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October 12-13th "Potential" Severe Outbreak


patrick7032

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Not really commenting on the severe threat at all since I haven't been watching closely enough, but hard to put any stock on NAM/SREF yet. They will all be at a huge vertical levels/resolution disadvantage compared to the ECMWF/RGEM/GEM/GFS. The speed at which they simulate the upper low seemingly "skipping" across the intermountain W complex terrain seems unrealistic. The ejection point seems even more unlikely (NW NE and S SD?) as vertical stretching would tend to keep the ejection farther S. Seen it time and time again with a northward and fast bias in NCEP guidance (NAM especially) under these setups...with all else being equal.

This will be a big big test for the SREF tornado ingredients probability parameter, anyway. Those threshold changes yielded a massive failure by making it run WAY too hot on 10/1 around here. I don't know why 1500m is being used as the threshold MLLCL, but between that and the CAPE adjustment, I doubt it will be useful at all anymore.

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Yes

Yeah not ending well is a bit of an understatement for that one...

If Rich from the SPC comes on here (as he periodically does), I'd like to ask him what spurred the change in the SREF STI thresholds, although something tells me (for the CAPE change at least) that some of the low CAPE, high shear events we've seen recently influenced it.

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Not gonna post as detailed an analysis as I wanted to, but a couple of points.

1) Chasers' nightmare. On Friday, the day where storm mode and likely storm structure are to be the most favorable, I think the bigger threat by far will be after dark. On Saturday, a more linear storm organization and wicked storm motions (50kt?), with perhaps some moisture issues (though I think, by and large, these are of somewhat lesser concern than I had originally thought). In short, the only real option for quality chasing will be late Friday afternoon in eastern NM, but with cyclogenesis not really likely to begin in earnest until around 00z, I don't know that the shear profiles for tornadoes will be there.

2) Overnight threat? We've already had a couple of these events this year, with the tragic EF3 tornado at Woodward during the midnight hour of 4/15 and a couple EF2 tornadoes in SE CO during the pre-dawn hours of 4/27. These events are uncommon but not unheard of on the plains. With cyclogenesis occurring during the overnight hours of Friday night into Saturday morning, you'll see a substantially intense wind field across the plains, which should suffice in keeping the PBL relatively mixed during the overnight hours and stemming much of a nocturnal inversion. This is shown pretty clearly in the recent GFS runs. As a related aside, the NAM (of which I don't take much stock past 48 h) shows significant overnight destabilization across western and central KS Friday night into Saturday morning. To destabilze locally overnight is rather rare. The destabilization is a product of the cyclogenesis, which leads to the retreat of the warm front northward across Kansas, as well as some weak CAA around 700 mb, which, combined, lead not only to a propagation of CAPE into KS but also an expansion and a slight elevation of maximum CAPE in the warm sector. There are likely other instances of this happening, but the only one I can think of didn't end particularly well (6 November 2005). Watch this in future model runs. If overnight destabilization continues to be modeled as we near the event, then I think the odds of an overnight severe weather outbreak will be substantially high.

3) Saturday is gonna be a tricky one. I think the odds are increasing that a more widespread severe weather episode is likely. Between moisture issues (I suspect the northward advancement of quality moisture is probably being overmodeled on the GFS and NAM) and sharper forcing, I am doubting just how substantial the tornado threat may be. However, I think that given adequate moisture and instability, along with very, very strong speed shear and good large-scale forcing, a widespread damaging wind event, likely with a few isolated tornadoes, is appearing more likely.

Bottom line: Supercell threat begins late Friday. Biggest tornado threat may be 03z-12z Friday night/Saturday morning. Most widespread severe wx threat is likely Saturday, from late morning through the afternoon and evening hours, until the storms reach the airmass farther east that will not likely recover due to the positioning of the ridge over the SE CONUS.

Great post, Tony. Thanks for your insights.

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Yeah not ending well is a bit of an understatement for that one...

If Rich from the SPC comes on here (as he periodically does), I'd like to ask him what spurred the change in the SREF STI thresholds, although something tells me (for the CAPE change at least) that some of the low CAPE, high shear events we've seen recently influenced it.

The CAPE adjustment makes sense if the goal was to tweak the parameter to account for more low CAPE, high helicity events. The MLLCL tweak just does not make sense to me as 1250-1500 m is a real stretch (pun intended) for significant tornado activity (outside of your non-supercellular, mega vorticity stretching setups like the May EF3 tornadoes outside Wichita). Still, I would have left both the same, given that yeah, you may underforecast some significant low-CAPE events, but most of your banner cases, cases you would want highlighted with your highest probability over a large spatial and temporal scale, usually have CAPEs over 1000J/kg.

