April 26th-30th, 2014 Major Tornado Outbreak

[andyhb]

It is likely that this thread will end up being moved given the potential areal coverage of this event, but for now we have a D5 and D6 outlook (and large ones at that) highlighted for this coming weekend, with the large upper trough that has been discussed to healthy extent in the mid range discussion. It appears one or more of 4/26-4/28 could end up as a potentially significant event, given high instability, rich moisture return and the magnitude of the incoming trough. We've got a busy week ahead.
 

DAY 4-8 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0345 AM CDT TUE APR 22 2014

VALID 251200Z - 301200Z

...DISCUSSION...
...SIGNIFICANT MULTI-DAY SEVERE EVENT EXPECTED ACROSS THE SOUTH
CENTRAL U.S. SATURDAY AND SUNDAY...STRONG TORNADOES ARE POSSIBLE...

22/00Z ECMWF IS EXHIBITING REASONABLE RUN-TO-RUN CONSISTENCY OVER
THE LAST FEW DAYS WITH THE GFS NOW TRENDING TOWARD ECMWF SOLUTIONS.
THIS FORECAST IS HEAVILY WEIGHTED TOWARD THE ECMWF.

EARLIER CONCERNS REGARDING A POLAR FRONT UNDERCUTTING STRONG SWLY
FLOW ALOFT OVER THE SRN PLAINS ARE LESSENING. WHILE UPPER BLOCK
OVER CNTRL CANADA WILL ULTIMATELY FORCE A DOMINANT SFC ANTICYCLONE
INTO THE MIDDLE OF THE CONUS...IT APPEARS THE LEADING EDGE OF THIS
AIR MASS SHOULD STRUGGLE TO ADVANCE MUCH SOUTH OF I-70 OVER KS/MO
THIS WEEKEND.

STRONG MID-LEVEL SPEED MAX IS FORECAST TO DIG SEWD INTO THE LOWER CO
RIVER VALLEY SATURDAY WITH UPPER DIFLUENCE EXPECTED ACROSS THE
PLAINS AS FAR SOUTH AS WEST TX AT 27/00Z. THIS EVOLUTION WILL ALLOW
SUBSTANTIAL MOISTURE TO ADVANCE POLEWARD ACROSS THE SRN PLAINS WHERE
PW VALUES SHOULD EASILY APPROACH 1.25 INCHES AS SFC DEW POINTS CLIMB
THROUGH THE MID-UPPER 60S ACROSS A BROAD WARM SECTOR. LATEST
THINKING IS MODERATE-EXTREME INSTABILITY SHOULD DEVELOP ALONG ERN
PLUME OF STEEP LAPSE RATES FROM WCNTRL TX...NWD INTO SCNTRL KS
SATURDAY WHERE MUCAPE COULD APPROACH 4000 J/KG. IF THERMODYNAMIC
ENVIRONMENT EVOLVES AS LATEST GUIDANCE SUGGESTS THEN POTENTIALLY
SIGNIFICANT SUPERCELL DEVELOPMENT WITH VERY LARGE HAIL AND TORNADOES
CAN BE EXPECTED. CONVECTION SHOULD INITIATE ALONG DRYLINE FROM SWRN
TX...NWD INTO WRN KS SATURDAY THEN REDEVELOP AGAIN SUNDAY FROM SERN
KS...SWD INTO CNTRL TX AS MID-LEVEL SPEED MAX EJECTS INTO THE SRN
PLAINS. SEVERE TSTMS SHOULD SPREAD INTO THE MS VALLEY BY EARLY
MONDAY. ORGANIZED SEVERE MAY CONTINUE INTO THE DAY7 PERIOD BUT
SUBSTANTIAL CONVECTION DURING THE DAY5-6 TIME FRAME COULD
POTENTIALLY DISRUPT THERMODYNAMIC ENVIRONMENT...THUS CONFIDENCE IS
NOT AS HIGH.

..DARROW.. 04/22/2014

[LocoAko]

As an aside, the Norman Music Festival (50,000-60,000 people), the OKC Festival of the Arts (~25,000 visitors), and the OKC Memorial Marathon (~25,000 runners) are all taking place this weekend. Bad weekend in the outdoor-events department for possible severe weather in the metro. 

