8/24/05: FCST: Central Plains

[nickgrillo]

A trof should move into the central/northern plains and into ample deep moisture, particularly from western NE and south into KS. While absolute flow is pretty weak in the mid-level, strong directional shear is yielding moderate deep-layer shear sufficiant for supercells. Both NAM/GFS showing the system moving into a rather JUICY airmass (65-75 Tds) in NE/KS, resulting in extreme instability of >4000 j/kg across western NE, and decreasing slowly southward from there. Low-level flow is pretty strong -- despite the weak mid-levels, the atmosphere is pretty prime for severe thunderstorms (including supercells).

Strong backing surface winds ahead of the boundary, veering to the south at 850mb, then strongly veering to the w/nw at 500mb (yet, at a rather weak 20-30kts)... However, despite the weak flow at 500mb, the flow present from the surface to 700mb is resulting in moderate low-level shear, with 0-1 km SRH >250 m2/s2 across much of NE. It is way too early to pindown a target area, but at this point, I'd have to say somewhere in western/northern NE.

Man, I wish I could get out this week.....

 

[Philip Flory]

I think the setup in KS tomorrow is worth keeping a close eye on as well. Weak mid-level winds (25kts) is the only major negative by late afternoon along boundry which will likely be somewhere across C KS. Weak cap along with SBCAPE values of 2500-3000 J/KG should contribute to sufficient instability for organized convection. Enhanced low-level shear/SRH to the north of frontal boundry in C KS may provide an enhanced tornado threat givin doable LCL's.

It's really early for a target city but I would go with Great Bend for now. SPC also has a 25% for tomorrow over the same general area. I am keeping a very close eye on tomorrow for a likely chase.

 

[Justin Turcotte]

NAM and GFS both seem to hit on Kansas but there are enough differences in the runs to make target placement difficult. NAM is further south with instability axis and appears to focus best surface convergence just east of the OK Panhandle. Southwest KS looks to be in the righ rear quad of the jet streak where lift will be enhanced. Looking further north both models agree on timing and placement of prefrontal trough in central NoDak. Surface flow appears to be sufficiently backed ahead of the fetaure and there is ample turning with hight through H50. The guts of the upper flow remain west of the trough. The usual caviats for ND is how much moisture gets advected in along with amount of solar in WAA regime. There will also be a cap, albeit relatively weak, with H70 temps in the +10 to +12 range.

 

[Simon Brewer]

If I had to make a forecast now I'd be in the Oklahoma Pan/SW KS Wednesday, but I can't go out on Wednesday. The combination of forecasted deep-layer shear, 20 knt southerly 850mb winds, 25 knt SW 700mb winds, and relatively strong instability will provide a good environment for supercells, and the tornado potential would be very good if a supercell were to ride any lingering outflow boundaries. Also, right entrance region of 50 knt 300mb jet is a good place to be. Too bad I can't chase on Wednesday.

 

[Amos Magliocco]

I'm still working through this, but a few things about tomorrow in Kansas caught my attention after I saw the slight risk on the SWODY2. I haven't finished my process here so I'm sure the fatal flaw is yet to be revealed.

However, midlevel flow is progged around 20-25 knots out of the west southwest, with SR anvil level winds around 25-30 knots on the western periphery of central and south central Kansas. Deep layer shear isn't great, but some 0-1k turning and a modest LLJ yield SRH values of ~100-150 m2/s2. These are interesting because it appears possible storms will fire in and around outflow boundaries generated by covection from earlier in the day, or which may be still ongoing per the newest SWODY2.

In addition, the current weak surface front currently stationary over southern KS (see backed flow on sfc chart) should still be hanging around.

As for that convection, the 40k NAM moves the rain shield well east of a potential target region by 21z. I think there's a chance that an outflow boundary could spend some time in the sun prior to later, more isolated thunderstorm development.

There are plenty of problems, like increasing cap strength and subsidence in the wake of the earlier disturbance. But moisture and instability are not a problem, and the shear values are interesting for August. As others have mentioned, the positiion relative to the upper jet is also favorable.

 

[Jeff Snyder]

Personally, I'm on the fence for tomorrow. Instability and moisture look pretty good, but the mid-level flow is paltry (20kts at 500mb), which helps yield pitiful 0-6km deeplayer shear values of 15-25kts (per the NAM --> http://128.121.193.153/CENTRAL_ETA_250M-6K...VECTOR_36HR.gif ). Now, it was 5000 CAPE and stronger low-level shear, then I'd be more optimistic. Otherwise, given weak mid-upper level SR flow and abundant moisture in the low-levels, I can't help but think that HP will be the preferred supercell type should the shear even be sufficient for supercells.

As Amos and others have noted, lee troughing will aid in the development of moderate 850mb and 700mb flow (15-20kts at 25-30kts, respectively) across most of the area-of-concern. Nice directional shear between the sfc and the mid-levels yields a profile that is supportive of supercells, but those mid-level wind speeds are making me wince. I suppose it could be worse...

