Historic Tornado Outbreak April 27, 2011

[cyclone77]

Wow not often do you see such amazing shear coinciding with large instability like this.

Yeah it's looking pretty ugly in that area.

FWIW the long range RUC forecasts cape in excess of 5000j/kg in this highly sheared environment over northeast Texas late tomorrow. Likely overdone, but I'm sure we'll be seeing monster sups traversing northeast Texas/southeast Oklahoma later tomorrow....

[Chicago Storm]

Tuesday's thread...

http://www.americanwx.com/bb/index.php/topic/17704-april-26th-severe-weather/

[cyclone77]

Tuesday's thread...

http://www.americanw...severe-weather/

Mods feel free to move my last few posts to the new thread for Tuesday.

[Superstorm93]

With the NAM and GFS looking so intense, I'm starting to wonder if we have enough to warrant a possible Day 2 High risk. I know the outlooks are trivial and the only things that matter are the reports, but it would still be interesting to see. Any thoughts?

We would be around the same area as it occurred last time way back in 2006:

[k***]

nice...broyles

[Stebo]

With the NAM and GFS looking so intense, I'm starting to wonder if we have enough to warrant a possible Day 2 High risk. I know the outlooks are trivial and the only things that matter are the reports, but it would still be interesting to see. Any thoughts?

We would be around the same area as it occurred last time way back in 2006:

There is certainly a potential of a High, of all 3 days Wednesday would be the one that has the highest potential get the High Risk designation.

[cyclone77]

Might as well change the title of this thread to just the 27th now that we have a 26th thread.

[SmokeEater]

DAY 2 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

0100 AM CDT TUE APR 26 2011

VALID 271200Z - 281200Z

...THERE IS A MDT RISK OF SVR TSTMS ACROSS A LARGE PORTION OF

KENTUCKY AND TENNESSEE...NORTHEAST AND EAST CENTRAL

MISSISSIPPI...THE NORTHERN HALF OF ALABAMA...AND NORTHWEST

GEORGIA...

...THERE IS A SLGT RISK OF SVR TSTMS FROM THE LOWER GREAT LAKES

REGION SWWD TO THE CENTRAL GULF COAST...

...SYNOPSIS...

A LARGE UPPER TROUGH INITIALLY OVER THE CENTRAL U.S. IS FORECAST TO

MAKE SLOW BUT STEADY EWD PROGRESS THIS PERIOD...WITH A POTENT SPEED

MAX FORECAST TO ROUND THE BASE OF THE TROUGH AND SHIFT ACROSS THE

MID SOUTH INTO THE OH VALLEY REGION WITH TIME.

AT THE SURFACE...A DEEPENING LOW IS FORECAST TO SHIFT FROM THE

VICINITY OF AR EARLY NNEWD TO THE GREAT LAKES REGION...WHILE A

TRAILING COLD FRONT ACCELERATES EWD ACROSS THE OH/TN/MID AND LOWER

MS VALLEYS. BY THE END OF THE PERIOD...THE FRONT SHOULD CROSS THE

APPALACHIANS -- WITH A ZONE BETWEEN THE MS RIVER AND THE APPALACHIAN

CREST LIKELY TO SEE A WIDESPREAD/POTENTIALLY DANGEROUS SEVERE

WEATHER EVENT THIS PERIOD.

...LOWER GREAT LAKES REGION SWWD TO THE CENTRAL GULF COAST...

***POTENTIAL FOR A SIGNIFICANT/WIDESPREAD SEVERE WEATHER EVENT --

INCLUDING THE POSSIBILITY OF A TORNADO OUTBREAK -- REMAINS EVIDENT

THIS FORECAST...CENTERED ON THE MID SOUTH/TN VALLEY AREA.***

SOME QUESTIONS EXIST ATTM WITH RESPECT TO CONVECTIVE EVOLUTION THIS

PERIOD...CENTERED AROUND THE ONGOING STORMS PROGGED TO BE SHIFTING

ACROSS ERN AR AND INTO WRN TN/NRN MS AT THE START OF THE PERIOD.

