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andyhb

May 7th-10th Severe Outbreaks

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So my concerns at this point are starting to diminish. As far as low-level shear is concerned, doesn't the GFS have a low bias in these cases?

 

Yeah, it has done this several times before, especially with a deepening low (fails to show enough of a low level wind response).

 

NAM, as I mentioned above, continues to look like a very high end event with some truly outrageous wind profiles for May.

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Actually, I like the fact that the heaviest convection over E OK before 12Z is much less widespread/moves out more quickly on the 12Z as opposed to previous runs. Plus, the overall evolution of the trough has been to favor a more rapid ejection and a broader, more consolidated mid-level feature, thereby greatly alleviating the veer-back-veer not just before 00Z, but also well into the overnight and early morning hours. The ECMWF and the GFS now seem to be inching toward better agreement on the timing of large-scale ascent and ejection, and the overall net trend is faster and more favorable for a significant severe outbreak on Saturday--one with a potentially long-lived threat for supercells/significant tornadoes overnight. So my concerns at this point are starting to diminish. As far as low-level shear is concerned, doesn't the GFS have a low bias in these cases? To me, what matters is the overall synoptic set-up, the improving thermodynamic prospects (at least before 18Z), and the strengthening low-level jet into the evening hours/overnight. Initiation time will still need to be nailed down, but fortunately there is ample time for change. The GFS is probably suffering from convective feedback due to WAA.

The GFS is looking better but hodographs are still kinked but definitely improving.

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I favor the OK target as well, given that the veer-back-veer issue will be less pronounced down there, as well as the likely more-isolated coverage and later initiation.  The eye-popping instability/shear combo over a large area would normally scream significant tornado outbreak to me, but the aforementioned veer-back-veer issue gives me quite a bit of pause.

 

The 5/24/11 event had some veer-back-veer concerns as well, though they were more subtle than this event if I recall correctly.  Things got pretty messy in N OK/S KS on that day.

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The 5/24/11 event had some veer-back-veer concerns as well, though they were more subtle than this event if I recall correctly.  Things got pretty messy in N OK/S KS on that day.

 

I believe this also had to do with a large amount of upper level backing north of the KS/OK stateline, since the trough was quite sharp and rather strongly negatively tilted.

 

The trough here seems to be broadening a bit with each run, which may help improve the wind profile aloft. For the record, I also like the OK target more as of now due to the reasons you mentioned.

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The northern target had veer back veer issues on April 14 2012 also. Probably other cases but remember many an unhappy NE chaser.

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I favor the OK target as well, given that the veer-back-veer issue will be less pronounced down there, as well as the likely more-isolated coverage and later initiation.  The eye-popping instability/shear combo over a large area would normally scream significant tornado outbreak to me, but the aforementioned veer-back-veer issue gives me quite a bit of pause.
 
The 5/24/11 event had some veer-back-veer concerns as well, though they were more subtle than this event if I recall correctly.  Things got pretty messy in N OK/S KS on that day.

 

Veer-back-veer was actually far less subtle on 5/24/11.  It was extreme, particularly in KS and then spreading south into OK after about 23z.  OUN didn't launch a 00z sounding (for obvious reasons), but DDC gives you some idea of just how severely the flow aloft backed.

 

DDC_00_obs.gif

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I'm starting to wonder whether a D2 moderate risk will be issued for tomorrow with the later D2 update this morning. Both the NAM and GFS have a favorable parameter space over a large area and the ascent from that lead impulse should be substantial enough to encourage fairly widespread initiation. Forcing is fairly diffuse too which may encourage a rather long-lived discrete mode.

 

NAM has over 4,000 J/kg banked up against the Caprock with 45 kt at H5 at 21Z tomorrow. Should support monster hailers if nothing else.

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Continuing with the 5/24/11 comparison for a second, besides veer-back-veer causing issues, you also had extreme upper-level difluence/divergence across the warm sector, leading to substantial upward vertical motion to help allow for widespread convection.  I just don't see this degree of difluence or the degree of veer-back-veer in the shear profiles that we saw on 5/24/11 being realized with this trough.

 

300_110525_00.gif

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Veer-back-veer was actually far less subtle on 5/24/11.  It was extreme, particularly in KS and then spreading south into OK after about 23z.  OUN didn't launch a 00z sounding (for obvious reasons), but DDC gives you some idea of just how severely the flow aloft backed.

 

Sorry, I meant more subtle over OK.  What the NAM has is probably comparable to 5/24/11 with regard to this issue, but the GFS is much worse.

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day2otlk_1730.gif?1431019925183

 

 

 

DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1226 PM CDT THU MAY 07 2015

VALID 081200Z - 091200Z

...THERE IS A MDT RISK OF SVR TSTMS ACROSS PARTS OF NORTHWEST
TX...WESTERN OK AND SOUTHERN KS...