[Radtechwxman]

I cant remember the last time I seen strong tornadoes mentioned 5-6 days out in an SPC outlook. That is a big deal. Looks like this thing is just getting in the range of nam now to. I am going to quote a post that chaser Gene Moore said on FB this morning:

"This weekend: The so called "weekend system" is now within the grasp of the NAM. Moisture return remains one of the big issues. What I'm seeing of consequence is the very weak high pressure following the Wed short wave. It appears it will not be a problem to impede return moisture flow. Here are the late Friday predictions for moisture showing a 60 dew point not far south of the Red. Associated CAPE in Texas is correspondingly high. Meanwhile we see the deep system hitting the west coast with 90Kts at 500Mb. Perhaps a bit slow for a daylight show Saturday, but it is in unimpeded zonal flow.

[Quincy]

The CIPS analogs hit Saturday and especially Sunday very hard, including significant tornado potential.

[jojo762]

The CIPS analogs hit Saturday and especially Sunday very hard, including significant tornado potential.

what dates/ analogues match?

[1900hurricane]

what dates/ analogues match?

 

Here are the links to the analog pages:

 

Saturday: http://www.eas.slu.edu/CIPS/ANALOG/stats.php?reg=SP&model=GFS212&fhr=F120&case=2014042200_SP_120

Sunday: http://www.eas.slu.edu/CIPS/ANALOG/stats.php?reg=SP&model=GFS212&fhr=F144&case=2014042200_SP_144

 

For Saturday, the big one that stands out to me is 4/14/12 and the big one for Sunday is 3/12/06. Lots of smaller events/busts in there too, but it definitely looks notable.

 

EDIT: Oh, 5/24/11 is a Sunday analog too. Missed that one the first time around!

2nd EDIT: Wow, there are a ton of impressive analogs for both days.

[tornadotony]

what dates/ analogues match?

Among those I saw for either day: 5/3/99, 5/4/99, 3/12/06, 4/14/12, 4/26/84, 4/25/94, 5/24/11, 4/28/02, 5/1/08, 5/27/95, 5/4/03, 4/26/91.

[1900hurricane]

Good list above. As I kept clicking through the days, I kept finding more and more impressive days that I missed on my first scan.

[andyhb]

I cant remember the last time I seen strong tornadoes mentioned 5-6 days out in an SPC outlook. That is a big deal. Looks like this thing is just getting in the range of nam now to. I am going to quote a post that chaser Gene Moore said on FB this morning

 

If I was targeting this upcoming weekend, I'd be pretty excited to have someone like him saying this, considering he is arguably the most experienced chaser on the planet.

[Thundersnow12]

Holy H at the 12z GFS for Saturday..

[andyhb]

Not only that, but 4/26/91 is a real good analog for Sunday with this run, the troughs looking nearly identical.

[LocoAko]

Holy H at the 12z GFS for Saturday..

 

Looks like it actually has dewpoints exceed 70F in NW OK by 21Z on Saturday -- does that look plausible? I'm vaguely suspicious based on nothing other than past events and climo, but that's not to say it can't (or hasn't) happened. 

[andyhb]

Nice EHIs (using SBCAPE) of 15+ with low LCL heights in that sweet spot with a backed 50 kt LLJ in south central KS at 00z Sat evening. Seriously if that doesn't cap over fast, boom goes the dynamite.

[Ground Scouring]

Holy H at the 12z GFS for Saturday...

Ironically, even the date 04/26 matches the analog. At this point, given the strong model agreement, the synoptic environment will certainly be generally favorable for a potentially significant event, so the biggest mesoscale questions will be related to timing. The 12Z GFS ad verbatim shows some early convection on Saturday that could also leave some outflow boundaries in time for peak diurnal heating, another factor present on some of the analogs like 04/26/1991. Of course such fine details will need to be watched as we get closer to Saturday.

[brettjrob]

Looks like it actually has dewpoints exceed 70F in NW OK by 21Z on Saturday -- does that look plausible? I'm vaguely suspicious based on nothing other than past events and climo, but that's not to say it can't (or hasn't) happened. 

