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05/17/05 FCST: Central Plains

GFS is showing a 994mb low over western KS at 0Z Weds with 30kt southerly surface winds over central/eastern KS and NE. 500mb winds are divergent at 40kt from the southwest in the same area but strongest in northern NE/SD. GFS also has precip breaking out in central NE with low 70Tds in most of central NE. I think Tuesday could be an interesting chase day. :)
0Z NAM showing a 992mb low pressure in NW Kansas and a greater area of SRH. 500mb winds are 30kts over most of the area but 60 Td's right in front of the low and some areas of 65. Seems like the NAM has been a little aggresive lately with the moisture return. I think NC KS, SC NE looks pretty promising.

We already have very good model similarities between the 12Z NAM/12Z GFS, they both pinpoint the same 994mb upper level system in the same place! How's that for 84 hours?

However, we have some model differences in the dewpoint temperatures, as the spread between the NAM and the GFS are in the range of 5-10 degrees at the moment, the NAM being a little more conservative than the GFS. The NAM predicts anywhere from the upper 50s to the mid 60s in the area, while the agressive GFS has the area in the upper 60s. Upper level support is somewhat weak by the NAM model at around 30-40 knots at the 500mb level, while the GFS shows stronger southwesterlies with a peak (50+ knots) in northern Nebraska and southern South Dakota. NAM shows an instability bullseye in SC Nebraska, looks like in the area of Grand Island/Kearney (have they had enough severe already?) Modest helicity will exist in the area via the NAM model of anywhere from 200-300 m2/s2. GFS also shows lower surface temperatures in combination with higher dewpoints which would equally as well support low level thunderstorms (surface based). Should the GFS model pan out, and moisture is higher, central Nebraska running up to South Dakota stand a better chance of severe weather than if the NAM verified. Let's hope the GFS verifies!

Graphic for this forecast found on the "My Chase Forecasts" link below.
I am fairly pleased with the way Tuesday is looking on the 00Z run. As of right now I am focusing on the area from the Nebraska/Kansas border South to central/southern Kansas along the dryline. The NAM does have precipitation across the target area along the dryline in Kansas by 00Z. Shear could be an issue with the NAM currently forecasting around 30kts, but I am banking on this improving a little. Dewpoints could be better. It is currently showing 60's stacking up along the dryline down to central Kansas. CAPE bullseye will be just North of the Kansas/Nebraska border at 2000-3000J/KG with 2000-2500J/KG available in front of the dryline in Kansas. Surface winds are forecast to be at 20-30kts out of the SSE along with great 850mb winds out of the South at 40-50kts. The big problem is the mid and upper level winds, but I an hoping they will improve. I am thinking that the good directional shear and better speed shear over central Kansas combined with relatively lower LCL heights should provide the best shot of tornadoes on Tuesday.
Seems mother nature favors Nebraska this year, or is frustrated with Nebraska, depending how you look at it. Given the ridge that looks to be setting in, if the current NAM run gets a little better, I'll probably take a day off to chase. It'd be dryline storms, too, which is a bonus. Right now there just doesn't seem to be enough shear; 30kts probably wouldn't cut it. Maybe it'll get better.
Looks like an interesting setup. Lack of significant deep layer shear may make the tornado threat pretty nominal. However a negative tilt trough ejecting into the Plains always catches my eye.

Thunderstorms should initiate along dryline in Central/Northern Plains by evening. Despite SRH values climing due to nocturnal intensification of the LLJ...BRN shear values suggest that supercells may be hard to achieve and that outflow dominated multicell clusters would be the dominant mode. Can't rule out an isolated supercell or tornado but main threat appears to be large hail. Unsure if it would be worth chasing. High potential for dents in car versus low potential of tornadoes and even lower potential of any significant tornadoes doesn't seem worth it. In addition, LCL heights will be remarkably high closer to initiation point when discrete modes are more likely. They drop down in E NE/E KS but by that time linear modes should dominate and it will be into the overnight hours.

Perhaps conditions will improve in the next several days...but I have my doubts.

