5/29/08 FCST: SD/NE/KS/IA/MN

[Derek Weston]

NAM currently showing very favorable setup for tornadic supercells over NW Iowa area.

Still wonder about timing and the cap, but it appears we may have the CAPE and shear needed to have some fun.

 

[Kevin Polston]

00Z NAM tonight shows eastern Nebraska/western Iowa as looking promising for supercells/tornadoes Thursday evening. I think this setup bears watching

 

[afischer]

The models are definitely trending over time toward a stronger/better set-up for Thursday. The 00Z ECMWF and 00Z UKMET are nearly identical in showing a 999mb lee-side low, and a 40kt LLJ maintained through the day over the middle/lower MO river valleys beneath a difluent upper jet. The NAM/GFS more or less echo this. Tonight's RAOBs show boundary layer dewpoints of 17-20C along the stalling cold front, which will return with ease and set the stage for a strongly unstable airmass. Plenty of mesoscale details to be ironed out yet... at this time I'm most concerned about sufficient heating/destabilization in the wake of elevated activity (which may be extensive), as well as having the favorable low-level shear/moisture (which will seemingly improve with eastward extent) co-located with the area of surface-based initiation/development. I have the day off for this one, so will be keeping a close eye on it.

 

[Darrin Rasberry]

WRF and GFS are still not quite in agreement over the placement of the low as of 00Z last night. The GFS's projected temps at the surface are somewhat less powerful than the WRF; I wish I could see the ECMWF or UKMET's projection for this to see which they agree more with. GFS also places a heavier cap, with which the WRF doesn't quite agree.

Higher helicity is due for eastern Nebraska and western Iowa, but I wonder whether storms are going to have to trail over there or whether some other form of initiation can occur along the IA/NE border, maybe S of Sioux City.

Based on the general disagreement currently, I'm personally boxing out an initial target area from North Platte to Thedford to Taylor to Broken Bow, to narrow down or move when I study more parameters and when the two models (hopefully) come into sharper agreement about the placement. I have a gut feeling this may be a little too far west of a box, but we'll see.

 

[Colin Davis]

I'd probably work a little east of that, Darrin, but I think you're in the right ball park. As down as I am about Friday's setup, I actually am a fan of Thursday's. More than adequate instability, and that shortwave will be the perfect trigger. If the NAM/GFS can hold that directional shear in place (and the LLJ kicks in by 0z, as expected), I see supercells and tornadoes tomorrow.

I said Grand Island to Lincoln yesterday...and nothing is pushing me away from that target just yet...

 

[Dustin Wilcox]

I'm super pumped for tommorow, looks like a strong moderate risk day with the potential for significant tornadoes. After just starting a new job and taking so many days off only a week in to catch last weeks action, I am going to try and work a full day tommorow, though provided my current target I should be able to make it in time. This morning's NAM has some amazing forecast Skew-T's in the area bound by Albion to Columbus and East of there towards Fremont. EHI's upwards of 10 with BRN shear values near the majic 15-16 range. Wouldn't be all bad to see a little better backing of the LL's, but provided the veered midlevels there's some pretty impressive turning as you work up the profile...

OLU looks impressive by 0Z.... http://beta.wxcaster.com/cgi-bin/parse_skewt_trace.cgi?model=NAM&fcsthr=036&STATIONID=KOLU

Based on current runs I like the Central City area working North-East from there, though don't want to overlook the area back to the SW either, by 3Z the 12KM likes to put a tornadic storm Near Yankton and another near Red Cloud....

 

[Mike Hollingshead]

I'm guessing I'll wind up somewhere near O'Neill or west of there for initiation. At least if there is that little bit of sfc convergence on the front prog'd west of town. It's pretty weak looking but would be enough.

ONL 0z

Cap gone by 21z there

Slowly making me think a little of June 9, 2003. Moisture returning late, but strong, boundaries in similar locations(very), similar mid-level flow strength. Seems like that day the low level backing wasn't prog'd all that strong, but once things happened they were really nice along and south of the warm front draping the NE/SD border. I guess the sfc trough is prog'd to line up quite a bit slanted ne-sw...and I rarely see anything good on that in this state. I will definitely be up near the SD border on this one.

 

[Michael O'Keeffe]

After taking a look at the latest NAM/ETA I am liking the extreme SC NE/NC KS area. Part of the reason is due to time restrictions, as the warm front in NE Nebraska looks interesting as well.

Anyway the 12z was putting an area from Concordia to Salina to McPherson under the gun for some very interesting severe weather on Thursday afternoon/evening.

Here is a link to the 0z Skew-T for KSLN.
http://beta.wxcaster.com/cgi-bin/parse_skewt_trace_all.cgi?model=NAM&STATIONID=KSLN

Pretty impressive with a nice hodograph. EHI around 10. SRH over 400. CAPE exceeding 4000j/kg. That all with very deep moisture we could see a widespread severe weather/tornado outbreak from SE SD into C KS. Bring it on!!

EDIT: The Concordia Skew-T is showing SRH at 719 at 03z tomorrow night!! That's ridiculous!!

 

[GPhillips]

EDIT: The Concordia Skew-T is showing SRH at 719 at 03z tomorrow night!! That's ridiculous!!

Something screwy is going on with the NAM environment in that area. Looking at the BUFKIT soundings for Fairbury NE just to the northeast of CNK, the 0-3 km SRH at 03Z is 391 m2/s2 and at 04Z it is 914 m2/s2. The 700 mb winds go from 27 to 66 kts in one hour. Looks as if model convection is creating some huge hodographs as the 04Z forecast sounding is completely saturated. Would think that this is what is causing the excessive SRH values at CNK at 03Z? GFS BUFKIT soundings have 371 and 551 0-3km SRH for 03Z and 06Z respectively with no convection.

