Memorial Day weekend severe threats

[jojo762]

I'll just move this over and start a new thread regarding the upcoming severe potential:

A diverse amount of questions remain, each of which could have varying degrees of effects on the threats/impacts/potential areas effected... such as subtle perturbation/Shortwave timing, synoptic boundary location, degree of moisture (low 70s or upper 70s?), and exact orientation of low-level flow, among other things... But fairly strong agreement from the various GEFS members in addition to the deterministic GFS on a robust/higher-end parameter space developing along and east of the dryline (wherever it sets up) on Friday/Saturday evening. As of now per the GEFS and GFS, friday would feature extreme instability, owing to very steep lapse rates and climatologically impressive moisture, sufficient effective shear of 40-50kts, substantial veering with height (especially in the lowest three kilometers AGL) producing a wind profile that is highly favorable for tornadic supercells... Big question for friday would appear to be the cap and the typically associated questions that come with it of IF and WHERE storms will/could form. Currently numerous GEFS members do not convect along much of the DL on friday, but several do in addition to the OP-GFS convecting several dryline supercells within a high-end environment... Saturday looks roughly the same as Friday environment-wise (perhaps with low-level winds AGL being a bit more veered than the previous day) BUT convective initiation as of now appears to be much more likely along a large part of the dryline on saturday per most GEFS members and the OP-GFS showing more substantial QPF than the previous day. Something to note is there is still fairly substantial run-to-run variability on which day *may* bigger or look better, at this point it is important to highlight both days as having higher-impact severe potential. 

Bottom line at this point... Questions that could dramatically effect the threat-level are still abound... But as of now high-end levels of moisture and instability juxtaposed to a favorable wind profile will likely exist on both Friday and Saturday, or perhaps just one of the days, along and east of wherever the dryline sets up. As of now, population centers would certainly be within the threat areas of potentially high-impact storms.  

[1900hurricane]

Considering EML strength, Thursday, Friday, and Saturday are all at risk of a cap bust. I think the day most likely to convect is Saturday since it will probably have the strongest impulse in the 500 mb flow and the best low level moisture due to days of (possibly undisturbed) return flow. Wind fields don't look amazing, but could be sufficient in the presence of big time CAPE. Both the GFS (below) and ECMWF have been consistent in showing a scattered/widely spaced QPF signal by 00Z Sunday.

[bjc0303]

Having a pretty hard time seeing Friday cap bust at the moment. ECMWF does not convect, several GEFS/op member do. 

[CGChaser]

At this point my concern would be geographic location.. GFS in particular is slowing pushing the threat east into that's reaching nigh-unchaseable.

[bjc0303]

17 minutes ago, CGChaser said:

At this point my concern would be geographic location.. GFS in particular is slowing pushing the threat east into that's reaching nigh-unchaseable.

I feel like there should be a pinned post on GFS being too progressive generally on eastward placement of dry line, sfc features etc. Saw it last week, and if you pay attention long enough you'll see it's a constant theme of the GFS and its ensemble members which ALL exhibit a mixing bias.

 

NAM, ECMWF, and GFS (accounting for its bias) all point to southern, SE OK. Early in the period a dominant surface low will be moving east.. before it starts to fill as renewed cyclogenesis occurs with the arrival of a stronger speed max/impulse.

 

As this occurs, winds will back along the dry line. Most of the dry line looks to remain capped on Friday. However, there does appear across NAM/GFS forecast soundings that a window of opportunity will exist for supercell development in the area of convergence along the dry line/sfc low interface in east OK. Looks like a conditional risk for significant severe, as wind profiles and thermodynamics, even with a modest low level response, would look very favorable for supercells capable of destructive hail and perhaps a few tornadoes. 