...Alex Lamers...
The NAM surface set-up reminds me a bit of the June 9, 2003 fest in Nebraska. By 00z Tuesday evening the NAM places the warm front along the Missouri River at/near the SD/NE border. Somewhat narrow band of CAPE where axis of moisture is expected. Best low level directional shear appears to be along the warm front boundary. The best upper level spped shear will be further south where upper level jet max ejects out of the Rockies. Cap is a concern along the dryline where H85 temps are advertised in the +25C range but the solid vort max entering the plains I wouldn't think the cap is indestructible. The SPC SWODY3 seems to cover all the areas well.
One good thing is the GFS coming closer to NAM re: moisture; it looks much better today. Still no area of ~65Td, but 2 more days to improve that situation.

Here's what I don't get: the GFS development of the SFC lows relative to the upper low, specifically the nothern SFC low, which looks like the primary SFC low. The low appears to move upstream with descending height, which I totally do not understand. I like the potential setup of the W KS satellite low to add some LL shear, but I wonder if the GFS is simply smoking crack. The NAM doesn't show this, instead presenting the intuitive setup. :? ?
The OZ NAM is looking a tiny bit better. Still too far out to zero in on the where, but right now west or southwest of Kearney, NE looks okay at around 0Z. That's about as far south as the good instability goes, and deep layer shear increases as you get further south and east (30kts, ~200 helicity). Still looks like a pretty marginal day at the moment, but again, with our good friend D. Ridge knocking on the door, may as well take what you can get while the gettin's good. :)
With the 00Z run not a whole lot has changed. An elongated surface low will run from western Kansas into western Nebraska. The NAM is showing dewpoints at 60 stacked up against the dryline from the surface low to central Kansas where the gradient widens and 60 is located farther out into the warm sector. It is showing precipitation breaking out by 00Z all the way down to west central Kansas. CAPE should be >2000J/KG in northern Kansas and scentral Nebraska. Good low level shear will create helicity values that are more than adequate for tornadoes, but deep layer shear is going to be the limiting factor. Currently 250m-6km is forecast to be at 35kts over northern Kansas with shear weakening further North into Nebraska. LCL heights are a bit high as well.

I agree with Ryan that the new run looks a little better, but the setup still looks marginal. I am liking the area along the Kansas/Nebraska border, which provides a good compromise between the better CAPE over Nebraska and the better shear in Kansas. Shear and moisture with most likely be the major limiting factors with Tuesday. I think I am more optimistic than most when it comes to tornado potential. The models may not be showing much tornado potential now, but it is May and things have a way of working out this time of year. Therefore, I am expecting a couple of tornadoes. I am thinking that we might get a discrete storm in the Nebraska/Kansas border - West central Kansas region due to the cap becoming stronger with southern extent. If we could get an isolated storm in this area and shear increases a little bit, we could be in business. At least that is my thinking on the matter. My optimism could be misplaced though.

Looks like a good day if we had stronger mid-upper level flow, and more moisture. Strong low-level shear (250-350 0-3km SRH) below marginal, though sufficient, 30-35kt deep-layer shear, and 1500-2500 CAPE, may yield a supercell and enhanced tornado threat (owing to the low-level shear). NE looks good, but deep-layer shear is below the supercell range at 15-20kts courtesy of 25-30kt SSW 500mb and 250mb flow. So, the southern KS / eastern TX panhandle area may be pretty good, but stronger capping may provide for a bust. Additionally, low-60 Tds may result in high LCLs, though the strength of the low-level shear may do something. Even this target, however, will likely only have 30-40kt 250mb flow, so ventilation will be a problem. I'm kind of grabbing here, but it doesn't look like there'll be much to chase for at least the next week. We'll see.
Cheer Up

:wink: DONT BE SUPRISED, these slight risk day's or if they look like crap have the potential to be SOMETHING UNEXPECTED, thats my hopes im targeting Grand Island Kearney, area right now, but that could all change, something has to be going on though ive never seen a area get severe weather in 2 days in a row then forcasted more severe weather another day.... Hmmm mother nature is up to something? just keep your eyes out and maybe these model runs/forecasts will get better its ONLY MAY yet to theres June/July/ psb Early August to go through yet, and if this was supposed to go in the TALK thread im so sorry.
LCLs still look much too high for tornado spinups. Would not doubt a few rotating cells...but they should mainly act to enhance hail size. Experimental formula I'm using to predict hail size is saying about up to 2.75 inches max, not counting the effects of the ever-dangerous supercellular mode. With excellent mixed sub-cloud layer, would not be surprised to see dmg winds either. Still not sold on tornado potential. If you're lucky you could catch some good structure shots though.