WHILE THE COVERAGE AND PROGRESSION OF THIS CONVECTION COULD HAVE

EFFECTS ON TIMING/LOCATION OF LATER DAY STORMS AS WELL AS THE

QUALITY OF AIRMASS IN SOME AREAS...IT APPEARS ATTM THAT EARLY STORMS

SHOULD CONTINUE ENEWD ACROSS WRN KY/WRN TN/NRN MS THROUGH THE

MORNING...POSSIBLY WEAKENING SOME WITH TIME.

HOWEVER...AS HEATING OF THE MOIST BOUNDARY LAYER COMMENCES AHEAD OF

THE ADVANCING FRONT AND ASSOCIATED UPPER FEATURE AND AWAY FROM ANY

CONVECTIVELY-COOLED AREAS...STORM REINTENSIFICATION AND NEW STORM

DEVELOPMENT WILL BE LIKELY BY EARLY AFTERNOON.

STRONG SLY SURFACE FLOW AHEAD OF THE FRONT INCREASING TO 80 TO 100

KT FROM THE SW AT MID LEVELS WILL PROVIDE SHEAR HIGHLY FAVORABLE FOR

VERY INTENSE/LONG-LIVED ROTATING UPDRAFTS. THIS -- COMBINED WITH

FAVORABLE THERMODYNAMIC ENVIRONMENT...APPEARS SUPPORTIVE OF

SIGNIFICANT LONG-TRACK TORNADOES AND -- DEPENDING UPON STORM MODE --

THE THREAT FOR WIDESPREAD/VERY DAMAGING WINDS ALONG WITH LARGE HAIL.

ATTM...GREATEST POTENTIAL FOR TORNADOES APPEARS TO EXIST ACROSS

TN/KY/NRN MS AND NRN AND CENTRAL AL...WHILE POTENTIAL FOR WIDESPREAD

WIND DAMAGE -- ALONG WITH TORNADO THREAT -- WILL EXTEND NNEWD ACROSS

THE OH VALLEY TOWARD THE LOWER GREAT LAKES REGION AHEAD OF THE

FRONT.

OVERNIGHT...SOME TORNADO POTENTIAL WILL CONTINUE...THOUGH STORMS MAY

BECOME MORE LINEAR AHEAD OF THE ACCELERATING FRONT AS IT MOVES TO --

AND POSSIBLY ACROSS -- THE APPALACHIANS. EXPECT POTENTIAL FOR

DAMAGING WINDS TO PERSIST WITH THE CONVECTIVE LINE...GIVEN THE

OVERALL STRENGTH OF THE LARGE-SCALE SYSTEM.

..GOSS.. 04/26/2011

[ALhurricane]

Still don't think they've expanded the MDT risk far enough south. The threat for strong tornadoes extends into southern AL/MS.

[Xenia74]

Well the setup is ideal for Wendsday to say the least....I believe this will be a very widespread outbreak much like 1974. I think we will see multiple large tornadoes and tornado families. The bullseye in my honest opinion is northern alabama, northern mississippi, and all of central tennessee with the highest threat. Not only do I think this threat will be extended south but to the north as well.....

[Hoosier]

12z NAM goes nuts with the surface low tomorrow. 984 mb basically right over me and then around 980 mb in western lower Michigan.

[TheWeatherPimp]

Stronger surface reflection could also cause a farther NW Track, might not be much, but at least some. The NAM showing some impressive STP numbers across Eastern Indiana/Western Ohio.

[OHSnow]

Stronger surface reflection could also cause a farther NW Track, might not be much, but at least some. The NAM showing some impressive STP numbers across Eastern Indiana/Western Ohio.

Hopefully, some of that shear and convective available potential energy will spread north so we can get in on some action here in Ohio. It's been really boring here severe-weatherwise, while elsewhere has been getting hammered.

[earthlight]

Moderate Risk extended way north

DAY 2 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1230 PM CDT TUE APR 26 2011

VALID 271200Z - 281200Z

...THERE IS A MDT RISK OF SVR TSTMS WEDNESDAY ACROSS PARTS OF

EASTERN INDIANA...MUCH OF OHIO...WESTERN PENNSYLVANIA...WEST

VIRGINIA...MUCH OF KENTUCKY AND TENNESSEE...NORTHEAST

GEORGIA...CENTRAL AND NORTHERN ALABAMA AND MISSISSIPPI...