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This will be a big big test for the SREF tornado ingredients probability parameter, anyway. Those threshold changes yielded a massive failure by making it run WAY too hot on 10/1 around here. I don't know why 1500m is being used as the threshold MLLCL, but between that and the CAPE adjustment, I doubt it will be useful at all anymore.

The 1500 m threshold is particularly interesting. Is there any documentation on the reasoning for the changes? My only guess would be potential biases noted by SPC/others from switching to all WRF members after droppping eta/RSM, but that is completely a guess.

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The CAPE adjustment makes sense if the goal was to tweak the parameter to account for more low CAPE, high helicity events. The MLLCL tweak just does not make sense to me as 1250-1500 m is a real stretch (pun intended) for significant tornado activity (outside of your non-supercellular, mega vorticity stretching setups like the May EF3 tornadoes outside Wichita). Still, I would have left both the same, given that yeah, you may underforecast some significant low-CAPE events, but most of your banner cases, cases you would want highlighted with your highest probability over a large spatial and temporal scale, usually have CAPEs over 1000J/kg.

Strongly agree. Lowering the CAPE to 750 J/kg to throw the SE and Midwest a bone would've been completely understandable. The MLLCL surprises me primarily because, if anything, the only areas that ever seem to get away with > 1100-1200 m are very sparsely-populated... e.g., the High Plains. Ironically, for supercell tornadoes in those very marginal LCLs, you probably need impressively-high CAPE.

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Anyone? Sorry I even asked. I take it the best action is going to be western TX, OK, but I'm wondering if the central parts will be worth traveling too.

There's good potential for severe storms within a 2-hour drive of TUL on Saturday afternoon/evening. Obviously I've made my opinion clear on the likelihood of tornadic supercells in this region, but several respected chasers and forecasters disagree, so take it as you will. If the ECMWF wins out on timing, it's possible the dryline will lag farther W and you'd have to drive 2-3 hours to get daytime storms. In my opinion, this is also rather unlikely, and you'll probably see storms by dinnertime right around there.

Friday's potential will most definitely be >4 hrs W of TUL.

TLDR: if you're just looking to see some severe storms, I'd say hang out through Saturday evening. If you want tornadic supercells to make it worth your while, you'll have to wait until at least Thursday to get a really good idea of that likelihood.

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Strongly agree. Lowering the CAPE to 750 J/kg to throw the SE and Midwest a bone would've been completely understandable. The MLLCL surprises me primarily because, if anything, the only areas that ever seem to get away with > 1100-1200 m are very sparsely-populated... e.g., the High Plains. Ironically, for supercell tornadoes in those very marginal LCLs, you probably need impressively-high CAPE.

Where did you guys hear about a significant tornado ingredients formula change? What type of improvement is this supposed to be?

I saw it from Tony, actually.

Is it 750 j/kg or 500 j/kg? SPC SREF plots depict 500 j/kg as the cutoff.

post-999-0-29225700-1349847044_thumb.gif

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Hmmm...

It's really insistent on that positive-tilt config through Saturday, which has large implications for the threat S of the KS/NE border. If the forcing and trough axis hold back through the afternoon, then low-level veering will be less pronounced, and tornadic supercells would be possible down the dryline. I still wouldn't consider the ECMWF verbatim to be an outbreak-ripe environment, but it does look substantially better than the GFS for places like E KS and possibly into N OK, IMO. Also, any secondary low developing along the front and inducing localized backing could be a game-changer in that scenario.

What a frustrating system... doesn't feel like we're making any progress with predictability as each day goes by.

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D3:

day3prob0730.gif

DAY 3 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

0229 AM CDT WED OCT 10 2012

VALID 121200Z - 131200Z

...THERE IS A SLGT RISK OF SVR TSTMS FRI/FRI NIGHT ACROSS PARTS OF

THE CNTRL AND SRN HIGH PLAINS...

...SYNOPSIS...

MODELS SUGGEST THAT THE MAIN BRANCH OF STRONGER WESTERLIES WILL

EXTEND IN A BROADLY ANTICYCLONIC BELT FROM THE EASTERN PACIFIC

ACROSS CANADA INTO THE WESTERN ATLANTIC DURING THIS FORECAST PERIOD.

