Late April severe weather risk ~Mon thru next Mon 4/24-5/01

[SmokeEater]

Cell east of OKC is starting to look interesting... Headed toward Tulsa.

The one south of OKC too, line broke up into supercells.

[jojo762]

KINX (Tulsa) VWP fwiw. 

[jojo762]

Rotation appears to be increasing on the Garvin Co. OK supercell. Looks pretty classic on radar.

[Stebo]

Early returns on tonight's NAM, through 39 hr it is not as aggressive with the push into the gulf and the return flow is already starting in southern Texas.

[aurora]

Just a lot of heavy rain, frequent thunder/lightning, and a bit of pea-sized hail here in Moore tonight, but the convective evolution has been fascinating.

[Radtechwxman]

0z nam lacks one thing that continues to worry me about Fri. Little to no LLJ. It is displaced way east. That would def put a much lower ceiling on tornado potential

[CGChaser]

6 minutes ago, StormChaser4Life said:

0z nam lacks one thing that continues to worry me about Fri. Little to no LLJ. It is displaced way east. That would def put a much lower ceiling on tornado potential

Wouldn't try to overanalyze the NAM @72hrs that much. I do agree, however still nice low-level curvature for the threat. With regards to <48hr trends, as pointed out already, moisture returns a tad faster after FROPA in Gulf.

[jojo762]

5 minutes ago, CGChaser said:

Wouldn't try to overanalyze the NAM @72hrs that much. With regards to <48hr trends, as pointed out already, moisture returns a tad faster after FROPA in Gulf.

In this case I do not feel like he is over-analyzing anything, it is a legitimate concern for Friday.

[CGChaser]

17 minutes ago, jojo762 said:

In this case I do not feel like he is over-analyzing anything, it is a legitimate concern for Friday.

I mentioned it's a concern, they're right. There are multiple concerns.. as with every system. But to me, at least this run, the NAM trended in favor a greater threat for severe wx than previous runs. All I was pointing out.

[jojo762]

Interesting part-- and probably should be taken with a grain of salt-- about this NAM run is that it starts crashing the cold-front on saturday morning... boundary ends up much further south than any other model at 12Z.

[brettjrob]

The lack of a robust LLJ at 00z Sat on the NAM is worrying because it's symptomatic of a real problem, perhaps the chief problem even: the second shortwave arrives about 3-6 h too late for primetime. This is true on the current model consensus, not just the NAM. At 00z Sat, it's clearly evident as a kink in the H5 height field over the TX Panhandle, which is 100-200 mi. W of the forecast extent of the warm sector. I have a bad feeling we may be looking at CI around dusk or later, unless we see changes that are fairly substantial for a 72 h lead time. Certainly still possible, though.

[bjc0303]

It's been a pretty consistent issue, too. GFS and ECMWF(and now the NAM) are just too late with substantial cyclogenesis.

[Jim Martin]

Would the tornado potential be lower given all these potential issues for this weekend?

[CGChaser]

Ya I gotcha, thanks. Good points, long day

[Quincy]

Tomorrow looks like a waste too with fairly strong odds that a squall line develops by midday/early afternoon and plows through the Arklatex to lower to mid-Mississippi Valley region. QLCS tornadoes certainly remain a possibility, but not much discrete activity is expected despite strong instability, >50kts deep layer shear and sharply backed low-level winds. (In some of the worst chase territory as well)

[Radtechwxman]

22 minutes ago, Jim Martin said:

Would the tornado potential be lower given all these potential issues for this weekend?

If all these issues came to fruition than definitely yes. Lack of a llj creates that weakness in the hodo that brett just mentioned. In regards to moisture I'm not as worried anymore. But capping, height rises, late jet arrival all spell trouble for even a low end tornado event. I mean this system is powerful and it wouldn't take much to reverse these trends. Only time will tell if these issues can be resolved. I really hope come May we get more spacing in between shortwaves because these great troughs being wasted is a shame esp with the Gulf so primed

[jojo762]

I'll take this... NW of DFW at 7pm Friday. This run actually shows the LLJ beginning to develop by 00Z, which is better than other runs.

[bjc0303]

Yeah, 00z GFS is a little more forgiving on timing and as a result low level mass responses begin a little earlier -> better late afternoon/eve shear. Still, this process happening a little earlier in the day would definitely help 1) bring the moist sector farther north, as the ECMWF does, and 2) increase low-level wind shear. Interested in tonight's 00z ECMWF.