[pbrussell]

1 hour ago, 1900hurricane said:

Considering EML strength, Thursday, Friday, and Saturday are all at risk of a cap bust. I think the day most likely to convect is Saturday since it will probably have the strongest impulse in the 500 mb flow and the best low level moisture due to days of (possibly undisturbed) return flow. Wind fields don't look amazing, but could be sufficient in the presence of big time CAPE. Both the GFS (below) and ECMWF have been consistent in showing a scattered/widely spaced QPF signal by 00Z Sunday.

Seen too many setups this year with fat cape but less than favorable wind fields not pan out. History seems to tell me it's better to have great wind fields with modest cape if you can't have the best of both worlds. Guess we'll see.

[jojo762]

12 minutes ago, pbrussell said:

Seen too many setups this year with fat cape but less than favorable wind fields not pan out. History seems to tell me it's better to have great wind fields with modest cape if you can't have the best of both worlds. Guess we'll see.

Directional shear in any number of forecast soundings picked out along the dryline on both Friday and Saturday is highly indicative/favorable for tornadic supercells with substantial veering with height... some question as to orientation and overall strength of LLJ on at least one of the days - but that won't be resolve until later on... perhaps some slight weaknesses in the 700-500mb layer also being indicated, but again this is something that won't really be nailed down until later. Effective shear is still forecast to be in the 40-60kt range in addition to ESRH of 200-400m2/s2, both of which are more than sufficient for strongly rotating mesocyclones with low-level rotation. 

[nrgjeff]

In late May high CAPE with modest upper level winds works out well. Wind fields are not the problem; it's the cap. Also, the LLJ and surface boundaries are a mess for Friday and Saturday. Finally, if the models are too far east, backing up to the I-35 corridor is a nightmare on weekends. Can't help the OKC circus, but an improving LLJ situation might break the cap with more authority.

[bjc0303]

1 hour ago, nrgjeff said:

In late May high CAPE with modest upper level winds works out well. Wind fields are not the problem; it's the cap. Also, the LLJ and surface boundaries are a mess for Friday and Saturday. Finally, if the models are too far east, backing up to the I-35 corridor is a nightmare on weekends. Can't help the OKC circus, but an improving LLJ situation might break the cap with more authority.

Would you care to explain more why you feel this way? I'm talking mostly about the bolded.

For Friday: modest response in the lowest levels, but still a pretty solid low-level wind profile. Strong storm relative winds through most of the profile. Cap will likely keep convective coverage extremely low Friday, but I certainly see no problem getting one or two supercells to initiate in the vicinity of the decaying surface cyclone and attendant dry line/front intersection where convergence will be maximized. 

For Saturday: Well it's a similar pattern, only perhaps less strongly capped. Same issue as above re: modest mass responses in the low levels, but still a sufficiently veering hodograph, only perhaps some weak storm relative winds in the 2-4 km layer... also uncertainties abound given possible enhancements or augmentations from prior day convection-related outflow.. But GFS/ECMWF highlight a severe-favorable parameter space in roughly the same areas as Friday. 

 

Both days (especially Friday) feature modest upper level forcing such that convective coverage should be low (Friday), or at least widely scattered (Saturday). Friday seems to me a very conditional day, but one with a bigger payout than Saturday. Given I live in Norman, I'll be there for both.

[Quincy]

Thursday looks like a severe threat in the western Kansas vicinity, even though boundary layer moisture may be modest, i.e. mid-50s Tds, and relatively weak 0-2km winds. Can't overlook a lower 990s mb surface low in the central high plains in late May. Analogs also show a fairly high incidence of Kansas severe reports with similar setups.

Beyond that, Friday and Saturday look like big CAPE days with solid potential, but too many details to work out to get focused on specific placement and severity. When you see high CAPE/modest shear setups as far south as as the southern Plains/Ozarks in late May, watch out. We know we've had some intense/violent tornadoes in such scenarios. 

[bjc0303]

1 minute ago, Quincy said:

Thursday looks like a severe threat in the western Kansas vicinity, even though boundary layer moisture may be modest, i.e. mid-50s Tds, and relatively weak 0-2km winds. Can't overlook a lower 990s mb surface low in the central high plains in late May. Analogs also show a fairly high incidence of Kansas severe reports with similar setups.

