4/24/08 FCST: NE/KS

[Van DeWald]

Setup for Thursday is looking better for Thursday afternoon/evening, especially across southwest Nebraska/northwest Kansas. Surface cyclogenesis should develop across western Kansas with the dryline intersecting a northward advancing cold front. Still a couple days out, but a target area from North Platte, NE to Hill City, KS is looking probable right now. Will likely be some good storms along the cold front further east into Northeast Nebraska, but think best tornado potential would be across the high plains 00z-03z.

 

[Eddie Natenberg]

Me and Gabe (who is is sitting next to me right now) both agree this day has potential. The models have been fairly consistent in the large scale features but I still think uncertainty is high and 3 day outlook seems appropriate given that uncertainty. "IF" the cap is weaker or the forcing is stronger with strong pressure falls to the NW will yield a classic tornado day in the plains giving the very deep moisture expected and the probable diurnal forcing since we should be dry slotted. This is definitely a day to watch. As for you OU capstone students I think the cap will hold most of the day.

Eddie

 

[Dustin Wilcox]

The GFS is still the most favorable model (at least over the NAM) for Thursday, which isn't all bad as the GFS typically seems to be a little closer to reality than the NAM. Its slightly further North with the placement of a 995mb sfc Low, somewhere near the Dodge or Garden City area. The Surface obviously backs nicely given the deepening pressure, but Low Levels also begin to back in the W KS area after Veering badly during the day due to the previous low racing NE. Its not the most impressive LLJ as the core of the LLJ is shoved to the East but I will take the 25-30kts progged. Moisture looks to not be an issue as all models have at least >60 dp's over the entire warm sector. The GFS progs >3000 j/kg CAPES North and then arcing NW near the sfc low in W KS. I typically like to be near the Nose of the strongest Theta Ridge which according to the GFS will be poking into the Great Bend area. Based on latest model data I will target the 281 corridor near Great Bend to Pratt KS. This is where the best Thermodynamics poke, and this area is right on the edge of the strongest upper level support, (hints of 50kts at 500 per 12Z GFS) with a 80kt streak rounding the trough at 300mb with SW flow and Westerly flow across the Panhandle regions. It's still slightly capped but willing to gamble, problem is when one looks at the NAM forecast Skew-T for same station it's a whole other story. http://68.226.77.253/skewts/GFS-test/060/SKT_GFS__KGBD.png

 

[Mike Hollingshead]

That veering low level jet through 1pm is a little concerning for the dryline I'm thinking. So is the warming prog'd between 800-850mb. Seems as though we'll wind up with shallow moisture along/just ahead of the dryline. Might be why the NAM fires nothing in KS through 1 a.m. My thought now is perhaps follow the west edge of any better moisture north to the warm front in NE and hope for the best. Of course that may even be a challenge with morning activity recently exciting the plains. I'm probably over thinking this, but having some concerns.

0z Hebron NE

Best moisture/thermo I've found near a front. Low level shear pretty crappy everywhere. I'm just not liking the prog'd air along/ahead of the dl. I could actually see something fire on that juice moving north, if it's very good air(it may very well not be with the potential precip leaving the area). Upper-level jet spreading over it by evening with cap gone.

 

[Kenny Drake]

I'm not going to post a too detailed forecast discussion at this point due to model inconsistency and my inability to run Kenny Drake's personal brain model. However, I will say this based on the 00Z NAM: SRH will skyrocket during the overnight hours into Friday across most of the warm sector. Good example: KICT @ 6Z shows 1km SRH ~600, over double from 3Z. The cap will be a major issue anywhere south of the KS/OK border (~100-200J/KG common.) However, the NAM also propagates the main 500mb vortmax across the area at the same time and in the process strengthens the sfc low and backs the 850 flow by sunset. Can any low level convergence aided with the vortmax overcome this progged cap south of KS? Not sure. North of KS the issue becomes the cold front plowing south and likely a quick linear transition. As for daytime chasing, unless vortmax timing changes (which it easily could given model inconsistency), I would stick well north as those have mentioned. Basically the problem for most of the day will be crappy shear (everywhere in warm sector) and a strong CAP (KS/OK border south.) Cells that will likely fire to the north will have rather crappy shear to utilize.