...THERE IS AN ENH RISK OF SVR TSTMS ACROSS PARTS OF THE SOUTHERN
AND CENTRAL PLAINS...

...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE SOUTHERN
AND CENTRAL PLAINS...

...THERE IS A MRGL RISK OF SVR TSTMS FROM SW TX TO IL...

...THERE IS A MRGL RISK OF SVR TSTMS ALONG THE CAROLINA COAST...

...SUMMARY...
SEVERE THUNDERSTORMS WILL DEVELOP ACROSS THE SOUTHERN AND CENTRAL
GREAT PLAINS. VERY LARGE HAIL...DAMAGING WINDS...AND TORNADOES ARE
EXPECTED...ESPECIALLY ACROSS WESTERN OKLAHOMA.

...SOUTHERN/CENTRAL GREAT PLAINS...

LARGE-SCALE HEIGHT FALLS WILL SPREAD INTO THE DESERT SOUTHWEST
DURING THE DAY2 PERIOD AS STRONG UPPER LOW TRANSLATES INTO THE
4-CORNERS REGION LATE. DIFLUENT HIGH-LEVEL FLOW WILL SPREAD ACROSS
THE SRN/CNTRL PLAINS AHEAD OF THIS FEATURE WITH SUBSTANTIAL FLOW
EXPECTED TO OVERSPREAD A PRONOUNCED DRY LINE DURING PEAK HEATING.
STRONG SHEAR AND FAVORABLE HIGH-LEVEL VENTING SHOULD RESULT IN A
CONCENTRATED CORRIDOR OF SEVERE THUNDERSTORMS...ESPECIALLY ACROSS
WRN OK.

BOTH THE GFS AND NAM SUGGEST A WEAK MID-LEVEL SHORT-WAVE TROUGH WILL
EJECT ACROSS THE SRN HIGH PLAINS AFTER 18Z...TRANSLATING INTO WRN OK
BY 09/00Z. THIS FEATURE IS EXPECTED TO AID CONVECTIVE DEVELOPMENT
ALONG DRY LINE BY EARLY-MID AFTERNOON ACROSS THE HIGH
PLAINS...SPREADING/DEVELOPING EWD TOWARD THE I-35 CORRIDOR BY LATE
AFTERNOON. SHEAR PROFILES FAVOR SUPERCELLS AND BOUNDARY-LAYER
MOISTURE...ABSENT DAY1 CONVECTIVE DISRUPTION...SHOULD EASILY ADVANCE
NWD TOWARD SYNOPTIC BOUNDARY THAT SHOULD ORIENT ITSELF IN AN E-W
FASHION ACROSS SRN KS/SERN CO. IF MOISTURE/INSTABILITY RETURN TO
THE SYNOPTIC BOUNDARY...AS SUGGESTED BY LATEST NAM...ROBUST SEVERE
THUNDERSTORMS WILL DEVELOP ALONG A FAVORABLY SHEARED BOUNDARY THAT
COULD ENHANCE TORNADO POTENTIAL WITH ANY SUPERCELLS. EVEN
SO...DISCRETE WARM SECTOR SUPERCELLS ARE EXPECTED TO DEVELOP ACROSS
THE AFOREMENTIONED REGION OF NWRN TX...NWD TO THE SYNOPTIC FRONT
WITHIN AN ENVIRONMENT SEEMINGLY FAVORABLE FOR VERY LARGE
HAIL...TORNADOES...AND DAMAGING WINDS. FORECAST NAM SOUNDINGS
EXHIBIT PROFILES CHARACTERISTIC OF STORMS CAPABLE OF PRODUCING HAIL
IN EXCESS OF 2 INCHES AND PERHAPS EVEN A FEW STRONG TORNADOES.

SCT SUPERCELLS SHOULD DEVELOP/SPREAD NEWD TOWARD THE I-35 CORRIDOR
BY LATE AFTERNOON AS FOCUSED LLJ AIDS INFLOW. DURING THE
EVENING...SEVERE CLUSTERS SHOULD EVOLVE ALONG THE E-W SYNOPTIC FRONT
WHERE LLJ IMPINGES ON BOUNDARY.

..DARROW.. 05/07/2015

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I'm starting to wonder whether a D2 moderate risk will be issued for tomorrow with the later D2 update this morning. Both the NAM and GFS have a favorable parameter space over a large area and the ascent from that lead impulse should be substantial enough to encourage fairly widespread initiation. Forcing is fairly diffuse too which may encourage a rather long-lived discrete mode.

 

Good call, Andy.

