2018-03-10 EVENT: TX/LA

Discussion in 'Target Area' started by Terry Tyler, Mar 8, 2018.

  1. Terry Tyler

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    Anyone have a take on the protuberances coming down Saturday evening?

    Thermodynamic values appear to be mildly healthy in the 1700-2600 j/kg range per 18Z NAM NEST run for Saturday. STP parameters also appear to be very high as well with ranges in the 5-7 range, but here is the burning question. The upper air patterns look like the isobars are pushing down, can anyone explain what this feature is or comment on the potential threat area in Texas and Louisiana on Saturday?


    Is this good for tornado potential? Yes or no, the upper air does not look like traditional set-up, but the STP values are high.
     
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  2. Mark Blue

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    Did you mean to say perturbations?
     
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  3. Quincy Vagell

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    This looks like an interesting northwest flow severe setup. Just looking at wind fields, the Arklatex region can expect a general WNW flow in the upper levels, backing to southerly near the surface. More than adequate deep layer shear supports enlarged hodographs and plentiful storm relative helicity across the area Saturday afternoon and night.

    In the upper levels, the main vortmax is progged to be transversing the central Plains at 00z Sunday with lead shortwave energy more or less sliding down the Red River Valley towards Arkansas/Louisiana, where a surface low is forecast to develop over North Texas.

    The focus for severe appears to be from East Texas into much of Louisiana and southern Arkansas. The models vary with placement of the surface warm front Saturday afternoon, but the consensus seems to be generally from the Red River to the AR/LA border region.

    Running through some of the red flags first, there is the potential for considerable ongoing convective activity from mid morning through midday Saturday in vicinity of the warm front, which may limit surface heating and lead toward messy/clustered storm modes near the warm front. While the warm sector from East Texas into Louisiana is forecast to see much better surface heating, the 3km NAM insists that warm air (14-18C) in the 800-850mb layer noses across Texas. This scenario creates a capping inversion over East Texas, despite large CAPE values over 2,000 J/kg and higher end severe parameters, i.e. STP and SCP.

    I think with all things considered, central to northern Louisiana seems to have the best shot at favorable thermodynamics, working in concert with large, clockwise curving hodographs, assuming the area isn't convectively overturned during the morning/early afternoon period. Even in that worst case scenario, you could probably still have embedded supercellular structures given wind profiles (WNW at 500mb to S near the surface is going to allow updrafts to rotate fairly easily), as well as the potential for some discrete warm sector activity, south of ongoing convection, but east of the capping inversion.

    If the lead shortwave/perturbation dives in only a little bit quicker, I could see an MCS developing over the Arklamiss area by midday Saturday and racing across Mississippi with damaging winds and marginally severe hail being the main threats. That scenario would leave limited forcing in the wake of this convective system, despite strong surface heating in areas like East Texas and western Louisiana.

    One also has to consider the position of the warm front. If morning precipitation is widespread, you may see a convectively reinforced boundary/effective warm front remain farther south, perhaps across Louisiana, as advertised on the last few runs of the 3km NAM. That would keep most, if not all of Arkansas on the cool side of the system, mitigating any severe threat there. However, if early day activity is not as widespread as currently progged, the warm front could easily lift into Arkansas and you may have a better shot at discrete warm front activity, but I don't think that scenario is probable at this juncture.

    It's tough to find any really close analog matches, but I came out with a couple that are loosely matched to Saturday:
    3/26/11 - Widespread severe hail across the mid-South/Southeast with a few tornadoes across AL/GA. Only a few isolated severe reports farther west near the surface low and cold front in TX. This analog is displaced at least a few hundred miles to the east, so the geographic placement isn't perfect.
    3/28/14 - A better matchup based on the surface features and geographic placement, although one large caveat is that surface heating across East Texas was more impressive than we'll probably see on Saturday, as midday surface temperatures had warmed into the mid-70s as far north as the Metroplex by 17z. Temperatures this time around will probably be 5-10 degrees cooler at the same time. Nonetheless, that event produced quite a bit of large hail from East Texas into southern Arkansas, with damaging winds common into Louisiana.

