6/20/07 FCST: NE/SD/MN/IA

[Mike Hollingshead]

I have a feeling this could be a very rewarding day out in north central NE or southern SD.

O'Neill fcst sounding at 21z

Big cape and nice turning. All one needs is to pop a storm near or east of the dryline in the area. NAM does indeed fire storms in the area too.

 

[Dave Winslow]

The Oneill area does seem to be a hot spot right along stalling cold front, becoming stationary along the NE/SD border. Tough part here will be whether to go SD or NE with rivers to cross. That sounding shows a dewpoint of 72 by 4 pm...wondering we can get that kind of juice that early but it does seem possible that we get a 70 degree dewpoint by the evening as the really good moisture pushes in later. Some worry for moisture transport will be if a big MCS develops tonight in SW Nebraska or KS, that could throw things off a bit.

Excellent High CAPE/moderate direction and speed shear combination for HP supercells

 

[Derrick Herndon]

I think the question for me (and others) is a trigger. Based on that sounding the convective temperature is 88F based on the dew of 72F. NAM continues to want to over-forecast the dews so something closer to 68-70F may be realized. That bumps convective temp up to 90F which even then still seems doable for tomorrow. Dryline may also help with a trigger mechanism. Looks like less of a tornado threat thoough as LCL's may be on the high side. Could be some pretty storms if they can get going.

 

[Jason McKittrick]

I think the question for me (and others) is a trigger. Based on that sounding the convective temperature is 88F based on the dew of 72F. NAM continues to want to over-forecast the dews so something closer to 68-70F may be realized. That bumps convective temp up to 90F which even then still seems doable for tomorrow. Dryline may also help with a trigger mechanism. Looks like less of a tornado threat thoough as LCL's may be on the high side. Could be some pretty storms if they can get going.

Just wanted to point out that all of the NAM runs over the last 48hrs including this morning 12z run under forecast the Dewpoint at Norman by almost 10 degrees F. Perhaps the NAM dewpoint problems are related to some seasonal parameters. If the dewpoints are under forecast for Oneil tomorrow like they were in Norman today things may indeed be very interesting.

 

[Mike Hollingshead]

Good point Dave. I'd say tomorrow could very well be screwed now, thanks to that monsterous wall of severe storms down there in southern KS, northern OK.

 

[Bill Schintler]

Chase target for Wednesday, June 20

Chase target:
Miller, SD (15 miles WNW of Huron).

Timing:
Storms will initiate along US-12 in the late morning hours near Aberdeen, and develop and move towards the south into the increasing moisture and instability through 4 PM CDT. A full spectrum of severe WX appears likely, including a few tornadoes.

Synopsis:
UA map indicated EWRD moving H5/H7 trough over the Great Lakes while convection has fired in an unstable environment along several boundaries ahead of this feature in PA and NY. In the SERN US, a weakening H5 circulation over MS and AL has essentially washed out. Further to the west, convection had initiated in an unstable environment over NWRN KS at the nose of the 15C Td H85 moisture axis. This was apparently in association with upsloping SERLY SFC flow along with lift within the right-entrance region of a 60kt H5 speed max over NEB. This mature convective complex has pushed a gravity wave to the eastern front-range of the Rockies in CO, while WV imagery also indicates increasing assent over WRN CO. Area soundings (DDC, OUN) showed steep lapse rates over deep moisture – in fact the OUN sounding indicated a 200mb deep moist layer. Further west in NM, CO, and SD (UNR), soundings indicated dry-adiabatic lapse rates in the lower half of the troposphere, and this airmass should be slowly advected towards the E on Wednesday, further contributing to the EML. The deepest moisture was still just nosing into NEB, as noted in the LBF UA observations.

Discussion – Wednesday:
Difficult forecast as aforementioned ongoing convection in TX/KS/OK complicates the picture with OFBs and extent of attainable airmass recovery with a large amount of debris CI lingering there. Further north, a fast-moving clipper system will round the ULVL ridge. It appears as though a compact wave/vort max along the SRN periphery of the stronger H5 flow along with strong heating in SWRN SD should result in SFC cyclogenesis south of Pierre while a SFC boundary develops E and NE of the low. Differential SFC heating will result in part from a broken CI shield that will be in place over NERN SD, while in the SWRN half of the state insolation will not be impeded by high clouds. Very impressive instability along with modest shear parameters should exist by late afternoon. Mid-level lapse rates of around 8.5C/km above increasing LLVL moisture should result in MLCAPEs reaching 4000J/kg. Backing SFC flow beneath an increasing SRLY LLJ will result in very impressive hodograph curvatures in the SFC-3km layer while deep layer shear will locally increase to 60 kts as the strongest H5 flow drops SE. Later in the evening, a strengthening LLJ will aid in the SEWRD propagation of a large MCS.

- bill
[FONT=&quot]10:30 PM CDT 06/19/07[/FONT]