8/17/05: FCST: Central/Northern Plains

I'm definitely seeing some supercell potential in the western halfs of KS/NE on WED. While the mid-level winds are a bit weak, Tds in the upper 60s to mid 70s in Kansas and Nebraska with temps in the upper 80s are yielding (according to the NAM) CAPEs in the 2500-3500 range. Additionally, a strong east-west Frontal interaction will enhance convection later in the afternoon. Continuous insolation should completely break the cap and increase instability.

Weak southwesterly flow through much of the threat region is yielding marginal helicity, however, more backed flow exists in northern KS... A nice veering wind profile above the surface, so low-level shear is VERY sufficent for supercells (KS/NE), despite the weak flow at 500mb. Overall - I'd say supercells and a tornado or two is possible, especially in KS/NE. This is a pretty good setup for this area, considering we are halfway through AUG here.

If I was chasing on WED, I'd be targetting NW KS/SW NE.
 
Upon review of the 00z model output I see a couple potential areas of interest. Both NAM and GFS advertise two H70 shortwaves in the flow regime; one in ND, the other in NE. Both models are in pretty good agreement on the timing and placement of the shortwave in western NE. There appears to be enough speed and directional shear ahead of the wave for supercell potential. Surface convergence also looks good. I don't see moisture being a problem as there is plenty of juice in southeast KS and OK to advect. The other area of interest is northeast ND/northwest MN (outside the scope of the thread title). Both NAM and GFS advertise a very robust H70 shortwave with the GFS taking it to the max with monster hodographs, a result of accelerating low to mid level winds. The area will be situated within the right rear quad of a fairly strong jet streak further enhancing lift. The models have some timing differences but are similar in magnitude. The primary caviats here will be quality of moisture and solar... otherwise one of the better synoptic set-ups I've seen in the far north this summer.
 
The latest 0z model runs are making a trip to northwestern KS a little enticing. Decent upper-level southwesterly flow (60-70kts) across northern KS and into NE and moderate westerly 500mb flow will result in sufficient lee troughing to yield 20-30kt 850mb southerly flow. As the situation currently appears, wind profiles will be favorable for supercells. The lee troughing will result in good moisture return, and >70 Tds should be into nw/nc KS and southern NE by afternoon. This moisture, combined with temps near 90, yields strong instability and "doable" LCLs heights. I haven't looked at this much, so I won't go into any more detail, but it's caught my eye.
 
I expect the genesis region to be along the dryline in KS/NE during the afternoon on WED, which will most definitely be supercellular. Winds back at the SFC, which is combined with a west-northwesterly 500mb jet of >30kts over a widespread area, particularly southern NE/northern KS (where it's in the 40KT range). There is very strong directional shear through the troposhere -- from SSE winds at the surface, to W-NW winds at 250mb. This, at least per the NAM, yield +40KTS of deep-layer shear, which when combined with 3000-4000J/KG MLCAPE, will likely prove sufficent for long-lived supercells.

This is by far the best setup across this area since early June, with a JUICY airmass (that's a meteorological term...) in place - and given such strong wind fields for this time of year, would lead to believe that there is potential for a few strong tornadoes, particularly in NW KS, along the dryline.
 
What isn't enticing is the ETA forecasted winds at 850 and 700mb... 15kts or so at both levels. Yuck! I'd wouldn't place any bets on seeing tornadoes... let alone strong tornadoes. Low level wind shear is very important... something that is lacking in this setup. 0-1km helicity is basically non existent due to the lack of winds at the lower levels.

Aaron
 
What isn't enticing is the ETA forecasted winds at 850 and 700mb... 15kts or so at both levels. Yuck! I'd wouldn't place any bets on seeing tornadoes... let alone strong tornadoes. Low level wind shear is very important... something that is lacking in this setup. 0-1km helicity is basically non existent due to the lack of winds at the lower levels.

Aaron

Well, I'd somewhat have to agree on the helicity...

Regardless, I'd think that this setup is pretty favorable, considering its AUG. Heck, 7/12/04 had SW Winds@<15kts at 850mb, and weak mid-upper level winds - yet, it still managed to kick out strong supercells, with one of them producing a violent tornado.
 
Regardless, I'd think that this setup is pretty favorable, considering its AUG. Heck, 7/12/04 had SW Winds@<15kts at 850mb, and weak mid-upper level winds - yet, it still managed to kick out strong supercells, with one of them producing a violent tornado.