    I won't be out chasing, but I think Louisiana would be the sensible play, unless you're an East Texas local and want to backyard chase, then you wait at home and see if the cap can be breached and perhaps isolated convection manages to form before going too far east into the jungle. Maybe something could go up on the dryline, but displacement from better forcing aloft and warm 850mb temperatures don't excite me.

    Overall, the setup looks to yield fairly messy results (and/or a cap bust in East Texas), but I always like northwest/west-northwest flow events, especially where surface winds are still southerly and instability is favorable.
     
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    #3 Quincy Vagell, Mar 8, 2018
    Last edited: Mar 8, 2018
  4. Randy Jennings

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    I'm with Qunicy on this one. Today's 18z 3k NAM makes capping look like a major problem, wasting those great hodos:

    20180308_18z_3kNAM_CIN_valid_20180310_21z.png

    Having said that, the 18z GFS says the cap is not as strong. We will see....
     
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  5. Terry Tyler

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    Possibly, Please explain.
     
  6. Terry Tyler

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    Great Write Up, Quincy, thank you for sharing the time to provide your insights.

    P.S. How did you know those events? Did you happen to watch them, or remembered a similar pattern before? What is the main benefit of Northwest flow into a system like this?

    - T.T.
     
  7. Mark Blue

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    We’re going way off topic here, but if you look at the word definitions you’ll find the answers. Plus the SPC uses perturbations quite often, while protuberance is simply not in their vocabulary.

    Perturbation: the state of being perturbed, agitation. In other words, a perturbation tends to upset the atmosphere and cause storminess.

    Protuberance: Something, such as a bulge, knob, or swelling, that protrudes. This could be anything from a Rhino horn to a wart on your knuckle.

    I’m just keeping you on your toes since it’s the TA. Hopefully I didn’t rattle your cage too much.
     
  8. Dan Robinson

    Dan Robinson WxLibrary Editor
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    Well, it looked good for a while! RAP, NAM, GFS and CAMs are now placing a large MCS in the Gulf overnight Friday/Saturday morning, decimating the northward moisture transport and completely hosing the setup. I don't see much of a play anywhere onshore now save for a brief window on the far western edge of the warm sector in E Texas where it looks like some of the higher DPs are making their way around the MCS outflow. It doesn't look like enough to overcome the aforementioned capping problem though. Still enough time for the models to be wrong, but unlikely when they agree this late in the game.
     
    #8 Dan Robinson, Mar 9, 2018
    Last edited: Mar 9, 2018
  9. David Williams

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    I'm thinking about pulling the trigger on today's Chase in East Central Oklahoma. Since I'm in Tulsa it seems like a local chase that could have the potential to be worthwhile. Most of the ingredients are on the marginal level including cape, low-level winds, moisture, and daytime Heating. However there are some cloud pockets opening up and cumulus starting to develop. Also a couple of the models including the hrrr have a stationary boundary along or just south of the I-40 Henryetta to Fort Smith area. I'm guessing it's some kind of baroclinic Zone like a modified warm front extending out of the low in Northern Texas Southern Oklahoma. That area also has some of the better moisture and helicity values. Potential is there so I'm taking my chances for my first Chase of 2018.

    Sent from my SM-G955U using Tapatalk
     
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  10. Quincy Vagell

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    They came up as analogs based on model forecasts.
    This link is very helpful: http://www.eas.slu.edu/CIPS/ANALOG/analog.php
    It's important to note that some analogs are better matches than others. For example, the upper levels may be a very close match, but the surface conditions may vary dramatically from the upcoming event, or vice versa. That's why I like to take a deeper look at the "closest" matches, to see what time of year they occurred, how the different panels line up to forecasts, the severe reports from a given event, etc.

    Northwest flow in mid to late spring and summer can create a very favorable setup for severe thunderstorms, assuming there is enough shear and instability in place. When winds are WNW or NW at 500mb, surface winds don't need to be as "sharply" backed to create large hodographs. For example, in a northwest flow event, it's entirely possible to have near-surface winds out of the southwest and still have large angle between 0 and 3km winds, sometimes as much as 90 degrees.

    The issue with this event (one of many) was that despite favorable deep layer wind shear, low level flow was weak (<15 knots of 0-1km shear) and instability was relatively low end.
     
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