Beyond that, Friday and Saturday look like big CAPE days with solid potential, but too many details to work out to get focused on specific placement and severity. When you see high CAPE/modest shear setups as far south as as the southern Plains/Ozarks in late May, watch out. We know we've had some intense/violent tornadoes in such scenarios. 

Yes, pushing a dry line/remnant dry line feature east of I-35 in late spring is rarely good for places out that way.

[nrgjeff]

2 hours ago, nrgjeff said:

In late May high CAPE with modest upper level winds works out well. Wind fields are not the problem; it's the cap. Also, the LLJ and surface boundaries are a mess for Friday and Saturday. Finally, if the models are too far east, backing up to the I-35 corridor is a nightmare on weekends. Can't help the OKC circus, but an improving LLJ situation might break the cap with more authority.

 

1 hour ago, bjc0303 said:

Would you care to explain more why you feel this way? I'm talking mostly about the bolded.

...Both days (especially Friday) feature modest upper level forcing such that convective coverage should be low (Friday), or at least widely scattered (Saturday). Friday seems to me a very conditional day, but one with a bigger payout than Saturday. Given I live in Norman, I'll be there for both.

Not really. I do agree there is hope; it's late May with high CAPE and moderate flow. I know the EHI, CAPE, helicity alone, and other indices look great, but there is more to analyzing low level shear.

[bjc0303]

3 minutes ago, nrgjeff said:

 

Not really. I do agree there is hope; it's late May with high CAPE and moderate flow. I know the EHI and other indices look great, but there is more to analyzing wind fields.

Then we must see different things, as I see hodographs favoring strong mesocyclone development when paired with strong-extreme instability, at least on Friday. 

850 flow is veered but still roughly parallel to remnant dry line such that detrainment won't be a huge issue, and with nearly westerly winds aloft, there is strong veering and strong SRW through most of the hodograph. That's just me.. and I do expect a storm or two to develop Friday.

[nrgjeff]

4 minutes ago, nrgjeff said:

Not really. I do agree there is hope; it's late May with high CAPE and moderate flow. I know the Hodographs, simulated soundings,  EHI, CAPE, helicity alone, and other indices look great, but there is more to analyzing low level shear.

I hope you are right. If the LLJ straightens out all of the other problems will go away. We'll see..

[David Reimer]

Another series of wasted potential, but that could be said for 2017's spring season as a whole from a chasing perspective. Getting down to 47 degrees tonight in Norman with a cold front pushing out into the Gulf as I type this. Thursday will probably feature some nasty severe weather in northeastern Colorado into western Kansas. Not much of a tornado threat the way it seems with garbage low-level profiles, but it looks like a rather nasty MCS will set up and attack Goodland. Friday, if the NAM is to be believed, has dews above 75 degrees east of a dryline in East-Central Oklahoma. SBCAPE values over 6500 with modest, but sufficient shear to support high-precipitation supercells. Low-level shear looks weak, but if that amount of CAPE verifies, there will be at least a low-end tornado threat. I haven't peaked ahead to Saturday, but I don't see why it won't underperform or feature a low-end tornado threat. Luckily for me I've been lucky enough to be on the 'mesoscale accidents' this spring, but most other chasers have not been. For those folks it has been a crappy spring tornado season. Still, if you're on the one storm that can do photogenically do it Thursday-Saturday, I suppose it will be a success. I'm just done being optimistic about any setup this spring, unless its in Georgia or the Carolinas... oh, and the Upper Midwest/Ohio Valley apparantly. 