 

I agree -- it's very unlikely we see that verified in the sfc obs Saturday afternoon. I don't know that I've ever seen 70s dew points that far W in April, actually. Regardless, if mid-upper 60s verify, that should be more than sufficient for one of the most impressive setups that region has seen in several years. The strong, backed 850 mb flow (as modeled by the GFS) should help some, though I'll never rest easy in this drought until I'm seeing the late-afternoon obs with my own eyes.

 

Overall, Saturday has the *potential* to be the best synoptically-obvious southern Plains chase day in years, in my opinion. That's not to say that, even in the best-case scenario, it will be the biggest outbreak in years (it almost certainly won't), or even that it will produce the "best" storm in years, i.e. topping the cyclic Woodward-Wichita storm on 4/14/12. But it's the type of classic, prototypical high-end High Plains setup we just haven't seen at all for several years (maybe even since 2007?), much to my dismay. Great terrain well away from the I-35 corridor; not overly dynamic; manageable storm motions; dryline will not blast eastward, even during the evening hours; exceptionally nice directional shear. The potential for jaw-dropping storms over relatively open country looks about as high as it can at this range on the GFS.

 

It's a nice fantasy, at least. Timing is still a major issue, as only the GFS is fast enough for this "dream scenario," so far. Interestingly, it did speed up even more from last night to this morning's run, though. If the Euro/CMC timing is accurate, Saturday could still be noteworthy, but probably not a multi-storm bonanza.

 

The trend for Sunday is pretty nasty on the new GFS for the I-35 corridor. It's remarkable how many high-end setups we've seen in that stretch from OKC to SLN just over the past four years; hard to imagine adding another, but those analogs listed above don't lie. Reads almost like an I-35 Greatest Hits list. Sunday is definitely the surer bet to end up as MDT risk (at minimum) and produce several or numerous dangerous storms.

[Ground Scouring]

Sunday *might* have a bigger problem with low-level veering along the dryline as the cold front picks up speed and the surface cyclone reforms over ern MT / wrn SD, at least during the day/before late afternoon, but that's just a "maybe." Sunday would have fewer problems with mixing due to the mid-level cold intrusion bolstering lapse rates, I think--correct me if I'm wrong. The GFS's timing would present a problem for the OKC metro especially on Sunday. Monday also looks potentially significant for Dixie Alley (lower MS Valley--nern LA, nrn/ctrl MS, swrn TN, ern AR, nrn AL) should the secondary low verify as on the ECMWF.

[andyhb]

Sunday *might* have a bigger problem with low-level veering along the dryline as the cold front picks up speed and the surface cyclone reforms over ern MT / wrn SD, at least during the day/before late afternoon, but that's just a "maybe."

 

I think it's likely that at least one of the states involved will have very favorable wind profiles for sustained supercells on Sunday, at least with how it sits currently. It might not be ideal along the entire dryline, but you don't need it to be for a dangerous event.

[tornadotony]

Sunday *might* have a bigger problem with low-level veering along the dryline as the cold front picks up speed and the surface cyclone reforms over ern MT / wrn SD, at least during the day/before late afternoon, but that's just a "maybe." Sunday would have fewer problems with mixing due to the mid-level cold intrusion bolstering lapse rates, I think--correct me if I'm wrong. The GFS's timing would present a problem for the OKC metro especially on Sunday. Monday also looks potentially significant for Dixie Alley (lower MS Valley--nern LA, nrn/ctrl MS, swrn TN, ern AR, nrn AL) should the secondary low verify as on the ECMWF.

Nope, nope, nope.  Let's try this again.

 

1) The sfc low isn't going to "reform" across ern MT/wrn SD.  The GFS is showing a new sfc low developing across SW KS.  Instead of causing winds to veer, that would lead to an increased likelihood of backing of low-level flow through isallobaric processes.

 

2) Steep lapse rates don't inhibit mixing.  They promote mixing, especially if the low-level lapse rates are steep.  Each day, as the sun heats the Earth diabatically, the Earth re-radiates that energy upward, causing the air near the sfc to warm.  That differential warming near the sfc leads to a steepening of low-level lapse rates, which in turn leads to an increase in turbulent eddy growth and subsequent growth of the boundary layer.  As the turbulence grows, mixing grows, until the boundary layer becomes well-mixed.

 

3) There's going to be so much low-level moisture advected N by Sunday that part (2) is moot with regards to moisture mixing out.  You'll be able to have a well-formed boundary layer, and it will still have more that sufficient moisture to promote low LCLs.

