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Historic Tornado Outbreak April 27, 2011


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I don't think the RUC will be right.

I don't either... but I'm not quite sure the SPC WRF is right with the morning convection being as widespread and organized as it shows. Everything in the activity north of the Louisiana supercell cluster is weakening... and this is going to make it harder and harder to get a large MCS with an organized cold pool... especially as capping increases ahead of these storms overnight. This will also make it easier for them to gain latitude as the mid/upper winds become more SW to NE as the trough deepens overnight and into the morning. I think there will be convective remnants... but I think the SPC WRF is too organized with it... and too slow (when looking at earlier panels, it's a few to several hours behind reality... and this is a very common bias in the model... with non-discrete convection). I think a halfway medium between the RUC and SPC WRF is probably the right way to go.

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A few honest points, as I do respect your opinion quite a bit.

I don't think you can say that yesterday and today underperformed (unless you were expecting a lot more). Yesterday ended up being a pretty decent outbreak with quite a few tornado reports, and today as well. The weakness of today's event (the weaker low-level wind field) was highlighted indirectly earlier, when it was stated that the timing of the trough was a bit slow, resulting in a weaker low-level response. Regardless, I am not a fan at all of the philosophy "the first day ended up underperforming, therefore the second day is more likely to underperform as well". Personally, I tend to treat every day as an independent event, because that's what they really are for the most part w.r.t severe weather. New geographic area, new instability/shear parameters, new storm mode questions, new morning precip issues, etc.

I don't think we'll have a problem of too many storms interfering with each other tomorrow. Unlike today, there is a clear focus for convective initiation, and given that the deep-layer shear vectors are perpendicular to the boundary (think April 15-16), it will ensure a sustained discrete supercell mode. There are questions regarding the extent of morning convection and its impact on the warm sector instability, but even the latest NAM run as Tony said is destablizing things very quickly following the passage of the first cluster. If there isn't a major tornado outbreak tomorrow, the morning convection is most likely going to be responsible.

Given the extreme rarity of seeing a strong mid-level 100 kt jet streak punching through a moist, unstable warm sector, with cyclogenesis ramping up the low-level wind fields and skyrocketing the helicities...the strong language used by me and many other mets regarding this event has been and is 100% justified. I try my best not to hype things up, and I'm brutally honest when I say I don't like a particular setup for reasons x,y,z. On the other hand, I know potential when I see it, and I have to be honest when I say that this might be the best outbreak setup I've seen in the dozen or so years I've been following severe weather. Whether it actually verifies that way or not, we'll just have to see...

Good points. I am by no means saying tomorrow will not be a big outbreak - there is definitely a high likelihood that it will be. All I am saying that with all the small mesoscale things that have gone "wrong" these past few days, it might pay to be cautious, especially since everyone is on the same bandwagon right now. Yeah day 1/2 doesn't affect day 3, but it is a good reminder of what can possibly go "right"/"wrong", and it is true that forcing has been stronger and initiation earlier than predicted on each of the days. Since tomorrow's event is associated with the main energy, it is logical to believe that forcing will be even stronger, and so we can make very crude comparisons with these past couple of days.

And I am somewhat concerned about the high temperatures across the SE tomorrow... LCLs will not be very favorable if temperatures rise above MOS guidance. And we all have to consider what if destabilization doesn't occur in time if forcing maintains numerous convection over the relatively uncapped areas of the TN Valley.

Anyway, just offering a second opinion here, since I've been hearing "big outbreak" across the boards from almost everyone in the know. Certainly not ignoring that, but as always convection is convection... and I always start with the philosophy of finding things that could go wrong. I usu don't post it when everyone has a shared consensus (for obvious reasons, as seen in earlier posts in this thread), but psychologically I do feel more bearish than usual on a forecast if the latest High risk just underperformed.

And you are one of the best so of course I know you are not trying to hype things up. It is justified for sure. However with a setup that looks extremely good on paper, with numerous forecasts out there talking about an outbreak... it won't take much "letdown" for forecasts to be seen as overblown... .thus reason why I am bearish.

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I don't either... but I'm not quite sure the SPC WRF is right with the morning convection being as widespread and organized as it shows. Everything in the activity north of the Louisiana supercell cluster is weakening... and this is going to make it harder and harder to get a large MCS with an organized cold pool... especially as capping increases ahead of these storms overnight. This will also make it easier for them to gain latitude as the mid/upper winds become more SW to NE as the trough deepens overnight and into the morning. I think there will be convective remnants... but I think the SPC WRF is too organized with it... and too slow (when looking at earlier panels, it's a few to several hours behind reality... and this is a very common bias in the model... with non-discrete convection). I think a halfway medium between the RUC and SPC WRF is probably the right way to go.