BUT A SIGNIFICANT CLOSED LOW WITHIN A REMNANT WEAKER BRANCH TO THE

SOUTH IS STILL FORECAST TO FINALLY ACCELERATE OUT OF THE

SOUTHWESTERN STATES. THIS FEATURE IS EXPECTED TO TAKE A

NORTHEASTWARD TRACK ACROSS THE CENTRAL ROCKIES INTO THE CENTRAL

PLAINS...AROUND THE NORTHWESTERN PERIPHERY OF A PROMINENT

SUBTROPICAL RIDGE CENTERED OVER THE GULF OF MEXICO. AS IT DOES...IT

APPEARS LIKELY TO CONTRIBUTE TO STRONG SURFACE CYCLOGENESIS TO THE

LEE OF THE CENTRAL ROCKIES...WHICH PROBABLY WILL COMMENCE AS EARLY

AS FRIDAY NIGHT.

A RIDGE AXIS EXPECTED TO EXTEND SOUTHWESTWARD FROM A COLD SURFACE

HIGH CENTER...OVER THE UPPER MISSISSIPPI VALLEY...INTO THE CENTRAL

HIGH PLAINS AT THE START OF THE PERIOD MAY INITIALLY LIMIT

CONVECTIVE POTENTIAL...PARTICULARLY NORTH OF THE TEXAS/OKLAHOMA

PANHANDLE REGION. HOWEVER...GUIDANCE INDICATES IT WILL WEAKEN BY

LATE IN THE DAY...AS THE HIGH CENTER REDEVELOPS EASTWARD INTO THE

GREAT LAKES REGION. IT IS POSSIBLE THAT A SHALLOW STABLE SURFACE

BASED LAYER MAY LINGER ACROSS MUCH OF WESTERN KANSAS AND NEBRASKA

UNTIL LATE FRIDAY EVENING...BUT EVENTUALLY 60F+ SURFACE DEW POINTS

ARE EXPECTED TO ADVECT NORTHWARD FROM THE SOUTHERN PLAINS...AS A

SOUTHERLY 850 MB JET STRENGTHENS FROM 30-50+ KT THROUGH MUCH OF THE

PLAINS.

...PARTS OF CNTRL/SRN HIGH PLAINS...

ALTHOUGH JUST TO THE SOUTH OF A CYCLONIC 70-90 KT 500 MB JET STREAK

EXPECTED TO NOSE INTO THE CENTRAL HIGH PLAINS LATE FRIDAY

AFTERNOON...ISOLATED TO SCATTERED STORM DEVELOPMENT DOES APPEAR

POSSIBLE ACROSS THE TEXAS PANHANDLE HANDLE...NEAR THE REMNANT

SURFACE FRONT AND A DEVELOPING LEE SURFACE TROUGH/DRY LINE. THIS

MAY INCLUDE DISCRETE SUPERCELLS...AS MODESTLY STEEP MID-LEVEL LAPSE

RATES AND BOUNDARY LAYER WARMING CONTRIBUTE TO CAPE ON THE ORDER OF

1000 J/KG...IN THE PRESENCE OF STRONG SHEAR.

SOME OF THIS ACTIVITY MAY PERSIST WELL INTO THE EVENING HOURS...

WITH NEW DEVELOPMENT ALSO POSSIBLE...SPREADING NORTHEASTWARD ACROSS

AT LEAST PARTS OF WESTERN KANSAS INTO SOUTH CENTRAL NEBRASKA...IN

RESPONSE TO LOW-LEVEL THERMAL/MOISTURE ADVECTION AND STRENGTHENING

LARGE-SCALE FORCING FOR UPWARD VERTICAL MOTION. LARGE HAIL MAY BE

THE MOST PROMINENT SEVERE THREAT...BUT THE RISK FOR TORNADOES MAY

ACCOMPANY LATE AFTERNOON/EARLY EVENING ACTIVITY.