[aurora]

1 hour ago, aurora said:

Just a lot of heavy rain, frequent thunder/lightning, and a bit of pea-sized hail here in Moore tonight, but the convective evolution has been fascinating.

The final round of storms tonight featured a nice hail storm at 10:10 p.m. or so. Max hail size looked to top out around quarters, with perhaps a few half-dollars mixed in. All in all, a very nice thunderstorm day.

[Radtechwxman]

Right now I'm liking this area for any CI Fri evening. The best cape, moisture, and low level shear seem to be maximized in this area. If we can get storms to fire 23z-0z before CIN increases again then we could see a few very discrete intense supercells capable of all hazards. A llj on 0z gfs is just getting established at 0z from the newly developed low. If this could happen sooner than the ceiling would go up even higher. 

[jojo762]

Also FWIW, something we haven't talked about too much, the last several runs of the GFS and Euro have been producing what would likely be a fairly significant flash flooding and river flooding event for southeast Oklahoma, the western half of Arkansas, Southern Missouri, and into southern Illinois. Widespread 3-5+ inch rainfall with isolated areas receiving perhaps up to a foot of rain.

[brettjrob]

Canadian trying to keep hope alive (a.k.a., prolonging the agony) with the faster s/w, lifting the WF to I-40 by 00z Sat with 30-40 kt flow from the SE at H85.

[thewxmann]

1 hour ago, Jim Martin said:

Would the tornado potential be lower given all these potential issues for this weekend?

It wasn't that great to begin with, but that's because of these issues which have been hashed out well in this thread.

The 12Z Euro advanced the warm front further north because the lead s/w on Thursday is very suppressed on that run. However, it is an outlier compared to the other models. We'll see what 0Z says.

Initiation or no initiation, the meager LLJ will be a huge limiting factor even if we do get the WF far north. We'll be highly dependent on the LLJ getting a boost at dusk.

[JoMo]

Large and Extremely dangerous TOG in NE OK?

[thewxmann]

1 hour ago, brettjrob said:

The lack of a robust LLJ at 00z Sat on the NAM is worrying because it's symptomatic of a real problem, perhaps the chief problem even: the second shortwave arrives about 3-6 h too late for primetime. This is true on the current model consensus, not just the NAM. At 00z Sat, it's clearly evident as a kink in the H5 height field over the TX Panhandle, which is 100-200 mi. W of the forecast extent of the warm sector. I have a bad feeling we may be looking at CI around dusk or later, unless we see changes that are fairly substantial for a 72 h lead time. Certainly still possible, though.

I could end up being wrong on this, but I'm not too concerned about CI. NW TX is on the favorable right entrance region of the upper-level jet streak and boundary layer moisture is rich, which helps with parcel buoyancy all else equal. The subsidence behind the initial Thursday s/w lifts out about at the right time, around 18Z. EML's have been manageable this year and for this particular system the EML source region is not too far south to advect in super hot temperatures in the mid-levels. I think the most likely scenario is supercells initiate but don't produce tornadoes, to the disappointment of every chaser who's drunk the hype cool-aid and scheduled a Friday chase.

[andyhb]

11 minutes ago, brettjrob said:

Canadian trying to keep hope alive (a.k.a., prolonging the agony) with the faster s/w, lifting the WF to I-40 by 00z Sat with 30-40 kt flow from the SE at H85.

UK should really appeal to most. Stronger H5 trough and more established cyclogenesis by 00z Sat with substantial pressure falls extending ENE along the warm front.

[Torchageddon]

1203 AM CDT WED APR 26 2017

...A TORNADO WARNING REMAINS IN EFFECT UNTIL 1230 AM CDT FOR
NORTHEASTERN MCINTOSH COUNTY...

At 1201 AM CDT, a severe thunderstorm capable of producing a tornado
was located 3 miles northwest of Stidham, moving northeast at 35 mph.

HAZARD...Tornado and baseball size hail.

SOURCE...Radar indicated rotation. The public reported baseball size
         hail at Tiger Mountain on I-40.

3 Tornado warnings and a mention of strong rotation in one of them, when its past most people's bedtimes.
 

[1900hurricane]

The 00Z GEFS mean looks very similar to the operational GFS solution, for what it is worth. The low level jet branches back to the west by 00Z over the axis of greater instability near the surface dry line in the vicinity of the Red River.

 

 

 

It might also be worth mentioning that parameters across several members of guidance look rather salty back to the east in the lower Mississippi Valley around the same time. There's no obvious surface trigger in that portion of the open warm aside from maybe the warm front to the north (largely parallel with the shear), but if cells can manage to get going in that environment, they could cause trouble.