[soonertrey]

I just checked out the forecast sounding on the 12z GFS run for Pratt, KS for Sat evening.  Pretty impressive.

[CUmet]

I agree -- it's very unlikely we see that verified in the sfc obs Saturday afternoon. I don't know that I've ever seen 70s dew points that far W in April, actually. Regardless, if mid-upper 60s verify, that should be more than sufficient for one of the most impressive setups that region has seen in several years. The strong, backed 850 mb flow (as modeled by the GFS) should help some, though I'll never rest easy in this drought until I'm seeing the late-afternoon obs with my own eyes.

 

Overall, Saturday has the *potential* to be the best synoptically-obvious southern Plains chase day in years, in my opinion. That's not to say that, even in the best-case scenario, it will be the biggest outbreak in years (it almost certainly won't), or even that it will produce the "best" storm in years, i.e. topping the cyclic Woodward-Wichita storm on 4/14/12. But it's the type of classic, prototypical high-end High Plains setup we just haven't seen at all for several years (maybe even since 2007?), much to my dismay. Great terrain well away from the I-35 corridor; not overly dynamic; manageable storm motions; dryline will not blast eastward, even during the evening hours; exceptionally nice directional shear. The potential for jaw-dropping storms over relatively open country looks about as high as it can at this range on the GFS.

 

It's a nice fantasy, at least. Timing is still a major issue, as only the GFS is fast enough for this "dream scenario," so far. Interestingly, it did speed up even more from last night to this morning's run, though. If the Euro/CMC timing is accurate, Saturday could still be noteworthy, but probably not a multi-storm bonanza.

 

The trend for Sunday is pretty nasty on the new GFS for the I-35 corridor. It's remarkable how many high-end setups we've seen in that stretch from OKC to SLN just over the past four years; hard to imagine adding another, but those analogs listed above don't lie. Reads almost like an I-35 Greatest Hits list. Sunday is definitely the surer bet to end up as MDT risk (at minimum) and produce several or numerous dangerous storms.

 

Nice post.  Saturday has a ton of potential, but I'm somewhat hesitant at this point myself given that the GFS is the fastest out of all the models.  Timing's going to be a big factor.

Sunday will depend on whether there's a secondary vort max that rotates around the base of the broad trough, which would induce a secondary surface low to form farther south.  This would back the surface winds and enhance the low-level jet, resulting in increased the low-level shear.  The models (except for the latest 12z GFS) have generally been pretty shy about this scenario, although I wouldn't expect the details within the broad trough to be worked out yet  Still a lot of uncertainties regarding Sunday, but I would definitely watch if this scenario starts showing up in the models as we get closer to the event.

[andyhb]

Hmm, 12z Euro drops a s/w through the OV at 72 hrs, with LL flow becoming temporarily disrupted.

[blizzardof96]

I just checked out the forecast sounding on the 12z GFS run for Pratt, KS for Sat evening.  Pretty impressive.

Here's a nice sounding valid 0z Sun from South-central KS. Very impressive run to say the least. Despite the elevated instability and sfc cap, 500-850mb wind vectors are nearly 80 degrees apart in some parts of SE KS and NC OK. Upper level support is also their with a 80-100kt 250mb jet nosing into the warm sector with a backed LLJ across KS.

[Thundersnow12]

Highest values I can remember since 5/19/13

 

[andyhb]

Euro closes off at H5 already at 96.

[andyhb]

Holy crap at 120, what a bomb...

[tornadotony]

Euro closes off at H5 already at 96.

It's closed but just as progressive as past runs.

 

Holy cow at 120h.

[Chicago Storm]

Sig changes on the 12z ECMWF.

[andyhb]

981 mb low in western NE...there's a new one.

[brettjrob]

Euro closes off at H5 already at 96.

 

980 mb sfc low at H120. Ski jump negative-tilt trough. Wow. Just wow. What an incredibly textbook synoptic setup for a mid-spring severe weather outbreak, details aside.

 

My personal benchmark for awesome chase days is an H5 low center sitting over the monument at the Four Corners at 00z. Interpolating between 96/120 suggests this ought to be fairly close for Saturday evening.

[A-L-E-K]

nice run