Completely agree that the middle road is probably the best option given each of the model biases. The latest HRRR run that goes out to 17z probably represents my own thoughts pretty well.

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Good points. I am by no means saying tomorrow will not be a big outbreak - there is definitely a high likelihood that it will be. All I am saying that with all the small mesoscale things that have gone "wrong" these past few days, it might pay to be cautious, especially since everyone is on the same bandwagon right now. Yeah day 1/2 doesn't affect day 3, but it is a good reminder of what can possibly go "right"/"wrong", and it is true that forcing has been stronger and initiation earlier than predicted on each of the days. Since tomorrow's event is associated with the main energy, it is logical to believe that forcing will be even stronger, and so we can make very crude comparisons with these past couple of days.

And I am somewhat concerned about the high temperatures across the SE tomorrow... LCLs will not be very favorable if temperatures rise above MOS guidance. And we all have to consider what if destabilization doesn't occur in time if forcing maintains numerous convection over the relatively uncapped areas of the TN Valley.

Anyway, just offering a second opinion here, since I've been hearing "big outbreak" across the boards from almost everyone in the know. Certainly not ignoring that, but as always convection is convection... and I always start with the philosophy of finding things that could go wrong. I usu don't post it when everyone has a shared consensus (for obvious reasons, as seen in earlier posts in this thread), but psychologically I do feel more bearish than usual on a forecast if the latest High risk just underperformed.

And you are one of the best so of course I know you are not trying to hype things up. It is justified for sure. However with a setup that looks extremely good on paper, with numerous forecasts out there talking about an outbreak... it won't take much "letdown" for forecasts to be seen as overblown... .thus reason why I am bearish.

I'm usually one of the first to pick on you when we have a higher end threat, and I know you've seen some very ominous comments from me on other forums today.... but I understand and really respect this more conservative approach... especially when we do have mesoscale questions that need to be answered. However, the one point I want to make about the early initiation.... is that both of these cases so far... have been along west to east oriented boundaries... and it's extremely easy to get initiation in these cases, as compared to along a more north to south oriented boundary... because, most of the time, you have a better chance at EML advection overtop the warm sector in the more north to south scenario. It's an understandable concern, but it may not be quite as comparable. At any rate... especially with convection just upstream... I definitely understand the conservative approach... but with several ongoing things that are leaning away from the convection developing into this big cold-pool driven B.S.... there are some times, as we've seen before, when you just don't need to ignore the synoptic environment that is ultimately driving the mesoscale. I'm sure most people thought, by 4 pm, last April 24th... that the Alabama portion of the HIGH would be a bust... but we had an EF4 and multiple EF3s that night.

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I would think the southern area (TN, AL, MS) is pretty much a High Risk slamdunk.

No I would agree, I was just stating if there was anything that would cause them not to go High, it would be that. I personally would go high for S. KY south into MS/AL.

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Hey thanks for the support Fred... I know you are definitely doing your best to inform the people on the other forums... and that of course requires a different approach than informally chatting about this.

You do bring up a good point that initiation is easier to do on a W-E boundary. Not only do mixed layers propagate W-E and thus can overspread a N-S boundary, LLJ trajectories often lie more orthogonal to E-W boundaries which maximizes forcing there. But I'm still a little cautious. Nevertheless, I believe it forcing was only a small part of the equation these past few days, as the weak low-lvl flow today and higher LCL heights yesterday were bigger factors. However, it would've been better for the storms to have more separation in either case... especially since a less contaminated warm sector would've supported damaging tornadoes as the low-lvl flow strengthened and LCL heights lowered in the late evening.

My point overall was trying to illustrate that forcing could potentially go wrong given the strong dynamics of this system and the weak capping north of the AL-TN state line.

And yes, I'm a big proponent of the synoptic-driving mesoscale approach, which is why tomorrow should not be a total bust, and that at the very least we ought to see a MDT-sized outbreak. I'm just bearish on this being a very large, damaging outbreak, but of course I'm not saying it won't happen either.

(And yes, I really thought AL on 4/24 was a bust by late afternoon, very surprised when I logged on that night and saw that a last round of sups initiated and dropped those damaging tornadoes.)