[jojo762]

Any potential we had for a larger-scale/more widespread tornado threat seems to be dramatically diminishing, as of now... Nonetheless, still feels like a period, especially on Friday,  in which we will see one storm, or two, go berserk within this extreme instability/favorable (for supercells) directional shear environment. Friday looks to have a better shot at this as SFC winds will be backed near the triple point, but low-level flow is progged to be fairly weak in addition to concerns over CI (but given the area of enhanced convergence near the triple-point, I like the chances of CI near there), nonetheless low LCLs and big-league CAPE could accommodate-- to an extent-- for that lack of stronger low-level flow. As has been mentioned numerous times, we have seen in the past what these high CAPE environments are capable of, not that those type of results will be seen this time, just highlighting potential.  Still a lot of uncertainty regarding Saturday, so will refrain from much detail on it, but CI looks more likely however have seen a  trend toward SFC winds being more veered/ nearly paralleling the front in NE OK/SW MO, with perhaps some more southerly flow in SE/SC OK - wait and see. 

[nrgjeff]

Thursday Palmer Divide. Friday Raton Mesa. Saturday will go but perhaps in chaser challenging areas such as the OKC Metro or Ozarks.

Outside of Colorado Thursday is such a waste of a lifting warm front and excellent turning with height. JIT dews might work in late May vs April, but this is NOT in time dews. Unbelievable! Thursday I would try the Palmer Divide and early. With just moderate dews, cool pool may undercut.

Friday should be capped in Oklahoma. I know the GFS has storms but I believe by afternoon strong WAA is back in effect at the mid-levels, but no LLJ. Try Raton Mesa or maybe southwest Kansas. Friday looks like an easterly flow day not really a DL day.

Saturday will go, but so many details to work out. This year if something can go wrong it will. Plus it's the Saturday of Memorial Day weekend in central Oklahoma!

[bjc0303]

I will say, one thing we can probably be pretty damn sure about for Saturday is moisture, and instability. The generally conservative ECMWF forecasts low-mid 70s dewpoints east of the OK dry line. The GFS and NAM depict mid-upper 70s, likely an overforecast. However, SREF mean forecasts of surface dewpoints east of the dry line Saturday suggest middle 70s dewpoints likely (seeing means of 75 or so).

[Weatherdemon]

How meteorologically similar is Saturday to May 10, 2008 when just a few storms exploded and an EF4 wiped out most of what was left of Picher, OK?

[Quincy]

59 minutes ago, Weatherdemon said:

How meteorologically similar is Saturday to May 10, 2008 when just a few storms exploded and an EF4 wiped out most of what was left of Picher, OK?

While there are some broad upper level similarities, the approaching shortwave was more robust. There was also an extensive warm front draped from OK to GA, where widespread severe/tornado reports where seen. The atmosphere over OK was largely uncapped by midday.

Despite a stronger cap progged for this time around, relatively high confidence in extreme buoyancy, i.e. CAPEs over 4000 J/kg, would still highlight a threat for something significant going up. In fact, delayed initiation would further allow the atmosphere to destabilize to high-end values given the time of year and location. 

The key difference here is that the setup looks more strongly capped and significant severe will be more conditional, pending mesoscale details. 

[Weatherdemon]

3 minutes ago, Quincy said:

While there are some broad upper level similarities, the approaching shortwave was more robust. There was also an extensive warm front draped from OK to GA, where widespread severe/tornado reports where seen. The atmosphere over OK was largely uncapped by midday.

Despite a stronger cap progged for this time around, relatively high confidence in extreme buoyancy, i.e. CAPEs over 4000 J/kg, would still highlight a threat for something significant going up. In fact, delayed initiation would further allow the atmosphere to destabilize to high-end values given the time of year and location. 

The key difference here is that the setup looks more strongly capped and significant severe will be more conditional, pending mesoscale details. 

Thanks for that man!

[jojo762]

Watching storms explode shortly after CI on Saturday will be fun... MLCAPE values approaching nearly 7000J/KG on the 12z NAM across northeast OK by 00Z. Degree of CINH should help storms stay discrete, i'd think, but the dominant storm-mode is still TBD as some uncertainty in what the wind profile will look like exactly still exists. Level of veering throughout the column will have a dramatic effect on storm-mode and attendant threats. TSA currently highlighting potential for significant severe in their AFD, OUN also highlighting potential for giant hail. 