 

[Michael O'Keeffe]

This may be the reason.

However, look at all the helicity the NAM is showing at 6z.

Hopefully that precip model is of a long track supercell which is very possible, if it is I will be on the KS/NE border.

 

[Derek Weston]

Certainly overlooked Kansas in my initial posting -- NC Kansas looks great on the 0z NAM tonight....

Then all the way up through SE SD -- watch the WF.

As long as the instability develops alright, it appears we're on our way to another busy severe weather day in the plains.

 

[Bill Schintler]

Chase Target for Thursday, May 29

Chase target:
Bartlett, NE (55 miles north of Grand Island)

Timing and storm mode:
High-based convection will initiate over the front range of the Rockies by early afternoon and then spread into western Nebraska where instability is limited. Storms will then develop into the favorable moisture and shear of target area at 5:30 PM CDT, with supercells and tornadoes likely. Storm motion will be east at 25mph.

Synopsis:
The WRN CONUS trough was shifting slowly E, and this feature will deamplify over the next 48 hours while a parade of compact impulses move E through this flow. Moisture return is a concern. At 00Z, H85 dewpoints AOA 15C were confined to a narrow axis over CNTRL KS, however strong moisture advection is underway courtesy of a strengthening LLJ and SFC dewpoints of 60F had already reached SERN NEB. The 21-member NCEP SREF ensemble means are handling current and FCST LLFL moisture well and FCST return appears reasonable. The latest WRF is poorly handling the OFB that should result from tonight’s MCS FCST to traverse the NRN half of NEB.

Discussion:
FCST is complicated by evolution of mesoscale S/WVs and details of the resulting forcing. Also of concern is position of OFBs from tonight’s storms. The first round of elevated convection will fire between 04Z-07Z in CNTRL and NCNTRL NEB at the nose of a 50kt LLJ along with help from forcing with a lead wave. A plume of steep mid LVL lapse rates will spread across the region, with H8-H7 MUCAPES AOB 1000J/kg. This convection will move rapidly E of the area during the morning with the veering LLJ, while ST will persist across most of the region into mid-afternoon. Attention then turns to the main event as the primary low pressure strengthens in NWRN KS with a second low in NCNTRL NEB. Dewpoints will rise to AOB 65F but subsidence will be the rule until a second impulse approaches the region and the H7 thermal ridge shifts SE. Prior to initiation, an area of CI will approach from NERN CO and spread into WRN NEB while the NLGN profiler will indicate increasing strengthening and backing of the flow in the H6-H4 layer. Both of these are a signal of the approaching S/WV. H7 omegas increase to -30ub/s after 20Z and high-based storms will initiate first in SWRN and WRN NEB and then build NE. SVR potential will be maximized along an outflow-enhanced WF immediately SE of a mesoscale low at the target location.

A potential negative will be a tongue of dry air in the H8-H7 layer, and updrafts may struggle early in convective evolution. Eventually, however, persistent backing of LLVL flow with a SFC dewpoint of 66F at 22Z will maintain sufficient storm inflow to overcome dry intrusion into updrafts. Stability will increase between 22-01Z as CAA in the H5 layer increases mid-level lapse rates to 7.5C/km, with MLCAPE increasing to 3000J/kg. Significant turning of winds with height is noted in the SFC-3km layer with the WRN periphery of strongest H85 flow above outflow-enhanced backed SFC winds near the OFB. Moderate tornado threat will exist with a 100mb LCL of about 1200m AGL. Tornado threat will increase near nightfall with an increasing LLJ and lowering LCLs. During the evening, convection will increase in coverage on a strengthening LLJ as the CF pushes SWRD, and heavy rainfall will become the primary hazard.

Please feel free to PM me for nowcasting.

- bill
10:58 PM CDT, 05/28/08

 

[Paul Knightley]

Will refine in the morning - currently in Salina, but thinking that Hastings, NE, could be a good starting point. WRF has what appears to be a supercell track nearby, with good SREH etc. Don't fancy going too far north really!

 

[Darren Addy]

Big factor tomorrow looks to be where/when the stratus deck clears, allowing surface heating in the longest. The LLJ looks to be cranking overnight and we've already got 64 degree DPs as far north as Hays. If the low sets up in northeast CO, we could see the warm front running right along I80 in Central Nebraska.

The nose of the richest moisture and the surface winds look to back best right around the the NE/KS border and looking at the 500mb winds we should get some supercells to travel parallel to the boundary and into a big area of decent helicity (if models verify).

Thursday is the worst day of the week for me chase-wise, so naturally every Thursday provides an opportunity. I'd target Phillipsburg, KS and expect things to heat up by the time they get to the Holdrege/Minden area of south-central NE. (They always do).

We've got a lot of saturated ground around here (when was the last time you heard of the Platte River being out of it's banks due to local rainfall?) so some transevaporation wouldn't hurt the cause a bit.

SPC looks like they agree. As I've typed this they've updated a nice bulleye over my backyard to a Moderate Risk.

 

[Chris Hayes]

Today is gonna be kind of hard to figure out where to target, in my opinion. The cloud cover over the region is quite worrisome. I'm not a fan of BIG SHEAR low cape days. If the cloud cover doesnt disperse or thin out in the next several hours, I feel this might be an overhyped day. Then there's the factor of if the cloud cover does disperse we could have very high LCLs. What to do?