..KERR.. 10/10/2012

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An interesting read:

AREA FORECAST DISCUSSION

NATIONAL WEATHER SERVICE AMARILLO TX

449 AM CDT WED OCT 10 2012

IT STILL APPEARS THAT BETTER CHANCES AND GREATER COVERAGE OF

THUNDERSTORMS WILL BE OVERNIGHT AS A 35-KT LOW-LEVEL JET INTERACTS

WITH A SOUTHWARD MOVING COLD FRONT. THIS FRONT IS EXPECTED TO STALL

ACROSS THE NORTH-CENTRAL TEXAS PANHANDLE OVERNIGHT. MOST UNSTABLE

CAPE VALUES UP TO 1500 J/KG AND DEEP LAYER SHEAR VALUES UP TO 30 KT

WILL STILL SUPPORT THE POSSIBILITY OF ELEVATED THUNDERSTORMS CAPABLE

OF PRODUCING SMALL TO MARGINALLY SEVERE HAIL OVERNIGHT...MAINLY

SOUTH OF A DALHART TO BEAVER LINE. HEAVY RAIN WILL ALSO BE A CONCERN

WITH PRECIPITABLE WATER VALUES BETWEEN 1 AND 1.5 INCHES...WHICH IS

AT OR ABOVE 2 STANDARD DEVIATIONS. THIS HEAVY RAIN CONCERN COULD BE

EXACERBATED WITH ANY STORMS THAT TRAIN ALONG THE COLD FRONT OR

BACK BUILD. LOCATIONS SOUTH OF A DALHART TO CANADIAN LINE WILL HAVE

THE HIGHEST CHANCES FOR HEAVY RAIN.

FRIDAY-FRIDAY NIGHT: THIS CONTINUES TO BE THE TIME PERIOD OF MOST

INTEREST IN THE NEXT SEVEN DAYS...PARTICULARLY WITH RESPECT TO

SEVERE WEATHER CHANCES. THERE HAVE BEEN A FEW CHANGES NOTED OVER THE

LAST 24 HOURS...BUT FRIDAY AFTERNOON THROUGH FRIDAY NIGHT STILL

LOOKS VERY INTERESTING. WE STILL HAVE SOME UNCERTAINTIES WITH HOW

THE CONVECTIVE EVENT WILL EVOLVE. OUR PRIMARY CONCERNS STILL REVOLVE

AROUND:

