABLY

WARM/MOIST AIRMASS SURGING INTO THE AREA. WOULD ACTUALLY ANTICIPATE

RISING TEMPERATURES TONIGHT...ESPECIALLY CLOSER TO LAKE MICHIGAN

WHERE MARINE LAYER IS KEEPING THINGS CHILLIER THIS AFTERNOON.

NOT OVERLY EXCITED ABOUT CONVECTIVE CHANCES TONIGHT...HOWEVER STILL

PLAUSIBLE THAT CONVECTION DEVELOPING ALONG THE DRY LINE OVER THE

EASTERN PLAINS LATER THIS AFTERNOON COULD CONGEAL INTO A CLUSTER

AND SCRAPE OUR NORTHERN CWA. BETTER PROBABILITIES CERTAINLY LOOK TO

BE OVER WISCONSIN...BUT CLOSE ENOUGH CALL TO MAINTAIN A CHANCE POPS.

ANY THUNDERSTORM THAT DOES MEANDER INTO NORTHERN ILLINOIS WOULD HAVE

THE POTENTIAL TO PRODUCE DAMAGING WINDS/LARGE HAIL GIVEN THE STEEP

LAPSE RATES AND STRONG SHEAR.

BY ALL INDICATIONS NOW...SUNDAY LOOKS TO BE WINDY...UNSEASONABLY

WARM AND HUMID. CHANCES FOR ANY LINGERING ELEVATED CONVECTION (WHILE

NOT ZERO) LOOK LOW AND EVEN CLOUD COVER MAY BE AT A PREMIUM AND

MODELS SUGGEST CIRRUS WITH SUBTROPICAL JET WILL REMAIN SOUTH OF US

AND LITTLE IN THE WAY OF CUMULUS INITIALLY GIVEN THE STRONG CAP

RESULTING FROM THE STRONG EML.

MODELS HAVE COME INTO REMARKABLY GOOD AGREEMENT AS TO THE EVOLUTION

OF CONVECTION TOMORROW AFTERNOON AND EVENING. NEARLY EVERY MODEL

(CONVECTIVE ALLOWING AND CONVECTIVE PARAMETRIZED) SUGGEST

CONVECTIVE INITIATION DURING THE MID-LATE AFTERNOON NEAR OR JUST

WEST OF THE MISSISSIPPI RIVER ALONG WHAT APPEARS TO BE A PRE-FRONTAL

CONVERGENCE ZONE. STRONG HEIGHT FALLS AND LARGE SCALE ASCENT

OVERSPREADING THE WARM SECTOR DURING THE AFTERNOON IS FORECAST TO

ALLOW CAP TO WEAKEN SUFFICIENTLY FOR EXPLOSIVE SEVERE THUNDERSTORM

DEVELOPMENT ALONG THIS CONVERGENCE ZONE.

GIVEN THE 50-60KT OF 0-6KM BULK SHEAR WOULD ANTICIPATE STORMS TO

QUICKLY GO SUPERCELLULAR WITH ATTENDANT DAMAGING WIND AND VERY LARGE

HAIL THREAT. RELATIVELY HIGH LCL HEIGHTS AND RATHER DEEPLY MIXED

BOUNDARY LAYER WOULD SUGGEST A LESSER TORNADO POTENTIAL INITIALLY

BUT TOWARD SUNSET AS LOW LEVEL JET INCREASES FROM 30KT TO AROUND

60KT DURING THE EARLY EVENING...RESULTING IN LOW LEVEL SHEAR JUST

BECOMING SICK. IN ADDITION...A SLIGHT COOLING OF THE BOUNDARY LAYER

AND LESS MIXING COULD ALSO ALLOW FOR LOWERING LCL HEIGHTS AND

CREATING AN ENVIRONMENT PRIMED FOR STRONG OR EVEN VIOLENT TORNADOES

GIVEN SUPERCELLULAR STORM MODE.

SPEAKING OF STORM MODE...THIS REMAINS PROBABLY ONE OF THE MOST

DIFFICULT THINGS TO FORECAST AND NOT SOMETHING WE ARE PARTICULARLY

SKILLED AT. HAVING SAID THAT...INITIATION ALONG A MORE SUBTLE

SURFACE BOUNDARY (IE PRE-FRONTAL CONVERGENCE VS COLD FRONT) WOULD BE

MORE FAVORABLE FOR DISCRETE SUPERCELL. ALSO...SHEAR VECTOR

ORIENTATION WRT TO INITIATING BOUNDARY WOULD ALSO FAVOR CELLS

BREAKING FREE FROM BOUNDARY AND MOVING QUICKLY EAST.