IF there is a good OFB to ride along, I think we may have a good supercell develop over sw KS or extreme nw OK. Discrete supercells on boundaries can always make magic happen, so I'm not giving up yet. I just hope that the wave is stronger than forecast to help increase the midlevel flow and semi-associated deeplayer shear. Heck, Gabe and I got a nice supercell out of last Thursday's setup near Pratt, and that setup wasn't TOO much unlike tomorrow's. So, I suppose we shall see...

 

[Bill Tabor]

I'm not going to go into a lot of detail ( I didn't look that hard), but I pretty much agree with Jeff. Only looking at the southern area (around KS) I see some potential for supercells along the KS/OK border area. Each side has different things going for it (meaning neither really have all). KS gets better upper winds whereas OK has better Cape. Mid levels suck. Low levels are marginal. I don't see much short wave activity. Still near the supposed boundary close to KS/OK nearing 0Z the area (according to NAM will be sporting over 6 EHI, and -6 LI's. Various tornadic parameters I looked at do indicate potential although I don't believe them too much. I think they are many picking up the cape and helicity. I am down on the tornado potential for this part of the US on Wed during the day.

I'd look for a few severe storms forming near this area probably before 0z assuming convection can break out. KS looks pretty capped, and without much jet support, or midlevel activity plus low sfc convergence it may be relying on day time heating to break out. Large hail is probable in some storms, possible Hp's mainly south of KS (due to KS having better upper winds). Storms in northern OK may have potential for microbursts and wind events.

 

[Jeff Snyder]

The 0z NAM solution from tonight actually makes me more optimistic for tornado and supercell potential across the OK/KS border region tomorrow. While the mid-level flow will still be weak tomorrow afternoon, the NAM is picking up on an OFB stalling along the OK/KS border. This in turn has lead to the NAM forecast of strong low-level shear / SRH along the boundary. Given the shear profile, storms will likely move east-southeast, which may very well be nicely parallel to the boundary. The abundant moisture will aid in the development of strong instability by afternoon (NAM forecasts of >3000 j/kg SBCAPE), which may make for an interesting situation tomorrow. Depending upon the timing of subtle forcing mechanisms, tomorrow afternoon may be game actually. As should be noted, thing will almost be entirely dependant upon the final position of any OFBs from the ongoing convection across extreme nw OK into southwestern KS. Storm mode may be supercell IF we do get the 30kts 0-6km shear progged by the latest NAM, and I'd still bank on HP mode given the relatively weak mid-upper level SR-flow and abundant moisture which should lead to plenty of updraft water-loading. Forcing on OFBs almost always seems to be finicky, and without much midlevel support, the areal coverage of convection is certainly in question. If only we had more than 25kts at 500mb...

 

[Amos Magliocco]

Agree with Jeff on mode and the precarious nature of weak winds aloft. Still, I can't rule out something on a forward flank meso tracking along an outflow boundary given strong instability and favorable 0-1k shear values. I don't expect anything long-lived and an MCS should be underway probably ninety minutes into the event, if not sooner.

This morning's surface conditions are encouraging. Low clouds are clearing out of southwest Kansas in the wake of the MCS tracking east, and the reinforcement of the baroclinicity is more dramatic than I expected. At 1133z, the temp at Pratt was 66 and Alva was 73. Consistent easterly flow along and north of the boundary as well. I wish we could cheat another five (or ten!) knots aloft, or even five at 500mb and five at 850mb would make me feel a lot better.

All the same, I think it's shaping up nicely for discrete rotating storms, at least for a small window, and that's enough for me in late August. Again, I think the surface is encouraging attm.

 

[Glen Romine]

Can't really find anything I don't agree with in the latest SPC outlook and as noted above. Mid-level flow in KS/OK is just pathetic - almost as much so as the area soundings showed in terms of lapse rates. The more interesting area looks to be in the Dakotas - where impressive lapse rates and adequate moisture should lead to favorable instability, and a weak 'feature' evident in the WV imagery may be sufficient to trigger a few storms by mid afternoon. Weak storm-top winds don't help - but still think there is reasonable potential for a tornado or two across north central SD. Hope someone will be out there today.

Glen

 

[Bill Tabor]

Will note that as of 3pm cdt SPC Mesoanalysis Ruc based fields are showing up to 6000 cape in northwestern OK and LI's up to -10. This is quite a bit higher than last night's NAM was forecasting. Additionally I see SCP at 10 and even sig tor at 1 indicating it thinks some potential for tornado develpment. I'm still not overly optimistic about torns, but the stronger cape certainly helps if those magic boundaries from earlier convection do occur and interact. Very weak 300mb winds over OK and fairly low Lcl's with AVK showing a sfc spread of 84/81 and 10 from the se. Cu building south of AVK as well. I expect precip to break out in the next few hours here resulting similar to what I described previously.