...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING THE MODERATE RISK

AREA ACROSS MUCH OF THE EASTERN U.S....

..SYNOPSIS

..OUTBREAK OF SEVERE THUNDERSTORMS POSSIBLE

MODELS INDICATE THAT THE LARGE UPPER TROUGH...NOW EVOLVING OVER THE

CENTRAL U.S...WILL SLOWLY LIFT ACROSS AND EAST OF THE MISSISSIPPI

VALLEY DURING THIS PERIOD. AS IT DOES...IT APPEARS LIKELY TO TAKE

ON AN INCREASING NEGATIVE TILT...AS A STRONG SHORT WAVE IMPULSE

LIFTS OUT OF ITS BASE...AND GRADUALLY PIVOTS AROUND THE EASTERN

PERIPHERY OF A DEVELOPING CYCLONIC CIRCULATION OVER THE UPPER

MISSISSIPPI VALLEY/GREAT LAKES REGION. THIS SHOULD BE ACCOMPANIED

BY STRENGTHENING CYCLONIC MID/UPPER FLOW FIELDS IN THE WARM SECTOR

OF A SURFACE CYCLONE PROGGED TO DEEPEN AS IT MIGRATES NORTHEAST OF

THE OZARK PLATEAU TOWARD THE GREAT LAKES. THIS PROBABLY WILL

INCLUDE A 90-100 KT 500 MB JET CORE NOSING ACROSS THE TENNESSEE AND

OHIO VALLEYS BY LATE AFTERNOON.

VARIABILITY EXISTS AMONG THE MODELS CONCERNING THE DETAILS OF THIS

CYCLOGENESIS...PERHAPS IN PART DUE TO THE INFLUENCE OF CONVECTION

...WHICH COULD BE EXTENSIVE AND ONGOING AT 12Z WEDNESDAY. HIGH

RESOLUTION MODEL DATA IS STRONGLY SUGGESTIVE THAT A SIGNIFICANT

CLUSTER OF STORMS COULD BE IN PROGRESS WEDNESDAY MORNING EAST OF THE

MIDDLE AND LOWER MISSISSIPPI VALLEY. IF THIS VERIFIES...IT IS

UNCERTAIN WHAT INFLUENCE IT WILL HAVE ON SUBSEQUENT BOUNDARY LAYER

DESTABILIZATION ACROSS THE OHIO/TENNESSEE VALLEY REGION. THIS

UNCERTAINTY PRECLUDES AN OUTLOOK UPGRADE TO HIGH RISK PROBABILITIES

AT THE PRESENT TIME. HOWEVER...MOST OTHER INDICATIONS ARE STRONGLY

SUGGESTIVE THAT A MAJOR OUTBREAK OF SEVERE THUNDERSTORMS COMMENCING

TONIGHT ACROSS PARTS OF THE OZARK PLATEAU/LOWER MISSISSIPPI VALLEY

WILL CONTINUE AND EXPAND NORTHWARD AND EASTWARD TOWARD THE

APPALACHIANS DURING THE DAY WEDNESDAY. THIS WILL INCLUDE THE

POTENTIAL FOR LONG-LIVED SUPERCELLS CAPABLE OF STRONG

TORNADOES...AND ORGANIZED STORM CLUSTERS CAPABLE OF STRONG DAMAGING

WIND GUSTS.

..EASTERN U.S

IN AREAS NOT IMPACTED SIGNIFICANTLY BY EARLY MORNING CONVECTION...