True, but that was due to strongly deviant storm motion. As the name suggests, storm-relative helicity depends upon the motion of the storm. So, even in environments characterized by weak shear, a strongly deviant storm motion can locally "create" shear sufficient to produce tornadoes. So, tomorrow, there will need to be some strongly deviant storm motion to maximize the hodographs if there is any hope of tornadoes.

I'm somewhat disappointed by the 12z run of the ETA, which significantly veers the surface flow almost everywhere. Since the (ambient) shear is somewhat lacking anyway, it will be even more difficult to eek out a tornado or two in this environment. The good news, though, is that the boundary-layer will be characterized by a very juicy tropical airmass (check out the 12z OUN sounding). I have no doubt that supercells will be had tomorrow given the reasonably strong deep-layer shear coupled with the strong-extreme instability. The question is, will there be enough low-level shear to warrant anything more than a marginal tornado risk? We shall see.

Gabe
 
Well, this event certainly has the potential, assuming that the NAM at least mostly verifies, to be a pretty good severe weather event... Rich boundary layer moisture will stream into the area of concern along and ahead of the trof/dryline in western NE/KS. NAM is forecasting CAPEs in the 1500-3000J/KG range across of the forecast area (KS, NE, SD). LCLs should be sufficiently low, with T-Td deficits only in the 10-15 degree range. Deep-layer shear will be pretty strong at around >40kts invof the KS/NE border, although low-level shear is not too impressive, it should still be sufficant enough to produce a few isolated tornadoes (main convective mode should be supercells) -- especially near the boundary, where there is enhanced low-level shear. Wind profiles very much support supercell development, and given the instability, giant hail is possible - along with some tornadoes.

I'm guessing that there will be a 5% tornado risk, with a 25% hail/wind across the rest of the threat area... As of this point, I'd target McCook, Nebraska.
 
What a difference a day makes.

Yesterday I was pessimistic about low-level shear, and while this is still somewhat of a concern, the degree of 0-1 km helicity forecast by the ETA (>100 m2/s2) over Nebraksa today is do-able in terms of tornadoes.

In the near term, there is some concern about how much instability can develop due to a fairly widespread area of clouds over the Plains. While I think this would be more detrimental to an early spring chase day, I think this can only help tornado probs on this late summer day because it will help to generate differential heating boundaries which will promote the generation of low-level vorticity (don't typically have as much trouble removing pesky cloud decks during the summer!). Also, ongoing convection will likely lay down a decent outflow boundary which will also serve to enhance the low-level vorticity.

With instability and moisture issues virtually a lock, I would expect that today will be a quality chase day across much of Nebraska. My current target is tenatively Saronville, NE (adjusted for wherever the possible outflow boundary sets up).

Gabe
 
Today still looks to have some potential - though maybe the focus for me at least has shifted a bit further south into KS based on morning review compared with favored region based on last night's NAM model run. Overnight/ongoing convection certainly helping in the low-levels - with a cyclonic circulation now taking shape over western KS and favorable trough/ridge pattern at the surface to promote somewhat backed flow at low-levels. Mid-level vort max over sw WY this morning continues to chug ese - and will hopefully aid in improving mid-level lapse rates, which are rather pathetic in 12Z soundings in the region. RUC guidance offers a mesolow developing over w KS today with a dryline bulge favoring a nw->nc KS area target (say around Wakeeney) - and ths appears to be supported by morning observations of increasing sw flow over sw KS. Still some issues to be sorted out - but see no reason to say that a tornado or two isn't at least possible later today at this point. Agree with the SPC 5% - though not quite with the location. Expect a little bit of shifting with the 1630 outlook.

Glen
 
What do y'all think about Southeast South Dakota? Right now we have the Turner County Fair going on, one of the largest in South Dakota. My dad is the Fire Chief for the host town, and I'm good friends with the county Emergency Manager, and they're both looking at me right now. I think the main threat will be large hail/damaging winds, but I need a second (or a third, or a fourth)) opinion on this.

Right now, I recommended that they close down the fair midway and the demo derby in the grandstand if there is a warning in McCook County (1 county west), so they have time to clear out the estimated 20,000 people, and they seemed to have liked that idea.

It should be noted that at any time, only about 2,000 could fit in the morton buildings that make up the fairgrounds, and 500 under the concrete grandstand. That was a big part of my recommendation.

Anyways, whattya think of southeast South Dakota?
 
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