 

[Heidi Farrar]

The SPC has upgraded to a Day 2 Moderate Risk, with a 45% hatched area, in case anyone missed it. The WRF, NAM & GFS place the best winds well east of the moderate risk area at 0Z FRI, but they are also showing > 3000 CAPE in much or all of the mod. riska rea; the GFS upper air data seems to be unavailable for critical runs at every site I know of -- what's this?! I just got a 0Z FRI run! and it looks like the NAM & WRF. So yeah, I echo what Kenny says about the crappy shear. Given this, I am a little surprised by the upgrade and 45% hatched area, but I guess they think the hodograph is impressive enough & the conditions will come together at the right time. Still, I'm not sure I've ever seen a mod. risk with 30 knts max showing at the 850mb level.

 

[Mikey Gribble]

I'm also quite surprised with the Day 2 moderate and 45% hatched. I only glanced at the setup after dark, so the forecast I posted is for daylight hours. As mentioned by others, shear does increase from 00Z-03Z, so this time frame could be interesting if storms remain surface based, but I won't look into that until tomorrow.

For daylight hours I really don't see this being a moderate risk unless I'm missing something. I do agree with SPC in that any storms coming off the dryline over Kansas will probably pose a threat of multiple tornadoes, but I don't think there is much of a threat for strong tornadoes.
Deep layer shear is barely adequate for supercells, but with SBCAPE AOA 3000 the environment should be quite favorable for supercells by 00Z. The fairly high LCL's (>1000m at GBD and Pratt at 00Z) and marginal mid level winds are what is really hurting the tornado potential IMO. I don't have that big of a problem with 20kts at 850mb, but 10kts more would help out a lot IMO. GBD and Pratt hodographs both have decent curvature in the lowest 1-2km, but they are shorter (obviously) than what I'd like to see for good tornadic supercells.
Overall I think this is a decent setup for weaker tornadoes, but I just don't see it as being 45% hatched material. I will definitely be chasing tomorrow since it's in my coverage area. I also think we'll get some weak tornadoes, but I just don't see strong tornadoes being possible. If wind fields can increase a little bit then it may be a different story. My target is DDC by 3PM. I posted a forecast here if you're interested http://loadedgunchasing.com/blog.html

 

[Mike Smith]

With regard to Kansas tomorrow...

Keep in mind the heavy rains of today:

1. Much of Kansas east of Greensburg received at least a half inch of rain.

2. The area between Emporia and McPherson had between 2.5 and 5 inches.

The models don't know about this most-recent rainfall. That will have effect of lowering the actuals LCL's (compared to what a model would predict) and, to an extent, increasing the instability.

 

[Bill Schintler]

Chase Target for Thursday, April 24

Chase target:
Speed, KS (51 miles north of Hays).

Timing and storm mode:
Elevated convection east of I135 during the morning hours will move out of the area by noon. Surface-based storms will develop in the target area by 4 PM CDT, with a full spectrum of severe weather possible, including a few tornadoes.

Synopsis:
The WRN CONUS trough continues to shift slowly EWRD with a 60kt H5 streak and height falls concentrated over NRN CA. Further S and E, ongoing SVR convection in TX and OK is connected with a NRN-branch S/WV with interaction from the SRN-branch flow that tracked from NM into TX over the last 12 hours. Both the WRF and GFS seem to be overestimating SFC dewpoints be several degrees in KS. Even so, abundant moisture depth is verifying over much of TX into NEB. A number of 00Z soundings were contaminated by ongoing convection.

Discussion:
A N/S-oriented area of convection across CNTRL KS along and E of I135 during the morning on Wednesday will weaken and shift E by the afternoon as the LLJ veers and the first shot of ULVL support shifts E. This will set the stage for destabilization and a second round of storms late in the day. Low pressure centered in SERN CO early in the day will strengthen and move NE, reaching CNTRL KS by 06Z Thursday. MDLS indicate a WF extending SE from this feature. This WF is more of a wind shift as the temperature and moisture gradients are weak, however moderate convergence and backing SFC flow are indicated along and just N of the WF. Meanwhile, a DL will sharpen and move E during the afternoon. Strong inhibition should persist until late in the day with convective temperatures in the low-80s, and will erode first in NCNTRL KS in response to UVM provided by a second compact ULVL wave and attendant cooling. Initiation should take place first at the DL/WF triple point.