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Good discussion thewxmann, CUmet, Fred, tornadotony, etc. Going to be an interesting forecast for tomorrow no doubt. I don't think large convective cold pools will be a large worry by the afternoon in this flow regime and the strength of the low level theta-e inflow. The bigger questions, imo, seem to be on the synoptic scale flow and the way the models handle the upper pv anomaly and how it ejects north tomorrow afternoon and evening and the associated upper level jet configuration and associated mesoscale circulations/low level mass response (and subsequent hodograph curvature). I wouldn't be surprised if SPC holds on a high until tomorrow when some of the synoptic and subsequent mesoscale forcing details are better known. That said--seems that a high risk will be highly possible over portions of TN and northern AL at some juncture.

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BULLETIN - EAS ACTIVATION REQUESTED

TORNADO WARNING

NATIONAL WEATHER SERVICE JACKSON MS

1253 AM CDT WED APR 27 2011

THE NATIONAL WEATHER SERVICE IN JACKSON HAS ISSUED A

* TORNADO WARNING FOR...

HOLMES COUNTY IN CENTRAL MISSISSIPPI...

THIS INCLUDES THE CITY OF THORNTON...

SOUTHERN HUMPHREYS COUNTY IN CENTRAL MISSISSIPPI...

NORTHERN YAZOO COUNTY IN CENTRAL MISSISSIPPI...

THIS INCLUDES THE CITY OF YAZOO CITY...

EASTERN SHARKEY COUNTY IN WEST CENTRAL MISSISSIPPI...

* UNTIL 145 AM CDT

* AT 1252 AM CDT...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED A

SEVERE THUNDERSTORM CAPABLE OF PRODUCING A TORNADO 6 MILES

NORTHWEST OF HOLLY BLUFF MOVING EAST AT 50 MPH.

* OTHER LOCATIONS IN THE WARNING INCLUDE BUT ARE NOT LIMITED TO

MIDNIGHT...BENTON...MIDWAY...COXBURG...TOLARVILLE...TCHULA...

HOWARD...EBENEZER...BROZVILLE...LEXINGTON AND FRANKLIN

THIS STORM HAS ALREADY PRODUCED DAMAGE ACROSS PORTIONS OF WESTERN

MISSISSIPPI AND NORTHEASTERN LOUISIANA!

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DAY 1 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

0100 AM CDT WED APR 27 2011

VALID 271200Z - 281200Z

...THERE IS A HIGH RISK OF SVR TSTMS OVER PORTIONS OF NRN

MS...AL...FAR NWRN GA AND SRN MIDDLE TN...

...THERE IS A MDT RISK OF SVR TSTMS OVER MUCH OF CNTRL AND NRN MS

AND AL...NWRN GA...MUCH OF TN AND KY...WRN CAROLINAS...

...THERE IS A SLGT RISK OF SVR TSTMS FROM THE LOWER MS VALLEY/GULF

COAST STATES NWD TO THE CANADIAN BORDER...

..SYNOPSIS

AN UPPER TROUGH AXIS WILL BE CENTERED ACROSS THE SRN PLAINS AT 12Z

WED...WITH A 100 KT MID LEVEL JET ACROSS N TX. THIS FEATURE WILL

STRENGTHEN THROUGH 00Z AS IT EJECTS NEWD ACROSS THE LOWER MS AND

INTO THE TN/OH VALLEYS DURING THE DAY. AT THE SURFACE...LOW PRESSURE

IS FORECAST TO BE OVER W CNTRL AR AT 12Z...WITH A DEVELOPING COLD

FRONT TRAILING SWWD INTO ERN TX. THESE FEATURES WILL MOVE RAPIDLY

ENEWD...WITH THE SURFACE LOW NEARING SRN INDIANA BY 00Z AND A SHARP

LOW PRESSURE TROUGH EXTENDING SWD INTO CNTRL KY...MIDDLE TN...ERN MS

AND WRN AL.

AHEAD OF THE FRONT...A VERY WARM AND MOIST AIR MASS WILL ALREADY BE

IN PLACE BY MIDDAY WITH 68-72 BOUNDARY LAYER DEWPOINTS LIKELY FROM

THE GULF COAST INTO MS/AL...AND MID TO UPPER 60S F OVER TN AND SRN

KY WHERE SUBSTANTIAL MORNING CONVECTION WILL LIKELY BE ONGOING. WITH

RELATIVELY COOL TEMPERATURE PROFILES ALOFT...AN EXTREMELY UNSTABLE

ATMOSPHERE WILL RESULT ACROSS THE WARM SECTOR. WIND FIELDS

THROUGHOUT THE TROPOSPHERE WILL STRENGTHEN WITH THE APPROACHING

TROUGH/JET MAX...AND THE COMBINATION OF THE TWO IS EXPECTED TO

RESULT IN A TORNADO OUTBREAK...WITH LONG-TRACKED STRONG TO VIOLENT

TORNADOES POSSIBLE OVER THE HIGH RISK AREA.

