7/27/2008 FCST: NE/IA/ND/SD/MN

[Ryan Shepard]

Surprised to see noone has posted for this day yet. I was looking at NAM ETA (UCAR) and I like what I see for Northeast Nebraska. The dp will be in the 70s, bulk shear will be 40-50kts, 500mb winds suggets 45 kt winds from the west. The upper level disturbance is the most significant feature here. Cape values are in the 4000. The surface winds may be limiting here for tornadoes if they only stay at 10 kts, but there does appear to be a SE component. SPC says this could be a widespread episode if a bowing system evolves, got to chase the isolated cells before that happens. Anyone in colorado who is going and needs help with gas money, let me know! lol.

Ryan Shepard
www.cuttothestormchase.blogspot.com

 

[Bill Schintler]

Chase Target for Sunday, July 27

Chase target:
20 miles N of Yankton, SD. Be advised that bridge options across the Missouri river along the NE/SD border are limited.

Timing and storm mode:
Convection be ongoing early in the day over much of eastern SD and into MN. Storms will expand south and east during the afternoon, while becoming surface-based in the target area by 6 PM CDT. A full spectrum of severe weather seems possible, including a tornado or two. Storm motion will be eastward at 25 mph.

Synopsis:
ULVL ridging was dominating the NRN CONUS, with NWRLY flow over the upper Midwest and Great Lakes area. The remnants of Dolly continue to spin away over NM, and a strengthening LLJ will shear off this circulation and transport moisture towards ERN NEB overnight. All models agree on the progression of one or more compact waves just S of the main flow over the upper Midwest on day-2, but differ on mesoscale details. The bottom line is that they differ widely on the details of the phasing of the “sheared off” Dolly circulation, and the NRN branch jet. Another complicating factor is the MCS in WRN SD – with LLJ support, this should continue to track ESE through SD while pushing an OFB S to roughly the NE/SD border by 19Z on day-2. This FCST is based mainly on a balance of the SPC SREF and GFS, with lesser input from the WRF. Healthy LLVL moisture is in place, with dewpoints of around 70F noted over ERN NE. The 00Z ABR and OAX soundings indicate a moisture layer 150mb deep.

Discussion:
Moisture associated with the remnants of Dolly currently over NM will be transported NWRD overnight, courtesy of a 30kt SRLY LLJ. During the day Sunday, a couple of compact S/WVs are FCST to track from SD into NERN NE and NWRN IA, while enhancing lift and deep-layer shear. The presentation of the details differs from one model to another, but the consensus agrees on this impulse.

The WRF and GFS both paint an almost unbelievable picture of SFC and H85 moisture return, with the GFS favoring a more NRLY position of deepest moisture and other features. If moisture verifies, (SFC-100mb) MLCAPEs will easily exceed 4000J/kg along the ERN extension of an EML by 22Z. Capping will be strong, owing to warm air aloft, with H7 temperatures in the 13-15C range. Added to this concern is that of inhibited insulation due to a couple areas of CI. During the morning hours, an area of CI will enter WRN NEB from CO and WY, and track E through NE during the day while thinning out in the process. This should not inhibit SFC heating. Of larger concern will be an area of convective debris associated with the elevated convection that will track from SD into MN. Insolation will be impeded N of the SD/NE border as a result.

Shear parameters should be sufficient for organization of discrete cells, with SFC-6km deep-layer shear of 35-40 kts and SFC-3km SRH of 250m2/s2 indicated. An SPC Significant Tornado Parameter of 2 is FCST, suggesting the possibility of supercell tornadoes. The LLJ will strengthen to 50kts and veer to the SWRN IA overnight, while aiding the maintenance of a large MCS which will track though IA during the overnight hours.

Please feel free to PM me anytime after noon for nowcasting support - leave a phone number.

- bill
11:02 PM CDT, 07/26/08

 

[nickgrillo]

Seems like there is a pretty good shot for a few surface-based supercells along the OFB / warm front that's draped across southcentral/southeast SD into northern IA. RUC mesoanalysis shows CINH for a mixed-layer parcel ascent decreasing as diabatic heating and low-level moist advection continues, with mixed-layer CAPE 2500-4000j/kg invof the surface boundary, with backed southeasterly surface flow enhancing boundary layer hodograph curvature and subsequently increasing low-level SRH. 0-6 km deep-layered shear is more than favorable to support a strong supercell -- and baroclinic effects associated with the OFB may enhance the potential for low-level mesocyclogenesis and possible tornado development with any storm that becomes rooted along this boundary and can ingest the rich low-level vorticity.