Moisture of unseasonably impressive quality and depth for this time of year is indicated with an axis of H85 dewpoints of 15C developing along the I-35 corridor in OK NWRD into NCNTRL KS. MLCAPEs should locally increase to 3000J/kg with moderate mid-level lapse rates of 7C/km in place. Convection will then expand into a large MCS during the evening and overnight hours which will track E through NEB as a 50kt LLJ cranks up during the evening.

- bill
9:30 PM CDT, 04/23/08

 

[Mikey Gribble]

That's a good point Mike. I'm sure we'll get a degree or two added to our dewpoints from evapotranspiration, but I still would like to see LCL's below 1000m AGL and I doubt we'll make that. I don't think LCL's tomorrow are a show stopper or anything. This is a pretty balanced setup across the board. There aren't any blatant negatives that should ruin tornado potential (like other setups this year with poor moisture, veering low level winds, etc.). Every paramater is pretty much at the low to middle end of the spectrum for tornadoes.
I just got done glancing over the NAM. Everything looks about the same aside from convection over the warm sector in the morning being a little different(and mid level winds look a tad better compared to the 12Z). I'm not real sure about the focus for convection that shows up near Emporia at 18Z, but I'm not too worried about. I think I'll move my target South to the Oklahoma-Kansas border straight South of Dodge City. Between 18Z and 00Z temperatures rebound nicely in this area (low 80's). The NAM also seems to be lining up a little better with the GFS on having the best low-level shear ahead of the dryline in southern Kansas and Oklahoma where winds in the lowest 1km are slightly stronger than they are farther North.
I am slightly more optimistic after seeing the 00Z run. It is nice to see some continuity with the models after their performance over the last couple weeks. The setup is virtually the same, but there are some slight changes and they went in the right direction. I am waiting on forecast soundings and COD to update so that I can get a better look at CINH, but after getting the overview shot from RAP there doesn't seem to be any CINH left by 00Z. Not quite sure why it doesn't have precip breaking out. It will be interesting to see the 4.5km WRF precip. graphics tonight. I'm also curious to see if the GFS breaks out convection in southern Kansas again.
Anyways, aside from the cap holding there isn't much I'm worried about. I think any storms forming along the dryline near the KS-OK border (and even up towards DDC) will pose a tornado threat. I don't think strong tornadoes are likely, but I wouldn't rule one out. All in all it looks like we have a decent chance for a few tornadoes tomorrow. I'm ready for it too. This is the latest I've ever made it into a chase season without a tornado. As things look now I would plan on being in Dodge City by 2PM or so to fine tune the forecast. If this setup unfolds as advertised by the NAM then I would probably head South from there and set up near the OK-KS border. That may change slightly tomorrow though.

edit - just checked forecast soundings for Pratt and I'm starting to think the 00Z-03Z time frame could get pretty interesting. The hodograph enlarges significantly (especially in lowest 1km) and 0-1km SRH spikes from 86 up to 282. EHI also jumps way up with 00Z showing 1.9 1km and 4.3 on 3km. By 03Z that has risen to 1km EHI of 4.7 and 3km of 7.3. Given these kinds of paramaters and hodographs I would say a threat of strong tornadoes may evolve in the 00Z to 03Z time frame. I will definitely be staying on my storm after dark. SPC has to be banking on this time frame producing. I would go moderate too after seeing the 00Z soundings for 03Z.

 

[Gabe Garfield]

edit - just checked forecast soundings for Pratt and I'm starting to think the 00Z-03Z time frame could get pretty interesting. The hodograph enlarges significantly (especially in lowest 1km) and 0-1km SRH spikes from 86 up to 282. EHI also jumps way up with 00Z showing 1.9 1km and 4.3 on 3km. By 03Z that has risen to 1km EHI of 4.7 and 3km of 7.3. Given these kinds of paramaters and hodographs I would say a threat of strong tornadoes may evolve in the 00Z to 03Z time frame. I will definitely be staying on my storm after dark. SPC has to be banking on this time frame producing. I would go moderate too after seeing the 00Z soundings for 03Z.