FINALLY...HIGHER RESOLUTION CONVECTIVE ALLOWING MODELS (WHICH HAVE

SHOWN SKILL IN FORECASTING STORM MODE) ARE MOSTLY FORECASTING

DISCRETE CELLS. SREF MEDIAN SIG TOR PARAMETER OVER NORTHERN IL

CLIMBS TO OVER 6 TOMORROW EVENING...SO IF FORECASTS FOR DISCRETE

CELLS VERIFY IT COULD GET UGLY TOMORROW EVENING.

AFTER 6 PARAGRAPHS OF SAYING THE SKY IS FALLING...ITS WORTH NOTING

THAT IT IS EXTRAORDINARILY RARE TO GET EVERYTHING TO FALL INTO PLACE

FOR A TORNADO OUTBREAK. IT WOULD ONLY TAKE ONE PARAMETER TO FALL

SLIGHTLY OUT OF WHACK TO RESULT IN A GREATLY REDUCED SEVERE AND/OR

TORNADO THREAT...OR FOR STORM MODE ISSUES TO OCCUR. WHILE HAVING

INITIATION ALONG A PRE-FRONT CONVERGENCE ZONE CERTAINLY DOESNT FAVOR

RAPID EVOLUTION INTO SQUALL LINE...IF CAP WEAKENS TOO RAPIDLY

WOULDNT BE TOO HARD TO ENVISION SCENARIO WHERE SO MUCH CONVECTION

DEVELOPS THAT SUPERCELLS BEGIN TO INTERFERE WITH EACH OTHER. BOTTOM

LINE IS THAT WE ARE STILL MORE THAN 24 HOURS FROM THE EVENT AND A

LOT COULD CHANGE IN THE FORECAST TO RESULT IN A LESS GLOOM AND DOOM

SOLUTION...SO STAY TUNES.

IZZI

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Nice disclaimer from Gino:

AFTER 6 PARAGRAPHS OF SAYING THE SKY IS FALLING...ITS WORTH NOTING

THAT IT IS EXTRAORDINARILY RARE TO GET EVERYTHING TO FALL INTO PLACE

FOR A TORNADO OUTBREAK. IT WOULD ONLY TAKE ONE PARAMETER TO FALL

SLIGHTLY OUT OF WHACK TO RESULT IN A GREATLY REDUCED SEVERE AND/OR

TORNADO THREAT...OR FOR STORM MODE ISSUES TO OCCUR. WHILE HAVING

INITIATION ALONG A PRE-FRONT CONVERGENCE ZONE CERTAINLY DOESNT FAVOR

RAPID EVOLUTION INTO SQUALL LINE...IF CAP WEAKENS TOO RAPIDLY

WOULDNT BE TOO HARD TO ENVISION SCENARIO WHERE SO MUCH CONVECTION

DEVELOPS THAT SUPERCELLS BEGIN TO INTERFERE WITH EACH OTHER.

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Nice disclaimer from Gino:

AFTER 6 PARAGRAPHS OF SAYING THE SKY IS FALLING...ITS WORTH NOTING

THAT IT IS EXTRAORDINARILY RARE TO GET EVERYTHING TO FALL INTO PLACE

FOR A TORNADO OUTBREAK. IT WOULD ONLY TAKE ONE PARAMETER TO FALL

SLIGHTLY OUT OF WHACK TO RESULT IN A GREATLY REDUCED SEVERE AND/OR

TORNADO THREAT...OR FOR STORM MODE ISSUES TO OCCUR. WHILE HAVING

INITIATION ALONG A PRE-FRONT CONVERGENCE ZONE CERTAINLY DOESNT FAVOR

RAPID EVOLUTION INTO SQUALL LINE...IF CAP WEAKENS TOO RAPIDLY

WOULDNT BE TOO HARD TO ENVISION SCENARIO WHERE SO MUCH CONVECTION

DEVELOPS THAT SUPERCELLS BEGIN TO INTERFERE WITH EACH OTHER.

What are the odds? I have no idea....best to prepare for the worst, and hope for the best. Tomorrow could be historic for someone.

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If something could roll off the prefrontal trough and get in here in the 0z-3z timeframe, I think it would increase the tor threat significantly as there's decent although not spectacular directional shear until then. That scenario doesn't look particularly likely but something to watch.

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