...Alex Lamers...
Well, based on the 0z NAM, it appears that I'll be staying home tomorrow. The low-level shear (as measured by 0-3km SRH) looks very favorable for signficant tornadoes tomorrow, but the very poor moisture return looks like it'll be the main ruin this setup. The latest NAM run realizes the near impossibility of getting the real juice into even the TX panhandles, so it looks like the area will be left with 55-62 Tds, resulting in only moderate instability (1000-2000 CAPE) and very high LCLs. In addition, 30-35kt 500mb flow atop 40kt 850mb yields pretty weak 0-6km deeplayer shear, despite the strong low-level shear. Additionally, the NAM fails to initiate convection south of I70 by evening. Since my domain tomorrow would only include the tx PH and southwestern KS, this looks like a no-go.

Yet again, as has been the story all year, we just can't seem to get any real juice into the plains in time. To think it's mid-May and we're still having severe moisture problems is frustrating to say the least.
Hmm... decent CAPE, TDs in the low 60s, decent theta-E... but no real shear to speak of... ETA runs this morning have a narrow band of 2000-2500J/kg from extreme North-Central Kansas up through Central Nebraska and into South Dakota. SBCAPE values near 2500 run pretty common in that area. Slapping the 3km SRH over it, a bulls-eye of good shear is sitting over in Iowa; most values in our good CAPE may hit 150 if you're lucky. The 1km is pitiful with 50 as good as it gets; I guess some good news is that its over our CAPE axis are opposed to of in Iowa! :lol: Winds as you move up don't change a whole lot; speeds are about the same from 500 down to 850 with directional shear pretty weak as well. Definately not a supercell day.

Further south along the dryline into Southwest Kansas, a very very tough cap sits over the area and lack of really strong forcing makes it hard to imagine anything will get through the cap. CAPE values along the dryline south of I-70 don't break 2000J/kg, either, and shear values don't amount to much down there either.

My play would be along and north of I-70 today; not exactly sure how far north yet, but preferrably along the dryline a bit further north into Kansas and perhaps Southern Nebraska...

Just a note about moisture, as Jeff mentions, the 13z SFC obs show only 50s TDs and even some 40s in Oklahoma in Texas advecting this way, so I wouldn't count on a huge upswing in moisture up here... gonna be an iffy chase day today, I'm afraid...
Chase target for today, May 17

Chase target:
McCook, NE.

4 PM, CDT.

Storm type:
Isolated high-based storms with hail to golf-ball size and winds to 80mph.

300mb analysis showed negatively-tilted trough over ID with zone of divergence spreading towards ERN CO/WY and WRN NE. Two shortwaves lifting through the SWRLY upper flow were evident in 700mb analysis and WV imagery. The first was lifting through SD while an upstream wave was diving through CO and WY. A 50kt LLJ was pulling 10C dewpoints N into KS/NE. Moisture was fairly shallow – about 50mb deep – as noted at both LBF and DDC 12Z soundings. DDC indicated a large hodograph loop below 700mb, although this zone of high helicities was lifting away from the target area. Last night, WWA resulted in an MCS as well as widespread precipitation in SD and MN, which is ongoing ATTM. Skies are mostly clear in NE and KS, with some CI spreading into the WRN areas. SFC dewpoints were meager, generally in the mid-50’s throughout the region.

Storms will fire along the surface trough which will be lifting E out of CO. The area will be strongly capped through the mid-afternoon, with 700mb temps AOA 10C, however rapid mid-level cooling will commence with the approach of the S/WV with 6hr temperature drops of 5C in the target area. Expect ample instability, with MLCAPE’s AOA 1500J/kg and LI’s around -6C. Straight hodographs along with deep layer shear of around 40kts will support decent storm organization.

- bill