TO THE E...THE BROAD SLY FLOW ACROSS THE WARM SECTOR WILL EXTEND

INTO THE MID ATLANTIC REGION...WITH A SECONDARY MOIST PLUME OFF THE

ATLANTIC NWD INTO THE CAROLINAS AND WRN VA BY AFTERNOON.

--- AN OUTBREAK OF TORNADOES AND DAMAGING WINDS IS EXPECTED TODAY

THROUGH EVENING OVER PORTIONS OF NRN MS...NRN AL...TN AND KY. ---

..LOWER MS VALLEY INTO TN/OH VALLEYS

A COMPLICATED FORECAST SCENARIO WILL UNFOLD TODAY...BEGINNING RATHER

EARLY IN THE PERIOD OVER SRN AND ERN AR AND NRN LA. MODELS ARE IN

RELATIVELY GOOD AGREEMENT WITH THE PLACEMENT OF THE SURFACE LOW OVER

AR BETWEEN 12-15Z...THEN DEVELOPING IT NEWD ACROSS TN/KY BY 00Z.

NUMEROUS THUNDERSTORMS WILL BE ONGOING...AND SOME WILL BE

SEVERE...FROM NRN LA/SRN AR EWD ACROSS NRN MS...WRN AND MIDDLE TN

AND KY. DESPITE TIME OF DAY...PRESENCE OF RICH LOW LEVEL MOISTURE

WILL RESULT IN LITTLE CAPPING AND SURFACE BASED EFFECTIVE PARCELS.

THUS...A TORNADO AND DAMAGING WIND THREAT WILL BE PRESENT WITH THIS

ACTIVITY.

SOUTH OF THE ONGOING ACTIVITY...STRONG DAYTIME HEATING WILL OCCUR.

WITH BOUNDARY LAYER DEWPOINTS IN THE LOW 70S F...THIS WILL CREATE

STRONG TO EXTREME INSTABILITY OVER MS AND AL NWD TO THE ONGOING

CONVECTION. A CAPPING INVERSION WILL LIKELY KEEP MUCH OF THE WARM

SECTOR S OF THE EARLY STORMS CAPPED...ALLOWING A BUILD UP OF

INSTABILITY TO BE RELEASED WHEN LARGE SCALE FORCING ARRIVES DURING

THE AFTERNOON ALONG THE COLD FRONT AND ESPECIALLY NEAR THE

FRONT/OUTFLOW BOUNDARY INTERSECTION. THE RESULT WILL BE FAST MOVING

TORNADIC SUPERCELLS CAPABLE OF STRONG TO VIOLENT TORNADOES. GIVEN

THE FAST STORM MOTIONS AND RELATIVELY WIDE WARM SECTOR...SOME

TORNADOES COULD BE LONG LIVED. THE GREATEST THREAT FOR THESE TYPES

OF STORMS WILL BE OVER THE HIGH RISK AREA...OVER NRN MS AND

AL...INTO EXTREME NWRN GA AND SRN MIDDLE TN. THIS ACTIVITY WILL

PERSIST TO THE E...BUT CELLS MAY EVENTUALLY MERGE INTO MORE OF A

QLCS STRUCTURE CAPABLE OF PARTICULARLY DAMAGING WINDS AND A FEW

TORNADOES.

..WRN NC/VA

A CONDITIONAL SEVERE THREAT WILL EXIST BY LATE AFTERNOON OVER WRN

NC/VA. STRONG HEATING WILL RESULT IN STEEP LOW LEVEL LAPSE

RATES...BUT FORECAST SOUNDINGS SHOW A CAPPING INVERSION AROUND 700

MB...WHICH SHOULD KEEP THE AIR MASS CAPPED. HOWEVER...MOISTURE

ADVECTION...AS WELL AS SELY UPSLOPE FLOW MAY RESULT IN ISOLATED

THUNDERSTORMS DURING THE AFTERNOON OVER THE HIGHER TERRAIN. WIND

PROFILES WOULD FAVOR SUPERCELLS...CAPABLE OF PRODUCING TORNADOES

HAIL AND WIND. WILL MAINTAIN LOW PROBABILIITES FOR THE DAYTIME

CONVECTION.