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I think two MDTs just broke the SPC website :P

 

HA! Yes, just noticed that. They still need to do some coding/banner work for multi-day outbreaks.  :lmao:

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Here we go:

 

mcd0533.gif

 

MESOSCALE DISCUSSION 0533
NWS STORM PREDICTION CENTER NORMAN OK
1252 PM CDT THU MAY 07 2015

AREAS AFFECTED...SRN OK THROUGH NRN TX

CONCERNING...SEVERE POTENTIAL...WATCH LIKELY

VALID 071752Z - 071945Z

PROBABILITY OF WATCH ISSUANCE...80 PERCENT

SUMMARY...STORMS ARE EXPECTED TO BECOME BETTER ORGANIZED AND
INCREASE IN INTENSITY FROM SRN OK THROUGH NRN TX THIS AFTERNOON.
LARGE HAIL AND DAMAGING WIND WILL BE THE MAIN THREATS...BUT A FEW
TORNADOES CANNOT BE RULED OUT. WW WILL LIKELY BE NEEDED SOON.

DISCUSSION...STORMS HAVE LARGELY EVOLVED INTO AN MCS FROM NWRN TX
INTO SWRN OK AND IS MOVING SLOWLY EWD. STORM EMBEDDED WITHIN LEADING
EDGE OF MCS OVER SWRN OK IS BEGINNING TO SHOW UPDRAFT ROTATION. THE
SFC LAYER DOWNSTREAM FROM THIS ACTIVITY IS UNDERGOING RAPID RECOVERY
AS AN OLD OUTFLOW BOUNDARY WEAKENS AND LIFTS NWD...ALLOWING RICHER
/UPPER 60S F DEWPOINTS/ TO ADVECT THROUGH NRN TX AND SRN OK. MIXED
CONVECTIVE MODE WITHIN THE MCS AND GENERALLY MODEST LOW-LEVEL SHEAR
LOWERS CONFIDENCE IN A MORE SUBSTANTIAL TORNADO THREAT.
NEVERTHELESS...DEEP SHEAR FROM 40-50 KT...A MOIST BOUNDARY LAYER AND
MODERATE INSTABILITY WILL SUPPORT EMBEDDED ORGANIZED STRUCTURES
WITHIN THE MCS INCLUDING SUPERCELLS AND BOWING SEGMENTS. MOREOVER A
FEW DISCRETE STORMS MIGHT ALSO DEVELOP SEPARATE FROM THE MCS.
AS
THESE STORMS INTERACT WITH THE RESIDUAL BOUNDARY...ISOLATED
TORNADOES WILL BE POSSIBLE DESPITE RELATIVELY SMALL AMBIENT
HODOGRAPHS. OTHERWISE...LARGE HAIL AND DAMAGING WIND WILL THE MAIN
THREATS AS THIS ACTIVITY DEVELOPS EWD THIS AFTERNOON.

..DIAL/HART.. 05/07/2015

 

Bolded an interesting bit that may affect TOR potential later, but we'll see.

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Severe Thunderstorm Watch goes up and the KFDR radar goes down per NWS Chat. Yeesh bad timing there.

I'm surprised TOKC didn't get slammed by damaging 70-90mph winds, as it seemed to be getting hit by a tornado at one point yesterday.

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Concerned dad here in edmond, ok. Anyone have any thoughts on where storms may fire first Saturday.

Have outdoor activities until 4:00 in N OKC

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A bit concerned about the outflow/cold pool from the ongoing MCS in OK/TX messing up tomorrow. Despite the fact that it is a long way in advance, cold pools can be very stubborn, since temps/dewpoints have dropped into the low 60s/upper 50s behind that thing. That said, wherever that boundary ends up setting up could see enhanced tornado potential.

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Veer-back-veer seems to have improved on the 06z/12z GFS runs, but it still seems to be breaking out an awful lot of convection before primetime on Saturday and still refuses to appreciably increase its low level shear values.

The ECMWF continues to do the same as well, and that's my biggest (and really only) issue with Saturday.

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That said, wherever that boundary ends up setting up could see enhanced tornado potential.

 

I am concerned that cold pool might push the boundary farther east into more populated zones than originally thought.

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A bit concerned about the outflow/cold pool from the ongoing MCS in OK/TX messing up tomorrow. Despite the fact that it is a long way in advance, cold pools can be very stubborn, since temps/dewpoints have dropped into the low 60s/upper 50s behind that thing. That said, wherever that boundary ends up setting up could see enhanced tornado potential.

Yeah...no.  Not unless that MCS magically backbuilds for the next 18 hours and the low-level cyclone never develops.

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Yeah...no.  Not unless that MCS magically backbuilds for the next 18 hours and the low-level cyclone never develops.

 

Alright, was just a thought.

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MCS contamination. Gobbling up those semi discrete cells. North Texas is going to junk. I'm heading west to play the dryline. The environment is decent SE of Lubbock with moderate destabilization and backed low-level winds.

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The ECMWF continues to do the same as well, and that's my biggest (and really only) issue with Saturday.

I just don't know how much it'll matter.  A lot of the early convection the models fire appears to be east of the main instability axis, allowing for maximum destabilization in the real zone of interest.  I wouldn't discount it as a factor, but it doesn't concern me quite as much as it would in most cases.

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