PRE-FRONTAL BOUNDARY LAYER MOISTURE OVER A BROAD AREA EAST OF THE

MISSISSIPPI VALLEY INTO THE APPALACHIANS...AND PERHAPS EAST OF THE

APPALACHIANS...WILL BE SUPPORTIVE OF CAPE ON THE ORDER OF 1000 TO

2000 J/KG WITH SURFACE HEATING. IN THE PRESENCE OF WIND PROFILES

CHARACTERIZED BY STRONG DEEP LAYER SHEAR...AND LARGE CLOCKWISE

CURVED LOW-LEVEL HODOGRAPHS BENEATH A BROAD 50-70 KT SOUTHERLY

LOW-LEVEL JET...A SIGNIFICANT TORNADIC SUPERCELL THREAT COULD BE

ONGOING OR RAPIDLY DEVELOP BY LATE MORNING ACROSS PARTS OF

NORTHEASTERN MISSISSIPPI/NORTHERN ALABAMA THROUGH WESTERN/MIDDLE

TENNESSEE AND WESTERN/CENTRAL KENTUCKY. HOWEVER...THIS WILL BE

LARGELY CONTINGENT ON EVOLUTION OF OVERNIGHT CONVECTION. IF AN

LARGE CLUSTER OF STORMS DEVELOPS AS SOME GUIDANCE SUGGESTS...AN

EXPANSIVE SURFACE COLD POOL COULD SLOW DESTABILIZATION AND MITIGATE

THE SEVERE POTENTIAL.

REGARDLESS...SUFFICIENT WEAKENING OF THE CONVECTION AND THE

ASSOCIATED COLD POOL IS EXPECTED TO OCCUR BY THE AFTERNOON TO

SUPPORT WEAK TO MODERATE DESTABILIZATION IN THE PRESENCE OF

STRENGTHENING LARGE-SCALE FORCING FOR UPWARD VERTICAL MOTION. AND

THIS PROBABLY WILL BE SUFFICIENT TO SUPPORT CONVECTIVE

DEVELOPMENT...INCLUDING TORNADIC SUPERCELLS...IN THE WARM SECTOR

...AND ALONG AND AHEAD OF THE COLD FRONT TRAILING SOUTHWARD FROM THE

SURFACE LOW. AS THIS FRONT ADVANCES TOWARD THE APPALACHIANS

...UPSCALE CONVECTIVE GROWTH INTO AN EXTENSIVE SQUALL LINE COULD

OCCUR BY EARLY WEDNESDAY EVENING...BEFORE SPREADING EAST OF THE

APPALACHIANS OVERNIGHT.

..KERR.. 04/26/2011

[earthlight]

Here are the graphics..

[Calderon]

So they're ready to pull the HIGH risk trigger, just destab. uncertainties from convection expected to push through here overnight. Some of the worst outbreaks for Mid. TN have had convection move through overnight into the early morning, get a break, and then BOOM.

It is going to be a long 36 hours for the Nashville area, to say the least.

[Stebo]

So they're ready to pull the HIGH risk trigger, just destab. uncertainties from convection expected to push through here overnight. Some of the worst outbreaks for Mid. TN have had convection move through overnight into the early morning, get a break, and then BOOM.

It is going to be a long 36 hours for the Nashville area, to say the least.

Yeah Tennessee is going to have a long day today and tomorrow, and its already been a busy time in the past 36 hours.

[VAwxman]

I certainly see why SPC didn't want to pull the "high" trigger yet. Models do show the storms hanging around into tomorrow, but I wonder if, as happens a lot, the storms move faster than progged and clear out. Nonetheless, on a day two outlook, I see the logic in refraining for now.

[Xenia74]

Looks like Dr. Greg Forbes is not afraid to issue a high risk area. He also happens to agree with me on the location...Looks very very bad....

[Calderon]

Yeah Tennessee is going to have a long day today and tomorrow, and its already been a busy time in the past 36 hours.

Yeah, and I got a front row seat to this.

[BrentO]

AFD from PAH ...........pretty strong language from them:

HOWEVER...TO MAKE MATTERS EVEN WORSE...A VERY SHARP NEGATIVELYTILTED SHORTWAVE WILL EJECT NE INTO MO DURING THE DAYWEDNESDAY. THIS SHOULD CAUSE A SURFACE LOW TO DEEPEN AND MOVE NERIGHT UP THE LOWER OH/MID MS RIVER VALLEYS DURING THE DAY WED. ANAREA OF STRONG TO SEVERE CONVECTION WILL LIKELY ACCOMPANY THISSYSTEM AS IT SWEEPS ACROSS THE REGION WED. THE DEGREE OFDESTABILIZATION THAT CAN BECOME ESTABLISHED BETWEEN THE EXITINGMORNING CONVECTION AND THE ADVANCING SFC LOW/COLD FRONT WEDAFTERNOON WILL BE THE BIGGEST FACTOR IN DETERMINING HOW BIG OF ATORNADO OUTBREAK OCCURS IN THE MID SOUTH WED.THE HIGHEST CHC FOR THIS OUTBREAK APPEARS TO BE ALONG AND SOUTHEASTOF THE OHIO RIVER...WHERE THE WARM SECTOR SHOULD BECOME MOSTESTABLISHED WITH 2000 TO 3000 J/KG CAPES AND SFC BASED LIFTEDINDICES FROM -5 TO -10 DEVELOPING BY EARLY AFTERNOON. IF THIS TYPEOF AIR MASS BECOMES ESTABLISHED...VERY STRONG/LONG TRACK TORNADOESWOULD BE POSSIBLE...ESP IN WRN KY ON WED. STAY TUNED.

[Xenia74]

Join the club...Another front row seat...Getting the camera ready here in Warren County TN

[CUmet]

I certainly see why SPC didn't want to pull the "high" trigger yet. Models do show the storms hanging around into tomorrow, but I wonder if, as happens a lot, the storms move faster than progged and clear out. Nonetheless, on a day two outlook, I see the logic in refraining for now.

I agree 100%. The morning convection is the one thing that can go "wrong", but like you said many times the storms do move well out ahead of the warm sector allowing destablization to occur behind it. This seems especially likely in this case due to the fast storm motions. There may even be some subsidence behind the lead shortwave that could facilitate this as well.

[DBriedis]

How does this compare to 1974? The general feeling I'm getting from reading this board and other sources on the web is that this is pretty high on the "Need to change pants" scale, but I'm no expert by any measure.

[iowahawkeyedave]

I'm also curious about how this compares to 1974. Was their morning convection in 1974 as well before the outbreak?

[Indystorm]

The Superoutbreak got its name because it is one of a kind and rarely invites comparison. However, I believe the progged low pressure on the NAM for tomorrow if it verifies is lower than the pressure of the system with the superoutbreak for what it's worth.

[Indystorm]

[CUmet]

How does this compare to 1974? The general feeling I'm getting from reading this board and other sources on the web is that this is pretty high on the "Need to change pants" scale, but I'm no expert by any measure.

From my perspective, I think it's very unlikely that this event will match the Super Outbreak, perhaps even if you count today's and tomorrow's tornadoes as part of the same outbreak. That was an incredible event on many levels, and it's extremely difficult to get amazing parameters over such a large area. With that said, there have been occasional attempts over the years at comparing some upcoming event to the Super Outbreak, and I've laughed off or scoffed at every single one. This time feels a little different.

[mempho]

From my perspective, I think it's very unlikely that this event will match the Super Outbreak, perhaps even if you count today's and tomorrow's tornadoes as part of the same outbreak. That was an incredible event on many levels, and it's extremely difficult to get amazing parameters over such a large area. With that said, there have been occasional attempts over the years at comparing some upcoming event to the Super Outbreak, and I've laughed off or scoffed at every single one. This time feels a little different.

This is a reasonable assessment and is very much in concurrence with my thoughts on it for tommorrow. It is highly unlikely yet it is eerily similar. I wouldn't exactly laugh at a 1974 comparison as my first thought when looking at guidance for Day 2 was "1974" but everyone needs to remember that nothing can be compared to 1974 until it's actually occurred. It's very rare..somewhere along the lines of predicting a hurricane season that will rival/surpass 2005. So, it's just not something we can ever expect to happen, but the similarities are there and are difficult for almost any wx aficionado to ignore.

[Xenia74]

This is a reasonable assessment and is very much in concurrence with my thoughts on it for tommorrow. It is highly unlikely yet it is eerily similar. I wouldn't exactly laugh at a 1974 comparison as my first thought when looking at guidance for Day 2 was "1974" but everyone needs to remember that nothing can be compared to 1974 until it's actually occurred. It's very rare..somewhere along the lines of predicting a hurricane season that will rival/surpass 2005. So, it's just not something we can ever expect to happen, but the similarities are there and are difficult for almost any wx aficionado to ignore.

Totally correct..Can't be compared yet but with these numbers this event could be the new standard of comparison....