..JEWELL/ROGERS.. 04/27/2011

post-147-0-70684600-1303884114.gif

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I was pretty sure when I woke up I'd see a few posts questioning today just because Tuesday was perceived as an underperformer, though you guys wound up making it into a pretty good discussion. In my view, Monday did not underperform. Tuesday may have been less than expected, but the best parameters were never supposed to come together until late in the evening, which isn't the most favorable time of day, and by then enough stuff was already out there that you got more of a "mess". Today has always, to me, looked the most favorable, if we get the current convection out of the way, so we'll see what happens. Hopefully for all in its path it will underperform, but no one should let their guard down just because of yesterday (to be fair I don't think anyone here said that though).

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Strong couplet heading right for downtown Birmingham.

BULLETIN - EAS ACTIVATION REQUESTED

TORNADO WARNING

NATIONAL WEATHER SERVICE BIRMINGHAM AL

529 AM CDT WED APR 27 2011

THE NATIONAL WEATHER SERVICE IN BIRMINGHAM HAS ISSUED A

* TORNADO WARNING FOR...

NORTH CENTRAL BIBB COUNTY IN CENTRAL ALABAMA...

SOUTHERN JEFFERSON COUNTY IN CENTRAL ALABAMA...

CENTRAL SHELBY COUNTY IN CENTRAL ALABAMA...

EAST CENTRAL TUSCALOOSA COUNTY IN WEST CENTRAL ALABAMA...

* UNTIL 600 AM CDT

* AT 529 AM CDT...THE NATIONAL WEATHER SERVICE INDICATED A SEVERE

THUNDERSTORM CAPABLE OF PRODUCING A TORNADO. THIS DANGEROUS STORM

WAS LOCATED NEAR WOODSTOCK JUNCTION...OR 17 MILES SOUTHWEST OF

BESSEMER...AND MOVING NORTHEAST AT 70 MPH.

* THIS DANGEROUS STORM WILL BE NEAR...

BESSEMER AND ALABAMA ADVENTURE AROUND 540 AM CDT.

RIVERCHASE GALLERIA...HOOVER...REGIONS PARK OR HOOVER MET AND

RIVERCHASE AROUND 545 AM CDT.

INDIAN SPRINGS VILLAGE...VESTAVIA HILLS...OAK MOUNTAIN STATE PARK

AND SAMFORD UNIVERSITY AROUND 550 AM CDT.

OTHER LOCATIONS IMPACTED BY THIS DANGEROUS STORM INCLUDE MCCALLA...

MCADORY...BESSEMER AIRPORT...JOE TUCKER PARK...SHANNON AND GOAT

ISLAND AT LAKE PURDY.

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The 08z HRRR mauls much of the northern 2/3rds of the state of AL and south-central/eastern TN after 20z this afternoon. It gives ample time for the airmass to re-destabilize behind this morning's action. Here's what it's showing for CR for 21z. We'll see how it does.

post-13-0-48649100-1303901039.png

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BULLETIN - EAS ACTIVATION REQUESTED

TORNADO WARNING

NATIONAL WEATHER SERVICE HUNTSVILLE AL

611 AM CDT WED APR 27 2011

THE NATIONAL WEATHER SERVICE IN HUNTSVILLE HAS ISSUED A

* TORNADO WARNING FOR...

NORTHERN FRANKLIN COUNTY IN TENNESSEE...

* UNTIL 645 AM CDT

* AT 609 AM CDT...NATIONAL WEATHER SERVICE DOPPLER RADAR WAS TRACKING

NEAR LOIS...OR JUST SOUTHWEST OF LYNCHBURG...MOVING NORTHEAST AT

55 MPH. WIDESPREAD AND SIGNIFICANT DAMAGE HAS BEEN REPORTED WITH

THIS STORM. THIS IS A LIFE THREATENING SITUATION.

* LOCATIONS NEAR THE PATH OF THIS TORNADO INCLUDE...

OAK GROVE.

WOODS RESERVOIR.

ALTO.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

A TORNADO HAS BEEN CONFIRMED. TAKE COVER IMMEDIATELY! DO NOT RUN

OUTSIDE TO FIND THE TORNADO...THIS IS A DANGEROUS AND POTENTIALLY

LIFE THREATENING SITUATION!

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