8/13/07 FCST: ND / MN / IA / WI

[Chad Cowan]

Potentially significant severe weather event shaping up for Monday afternoon through the night from central MN to western WI... extreme instability, forcing and shear will all be in place. Temps could reach the century mark into parts of MN, along with Tds over 75. The resulting CAPE values will be through the roof. Capping will be the concern with the nasty H7 temps, but it seems that the WF should provide enough forcing to compensate for that.

Check out this hodo for Alexandria (100mi NW of MSP) @ 0z:
http://beta.wxcaster.com/cgi-bin/parse_skewt_trace.cgi?model=NAM&fcsthr=024&STATIONID=kaxn
500 m2/s2 0-3km SRH
+5000 j/kg SBCAPE
Negligible Cinh


If the models remain consistent, I would expect an upgrade to MOD risk.

 

[Andrew Pritchard]

Wow, looks like I picked quite a time to be up here. This one has painful bust written all over it to me, however. Just seems like one of those days when we end up with extreme parameters and the cap will hold on for dear life. Another day of "if the cap breaks, things will go INSANE!" posts in the NOW thread, before everyone gives up and quits posting at 8 pm .

That low level jet really cranks up though to 45 kts, with a 55 kt streak atop it. The wrf does want to breach the cap by 0z around Minneapolis, actualy removing all the cinh by then. Looks like waa should pile up mid 70's dew points again along the warm front so it should be nice and sticky out there.

It would be ideal if something could go before sunset and utilize the extreme instability and shear, but I have my doubts. I do think that by dark we will be seeing another bow echo raging across western and central Wisconsin like last night.

Can any residents of MN pm me or include in your forecast how far north chaseable terrain goes? Being in next door Wisconsin til Tuesday I may venture over if I can get over the cap, but don't feel like taking my chainsaw along.

 

[Chad Cowan]

Here is an updated hodograph, for 03z at St. Cloud, NW of Minneapolis:
http://beta.wxcaster.com/cgi-bin/parse_skewt_trace.cgi?model=NAM&fcsthr=027&STATIONID=kstc

0-1km SRH according to this model is 481 m2/s2

 

[Bill Schintler]

Chase target for Monday, August 13

Chase target:
Litchfield, MN.

Timing and storm mode:
Storm initiation 8:30 PM CDT. Expect elevated supercells with large hail the primary threat following initiation. By late evening, a transition to a forward-propagating MCS will take place, with storms racing to the SE towards NE IA.

Synopsis:
Little change this evening in the large scale pattern which continues to be dominated by a broad ridge over the CNTRL/SCNTRL CONUS. A parade of disturbances continues to move E through the stronger band of westerlies along the NRN US. A deepening trough was noted over the NERN US while a decaying MCS/bow echo in SRN IA and NRN MO was associated with a S/WV over IA/MN. Of concern for tomorrows WX is a strengthening 50kt H5 streak nosing into WRN MT with associated assent spreading over WY which is well depicted on the WV loop. LLVL moisture in the Upper-Midwest was limited with dewpoints generally in the mid- to upper-50’s in CNTRL MN, however a LLJ was rapidly strengthening over the Dakotas, and WAA will increase rapidly overnight. At the SFC, a weakening stationary boundary was noted near the IA/MO border.

Discussion:
High pressure now over MN will shift to the E overnight as a WF develops and lifts NE towards the area. Significant moisture return will begin during the day Monday, with an axis of 70F dewpoints surging into CNTRL MN by 00Z. Primary concerns are cap strength and timing of subtle forcing features as strong mid-level WAA will commence tomorrow morning over MN. MDLs differ on the timing and strength of an H5 vort max advertised to arrive in WRN MN by early evening, with the WRF stronger and further S. A potential negative is that the strongest forcing will be slightly N of the instability axis and SFC features. Current reasoning is that SFC/based or slightly elevated convection will take place during the early evening hours as the LLJ begins to strengthen. Stability parameters and large hodograph curvatures certainly suggests supercell storms, while tornadoes seem possible with cells that root just NE of the SFC boundary in the cooler and more backed BL air. During the evening hours, a 40kt LLJ will aid in the SEWRD propagation of a large MCS which should track into SE MN, NE IA, and WRN WI overnight.

- bill
[FONT=&quot]11:00 PM CDT, 08/12/07[/FONT]

 

[Andrew Pritchard]

I hate caps. I'm eyeing the area from Willmar to around Mankato. WRF still wants to erode the cap by 0z with very strong VV's at all levels. Of course with extreme instability and effective shear of 60 kts, and good low level turning we'll be looking at supercells with whatever develops. However, even with it breaking the cap, I'm hard pressed to believe it. I'm not terribly worried about LCL's as they should be around 1400-1600, which would be barely enough if a storm really wanted to do it. It's only a 4 hour or so drive from here, but I really don't feel like sitting alone in the middle of nowhere Minnesota watching sunny skies and baking in the heat with no reward. I guess I'll just sleep on it and see how things look in the morning. I'm guessing exactly the same; great if the cap goes, but don't count on it going.

 

[Justin Turcotte]

Elevated supercell in north central ND is taking a right turn to the southeast. A couple smaller cells about as well. Environment ahead of the storms is highly sheared with ample elevated instability so the storms should hold their own. Pockets of strong heating this morning into the afternoon may allow the storms to become rooted closer to the surface in eastern Dakotas/westrern MN. If the big cell near Rugy, ND does stay togther and moves an hour closer to Grand Forks than I might give it a gander. I cannot stray too far as I have the evening shift today. Chasers comming into MN should be aware there is a lot of MT fire smoke in the air which could hinder surface heating and visibility.

 

[Craig Maire II]

Today

Looks like Iowa should be included in this thread, I'm looking at a nice bow echoe with embedded supercells to impact parts of north central and northeast Iowa sometime after 8pm, main threat looks to be damaging winds in excess of 70 mph.