6/25/07 FCST: ND / SD / MN / MT / WY / (MB)

[Bill Schintler]

Chase target:
Washburn, ND (25 miles north of Bismarck).

Timing:
Storm initiation 8 PM CDT. A few supercell storms will be possible early in storm evolution before a transition towards a linear complex takes place.

Synopsis:
SFC and Satellite analysis indicated that outflow-enhanced CF had pushed S of an HEI to BIS line in the Dakotas. In the upper-levels, a lead S/WV was exiting NERN ND while upstream 55kt speed max was rounding base of deamplifying H5 low. 18C H7 thermal ridge was nosing into WRN SD, leading to very capped EML soundings. Moisture is in question as depth was limited over WRN SD while 35kt LLJ was transporting 12-14C dewpoints across ERN SD and into ND. Meanwhile, a much drier airmass in the H95-H8 layer was lifting over WRN SD from the SW. An area of strongly capped instability was noted over the central Dakotas, as 12Z BIS sounding indicated steep mid-level lapse rates over a 150mb deep moist layer with convective temperatures well excess of 100F.

Discussion:
The entire area will remain strongly capped during the afternoon hours. Once storms fire during the early evening, modest shear and instability parameters will be sufficient for a few high-based supercell storms immediately to the NW of the SFC CF front. This front will provide the focus for convection during the early evening hours as it surges to the SE after 00Z in response to an H7 impulse, accompanied by mid-level temperature drops of several degrees. With the strongest ULVL flow to the north of the area, bulk shear will remain AOB 30 kts while a veering and strengthening 40kt LLJ over backed SFC flow NE of the low may result in a marrow axis of 200m2/s2 SFC-3km SRH. Just how far west the stronger H85 flow sets up is in question as the WRN edge of the LLJ may be too far east of the developing convection, which would locate the best hodographs east of the instability and H85 theta-E axis. MLCAPEs should reach 2500J/kG beneath the aforementioned steep mid-level lapse rates.

- bill
[FONT=&quot]9:30 AM CDT[/FONT]

 

[Justin Turcotte]

6/25/07 FCST: ND/SD/MN/MT/WY/(MB)

(Edit: Looks like Bill and I were typing at the same time. Mods should feel free to combine this thread.)

First a quick background. An ever so slowly southward sagging boundary has produced tornadoes three days in row. First in MB and north central ND and yesterday in extreme northwest MN. Severe storms have also fired further to the southwest each day. The same set-up is in place today.

Elevated MCS is currently propagating through northwest MN and should tend to dacay with time later this morning as LLJ slackens a bit. There doesn't appear to be significant outflow from the MCS per surface obs or readily apparent lines on the VIS (yet). Skies should clear quickly this morning allowing strong instability to develop along the frontal boundary. Northeast ND and northwest MN have had significant rainfall the last few days and dewpoints in this area have topped out in the 70's each of the last few days courtesy of moisture convergence along the boundary along with plenty of ET. It is possible the Hallock, MN sensor is biased toward the high side which generates some rediculous instability numbers on the SPC mesoanalysis but surrounding obs have supported low to mid 70's dews which still leads to very strong instability. Yesterday, a the RUC magically changed the H70 scematic around 20z, perhaps as a result of an 18z sounding or two. H70 temps in northeast ND and northwest MN were more in the +9 to +11 range rather the +12 to +14 earlier suggested. A late evening tornadic supercell eventually developed in that weakness. The 12z RUC and NAM advertise a weakness in the H70 cap over eastern ND and northwest MN this evening. Lift in the area should be enhanced this evening as it resides in the right rear quad of the upper jet streak. Bulk shear more than sufficient for supercells and the boundary will provide plenty of helicity for potential tornadoes as it has the last few days. I'll be heading out this afternoon boom or bust.

Further southwest storms are expected to pop off the higher terrain in northeast WY and Black Hills region. Bulk shear and instability is sufficient for some potent cells. An isolated tornado is possible along the boundary with moist northeasterly upslope flow in place otherwise wind appears to be the primary threat given larger Td-depressions.