I agree, Mike. The forecast soundings north of the OK/KS border show weak CINH even at 10 p.m. tomorrow. Thus, it's my feeling that any storms that fire north of the KS/OK will have a much better chance at being cyclic monsters after dark. With the high degree of moisture/instability that is already in place across the Plains, I believe the MDT is certainly justified. All in all, I think tomorrow has the potential to be one of the best chase days of the season (though, this season, that isn't saying much).

 

[Van DeWald]

Still looking at the northern target here, as best parameters seem to be coming together, still in the 00z-03z timeframe. Right now, would shoot for a Norton, KS target. Nam progs at 03z, about a +5 0-1km EHI, 375 0-1km SRH and about 25kts of 0-1km shear, and 0-3km shear around 60kts, and around 3000 CAPE (based on plan view progs from Earl's site). Dryline/frontal intersection still looks good near triple point.

Van

 

[Mikey Gribble]

The SRH from 03-06Z over south central Kansas is amazing. 0-1km of 448 and 0-3km maxed out over 500 on COD. Those are some big numbers. By 03Z there is still >2000J/kg on surface based CAPE left too in this same area. Any storms firing along the dryline in the Oklahoma-Kansas border area up to DDC will track right into thie area later in the evening. I have plenty of respect for the guys chasing the northern target, but if you are going to stay South picking a target is pretty easy IMO. If you get an isolated storm in the area I mentioned you should be in great shape.
SPC did a good job going moderate. In the 12Z run the 00Z-03Z threat looked a little uncertain IMO, but it looks a lot better now on the 00Z run. I think they probably deserve a pat on the back if this verifies. With the models being incredibly shakey lately who knows if this will hold, but I am actually optimistic that it will. Both the GFS and NAM are in pretty good agreement on this setup (at least 12Z GFS) and the NAM has been consistent with this setup for the most part. Maybe it is time for the NAM to redeem itself.... Man it's nice to finally have a good tornado setup.

 

[Van DeWald]

Well, the good thing about the northern/southern targets, they're not that far apart. A couple hours from either direction should put you in the hot seat. Will certainly know tomorrow with mesoanalysis, and can easily adjust south (or north) as needed.

*Edit, Just looked at the EMC 4km WRF. It indeed develops storms further south down the dryline through south central Kansas. Tough dilemma now, north or south. I know this happened last year for Greensburg as several folks got suckered on the northern convection as well.

 

[Bill Hark]

There is a definite potential for tornadic supercells Thursday. The approaching trough should produce 30-40 SW winds across Kansas and Nebraska per both GFS and WRF with a hint of some divergence. The low over the Kansas/Colorado border (more to the south per WRF) will give backing surface winds through central Kansas to south central Nebraska. Dewpoints are sufficient with 65 degrees pushing northward to near the Nebraska border by 00Z on both models though the GFS solution has it shifted slightly more east and it shows more of a dryline bulge over north-central Oklahoma. I really like the narrow tongue of forecast CAPE to 3000 approaching the Nebraska border. SRH increases progressively through the day. Picking a target area in the long area from central Kansas into Nebraska is difficult. Both the GFS and WRF show an explosion of precip by 600 April 25 in the North Platte area but they differ on precip farther south into Kansas. At this point, I would like to spend the night in WaKeeney, KS and then refine my target after the AM obs, probably northward to Hill City.

I enjoy looking at Earl’s model outputs for combined indices (ie STP) and am a bit concerned about a 00Z bullseye in SW Oklahoma that shifts northward and enlarges by 03Z into southern Kansas (new definition STP; NAM)
http://wxcaster4.com/nam/CENTRAL_ETA212_ATMOS_STP-NEW_24HR.gif
I definitely take these combined measuremts “with a grain of salt” due to accumulated errors with each ingredient.
For now, I would stick with my northern target of WaKeeney, KS.


Bill Hark
sitting at home