[brettjrob]

The LLJ Saturday is not only strongly veered (WSW), but fairly anemic by 00z along the DL, which puts a big question mark on the likelihood of multiple tornadic sups (provided multiple storms initiate). Since all signs are pointing toward a DL along and E of the I-35/I-44 corridor, super veered H85 isn't a dealbreaker, but it's not ideal. Days like 5/10/08 and 5/21-22/11 do come to mind as examples of the ceiling, consistent with a maximized threat from around Ardmore to Joplin. Just not sure yet whether the LLJ will be strong enough to approach that territory.

Overall, though... it's late May, low-mid 70s Td's, and I tend to agree with the general idea that drylines punching east into this region with extreme instability this time of year have an ominous history. Unless it's a cap bust, at least a few tornadoes appear probable, and sigtor's can never be considered a surprise in this type of environment.

[Radtechwxman]

1 hour ago, brettjrob said:

The LLJ Saturday is not only strongly veered (WSW), but fairly anemic by 00z along the DL, which puts a big question mark on the likelihood of multiple tornadic sups (provided multiple storms initiate). Since all signs are pointing toward a DL along and E of the I-35/I-44 corridor, super veered H85 isn't a dealbreaker, but it's not ideal. Days like 5/10/08 and 5/21-22/11 do come to mind as examples of the ceiling, consistent with a maximized threat from around Ardmore to Joplin. Just not sure yet whether the LLJ will be strong enough to approach that territory.

Overall, though... it's late May, low-mid 70s Td's, and I tend to agree with the general idea that drylines punching east into this region with extreme instability this time of year have an ominous history. Unless it's a cap bust, at least a few tornadoes appear probable, and sigtor's can never be considered a surprise in this type of environment.

I couldn't have said it better myself. Extreme cape can really do incredible things to supercells. We seen it time and time again. Def not a fan of the veered LLJ and SW surface winds however if we get an OFB in that mix it could certainly spell trouble. This has a high ceiling but def plenty of flaws. I feel like a mesoscale accident will be needed to see something very significant at least from a tornadic standpoint.

[bjc0303]

NAM forecast soundings exhibit extreme instability, even when accounting for a likely overforecast in surface dewpoints.

Agree with plenty posted above. Would like to touch on a few things in particular:

A veered low-level jet east of I-35 is not only not that much of an issue (typically), but also extremely common. S or SE 850s east of I-35 are pretty rare, from my experience. With winds WSW aloft, a veered 850 flow is not a dealbreaker at all.

What might be a deal breaker seems to be the low-level shear vectors. I'm not seeing dramatic turning of the shear vectors with height, which suggests to me that storm splits are probable, although NAM does show some regions more favorable in the low levels regarding shear vectors. Deep-layer shear vectors seem to lie at a 45 degree angle to the dry line, maybe less in some locations. Anvil level, SR flow is pretty solid, and would likely favor discrete storms. Could see a mixed mode event, with supercells evolving into clusters by/after 00z. Significant severe looks likely, especially with discrete supercells. As brett said.. Tornadoes do seem probable, but exact degree of tornado potential probably won't be solidified until much closer to the event.

Models have been inconsistent on surface features, and this will also be impacted by overnight MCS activity, so hard to gauge true tornado potential this far out. Overall, I live in Norman, so I'm excited. If I lived farther away, it'd be a little more complex... But living in Norman, Saturday is a no-brainer.

[jojo762]

25 minutes ago, bjc0303 said:

NAM forecast soundings exhibit extreme instability, even when accounting for a likely overforecast in surface dewpoints.

Agree with plenty posted above. Would like to touch on a few things in particular:

A veered low-level jet east of I-35 is not only not that much of an issue (typically), but also extremely common. S or SE 850s east of I-35 are pretty rare, from my experience. With winds WSW aloft, a veered 850 flow is not a dealbreaker at all.