[1900hurricane]

Can't say I expected the Day 1 Moderate Risk.

 

 

 

[div]

SPC AC 260548

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1248 AM CDT Wed Apr 26 2017

   Valid 261200Z - 271200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS THE
   ARKLATEX REGION...

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM NORTHEAST
   TEXAS TO EXTREME SOUTHERN MISSOURI...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM SOUTHERN
   LOUISIANA TO SOUTHEAST MISSOURI...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE UPPER
   TEXAS COAST TO NORTHWEST INDIANA...

   ...SUMMARY...
   Severe thunderstorms are expected across the Lower Mississippi
   Valley Wednesday.  Tornadoes, large hail, and damaging winds should
   focus from northeast Texas across southern Arkansas and northern
   Louisiana primarily during the afternoon hours.  Isolated severe
   thunderstorms may extend as far north as Illinois and into the
   central Gulf States during the overnight hours.

   ...Lower MS Valley...

   Early-morning water vapor/radar imagery depict a well defined upper
   circulation over northeast NM shifting east-southeast in line with
   late-evening model guidance.  High-level flow is increasing across
   the southern Rockies and 500mb speeds on the order of 75kt will
   translate into central TX later today.  This feature will induce
   significant mid-level height falls across the lower MS Valley and
   encourage strengthening LLJ over LA/AR by 18z.

   26/00z observed soundings across TX depict very steep mid-level
   lapse rates, on the order of 8.5-9 c/km, and this lapse-rate plume
   should spread across the Arklatex region prior to convective
   development.  Current thinking is thunderstorms will struggle to
   develop across much of TX due to significant CINH and this should
   allow air mass to destabilize across the MDT risk region as surface
   temperatures warm to near 80F with near 70F dew points.  Forecast
   NAM soundings by early afternoon depict SBCAPE in excess of 3000
   J/kg across northeast TX where convection should initiate prior to
   frontal passage.  Shear profiles strongly favor supercells, though
   mid-level height falls and slightly backed mid-level flow could
   eventually lead to a more complex MCS as the trough ejects toward
   the Mid-south after dark.  Very large hail could accompany the
   supercells across the Arklatex region and tornadoes appear possible.
   Strong/severe convection should spread into the central Gulf States
   during the evening/overnight hours as entrance region of upper jet
   shifts into this region during the latter half of the period.

   ...Mid MS Valley...

   Southwest-northeast oriented squall line has evolved from eastern OK
   into southwest MO early this morning.  Models suggest this activity
   should be ongoing at the beginning of the period but possibly a bit
   farther east than deterministic data would suggest.  This early-day
   convection will certainly disrupt and overturn buoyancy across much
   of the mid Mississippi Valley.  It's not entirely clear how unstable
   MO/IL region will be ahead of this activity and forecast soundings
   suggest poor lapse rates and weak buoyancy.  For these reasons have
   lowered severe probs across much of IL/MO due to limited
   instability.  Otherwise, forced squall line could certainly produce
   isolated severe wind gusts but the more robust convection should
   remain across the Ozark Plateau and points south.

   ..Darrow/Leitman.. 04/26/2017[/div]

[Quincy]

1 hour ago, 1900hurricane said:

Can't say I expected the Day 1 Moderate Risk.

The setup reminds me a little (only distantly similar) to 11/17/15, where a squall line 10% tornado outlook busted horribly. The key difference here is substantially better buoyancy is forecast thanks to steeper lapse rates in the 700-500mb layer. We're also talking about different times of year...

The theory makes sense: Substantial squall line evolves by early afternoon across Arkansas and due to residual capping and pronounced deep layer shear, isolated, robust convection develops on the southwestern flank of the convective system into Louisiana. Should convection remain discrete here, significant tornado potential should be fairly high given the parameter space, despite some kinking of hodographs in the mid-levels. 

I fully expected a 10% tornado zone, but the closer I look, the 0-2km wind fields appear very favorable for tornadoes by 18-21z over northern LA/southern AR. Also, the latest HRRR runs may be too overzealous in southward extension of the current Ozarks vicinity convective system in the near-term, allowing for better destabilization of the boundary layer. Assuming instability progs aren't too overdone, any discrete/semi-discrete convection that initiates near the Arklatex early this afternoon should rapidly become supercellular.

Overall, the tornado-driven moderate risk seems a bit conditional, but can be justified.