1. HOW MUCH ONGOING CONVECTION THERE WILL BE FROM OVERNIGHT STORMS

ON THURSDAY

2. IF AND WHEN ANY LOW STRATUS WILL BREAK

3. ASSOCIATED WITH 1 AND 2...HOW MUCH INSTABILITY WILL BE PRESENT

4. THE DEGREE OF CAPPING PRESENT DURING THE AFTERNOON AND EVENING

5. THE LOCATION OF THE WARM FRONT

THE CLOSED UPPER LOW WILL EJECT FROM THE FOUR CORNERS INTO THE

SOUTHERN ROCKIES THROUGH THE DAY. LARGE SCALE FORCING FOR ASCENT

WILL BE INCREASING THROUGH THE DAY AND OVERNIGHT. IN RESPONSE TO

THIS...THE COLD FRONT WILL SLOWLY RETREAT NORTHWARD AS A WARM FRONT

THROUGH THE DAY. HOWEVER...THE RETREAT OF THE WARM FRONT COULD BE

INHIBITED BY ONGOING CONVECTION ALONG AND ESPECIALLY NORTH OF THE

WARM FRONT. IT REMAINS TO BE SEEN HOW FAR NORTH THE WARM FRONT WILL

LIFT...BUT MOST MODEL GUIDANCE NOW PLACES THE FRONT INTO SOUTHWEST

KANSAS BY FRIDAY EVENING. THE NAM SOLUTION IS FARTHER SOUTH WITH THE

WARM FRONT AND KEEPS IT NEAR THE OKLAHOMA AND KANSAS BORDER. THIS

FRONT WILL SERVE AS A FOCUSING MECHANISM FOR ADDITIONAL SHOWER AND

THUNDERSTORM DEVELOPMENT THROUGH THE DAY. AS A RESULT...THERE COULD

BE WIDESPREAD CONVECTION ALONG AND NORTH OF THE WARM FRONT...WHICH

WILL SIGNIFICANTLY REDUCE THE AMOUNT OF INSTABILITY. SOUTH OF THE

WARM FRONT...THERE MAY ALSO BE ONGOING CONVECTION AND WIDESPREAD

CLOUD COVER THROUGH THE DAY. THIS SEVERE WEATHER EVENT WILL GREATLY

DEPEND UPON HOW MUCH HEATING CAN DEVELOP THROUGH THE DAY...SO WE

WILL HAVE TO CLOSELY WATCH FOR ANY BREAKS THAT DEVELOP IN THE

CLOUDS. DESPITE THESE UNCERTAINTIES...WE STILL THINK THAT HIGHS WILL

BE ABLE TO WARM INTO THE 70S FOR MANY LOCATIONS IN THE TEXAS

PANHANDLE. WITH THE DIFFERENCES IN HOW MUCH HEATING

DEVELOPS...MODELS ALSO DISAGREE ON HOW MUCH INSTABILITY WILL DEVELOP

DURING THE AFTERNOON HOURS. SOLUTIONS RANGE FROM MIXED LAYER CAPE

VALUES BETWEEN 600 J/KG TO 1500 J/KG. ALL MODELS AGREE THAT DEEP

LAYER SHEAR VALUES WILL INCREASE THROUGH THE DAY WITH VALUES UP TO

60 KT POSSIBLE FRIDAY NIGHT. BASED ON THE COMBINATION OF INSTABILITY

AND SHEAR...SEVERE STORMS SEEM TO BE A GOOD BET. JUST HOW SEVERE THE

STORMS WILL BE...AND WHAT MODE THE STORMS WILL BE...WILL GREATLY

DEPEND ON HOW MUCH INSTABILITY CAN MATERIALIZE.

ANOTHER COMPLICATING FACTOR IS WHETHER OR NOT ANY DISCRETE SUPERCELL

THUNDERSTORMS WILL DEVELOP DURING THE AFTERNOON AND EARLY EVENING.

NAM FORECAST SOUNDINGS SHOW LESS CAPPING THAN GFS FORECAST

SOUNDINGS. IF THERE ARE ENOUGH BREAKS IN THE CLOUDS AND SUFFICIENT

INSTABILITY IS ACHIEVED...DISCRETE SUPERCELLS WILL BE A DISTINCT

POSSIBILITY. IF THIS SCENARIO PANS OUT...LARGE HAIL...DAMAGING

WINDS...AND EVEN AN ISOLATED TORNADO OR TWO WILL BE POSSIBLE. THE

TORNADO THREAT COULD BE ENHANCED ALONG AND JUST NORTH OF THE WARM

FRONT AND ALSO IN PROXIMITY TO RESIDUAL OUTFLOW BOUNDARIES LAID OUT

BY EARLIER CONVECTION. UNFORTUNATELY...THE TORNADO THREAT WILL NOT

BE ABLE TO BE FULLY ASSESSED UNTIL FRIDAY WHEN MESOSCALE PROCESSES

CAN BE THOROUGHLY ANALYZED. REGARDLESS OF WHETHER OR NOT DISCRETE

SUPERCELLS DEVELOP...WE DO EXPECT A SQUALL LINE TO DEVELOP FRIDAY

EVENING AS STRONG FORCING FOR ASCENT ASSOCIATED WITH THE CLOSED LOW

IMPINGES ON THE AREA AND A PACIFIC FRONT BEGINS TO SWEEP EASTWARD.

THIS SQUALL LINE WILL MARCH ACROSS THE FORECAST AREA OVERNIGHT AND

WILL MAINLY POSE A RISK FOR DAMAGING WINDS. AN ISOLATED TORNADO

THREAT...ALBEIT SMALL...WILL STILL EXIST OVERNIGHT WITH THE SQUALL

LINE AS 0-1 KM SHEAR VALUES WILL BE AS HIGH AS 40 KT. THE HEAVY RAIN

THREAT WILL ALSO BE PRESENT DURING THIS TIME PERIOD DUE TO HIGH

PRECIPITABLE WATER CONTENT.

DESPITE A SOMEWHAT UNFAVORABLE TIME...CLIMATOLOGICAL LY

SPEAKING...FOR SEVERE WEATHER IN THE PANHANDLES...THIS STORM SYSTEM

CERTAINLY WARRANTS OUR CLOSE ATTENTION.

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Once again from a cursory glance, the 12Z NAM is printing out some impressive stuff for 00Z Saturday in the extreme northern TX panhandle. Here's the sounding for KDUX.

SKT_NAM__KDUX.png

Yeah it looks that way. I still wish the 500 jet max was faster and closer...and don't like the panhandle position in relation to the upper jet quadrant...but we will see. Saturday looks decent in a END-KHUT-KSGF-KTUL "box". Debating about a pahhandle/state line chase Friday or northern OK/southeastern KS chase.

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I wouldn't be so trusting of the 12Z NAM for Saturday since it just got into its range. Just a personal rule of mine, I don't generally trust the NAM until it gets into the 72 hour range.

Even that's kinda generous. ;)

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