What might be a deal breaker seems to be the low-level shear vectors. I'm not seeing dramatic turning of the shear vectors with height, which suggests to me that storm splits are probable, although NAM does show some regions more favorable in the low levels regarding shear vectors. Deep-layer shear vectors seem to lie at a 45 degree angle to the dry line, maybe less in some locations. Anvil level, SR flow is pretty solid, and would likely favor discrete storms. Could see a mixed mode event, with supercells evolving into clusters by/after 00z. Significant severe looks likely, especially with discrete supercells. As brett said.. Tornadoes do seem probable, but exact degree of tornado potential probably won't be solidified until much closer to the event.

Models have been inconsistent on surface features, and this will also be impacted by overnight MCS activity, so hard to gauge true tornado potential this far out. Overall, I live in Norman, so I'm excited. If I lived farther away, it'd be a little more complex... But living in Norman, Saturday is a no-brainer.

Highest tornado threat would probably reside in two different regimes. 1. Near any weakly-defined/quasi triple-point that may develop where surface winds may be at least slightly more backed, and 2. with any residual outflow boundary left over from overnight/morning convection. Both of these are details that will not be resolved until the day-of. Thermodynamics will be overwhelming favorable for low-level stretching with widespread 0-3KM CAPE of 100-250J/KG, so you can imagine what any left over vorticity pool or OFB could do with that... Lastly, 100% agree with the splitting storms idea as hodographs much above 2KM, in even the best of the various point forecast soundings, are straight line. Likewise, any left-splits (in addition to right movers) are likely to be prolific hail producers with the impressively steep lapse rates of 8.0-9.0C/KM helping result in the extreme instability, SHIPS parameter is highly indicative of giant hail as well. As details evolve, if they evolve in a favorable direction of course, wouldn't be surprised to see a MDT risk for very large hail at some point. 

[andyhb]

Not used to seeing this amount of instability juxtaposed with strong deep layer shear (40-60 kts depending on location). Hail is obviously my first thought when seeing the level of moisture being suggested underneath close-to-dry adiabatic mid level lapse rates. Depending on the level of veering with height and given the strength of the mid level jet, I certainly could see a derecho materializing out of this later on as well.

Seems like a pretty broad area of sig svr potential from the Plains eastward to the Ohio Valley, but I certainly wouldn't want to be chasing in the Ozark Plateau with 3"+ hail waiting to destroy my vehicle waiting in basically every stronger updraft that goes up.

[janetjanet998]

4 hours ago, andyhb said:

 

Seems like a pretty broad area of sig svr potential from the Plains eastward to the Ohio Valley, but I certainly wouldn't want to be chasing in the Ozark Plateau with 3"+ hail waiting to destroy my vehicle waiting in basically every stronger updraft that goes up.

might as well make this the main severe thread even for the OH valley portion

The SPC filtered storm reports from this could be very high....giving the potential for huge hail and a few tornadoes and wind later over a wide area with decent population

I think the highest I have seen in one day was just over 1000...from what ended up as a huge fast moving derecho in June..but that was still when they were counting 3/4 inch hail reports as "severe"(now 1 inch)..

 

 

[melonknee]

34 minutes ago, janetjanet998 said:

might as well make this the main severe thread even for the OH valley portion

The SPC filtered storm reports from this could be very high....giving the potential for huge hail and a few tornadoes and wind later over a wide area with decent population

I think the highest I have seen in one day was just over 1000...from what ended up as a huge fast moving derecho in June..but that was still when they were counting 3/4 inch hail reports as "severe"(now 1 inch)..

 

 

http://www.spc.noaa.gov/climo/reports/110404_rpts.html

 

If I'm not mistaken, that was the most severe reports ever received by the SPC in one day

[Chargers09]

Any thoughts on Sunday?