07/17/05: FCST: Upper Mississippi Valley

[nickgrillo]

As I mentioned earlier, the timing appeared to be flip-flopping a bit - now it appears to the best setup will be on Sunday...

Height falls are pretty strong with this setup... Wind fields are strong through the layer, and with good moisture pooling, instability gets up over 3000J/KG pretty easily. Shear is pretty impressive, and it will likely lead to severe thunderstorms / supercells. As for a strong squall line to develop, we have moisture pooling and high theta-e, intense height falls, pretty good wind fields, strong instability, etc., so we shall see...

Target: Southwest MN/northeast NE (LOL way too early to pin it down anyways)

 

[Jayson Prentice]

Nick, I'm watching this day closely beings it may be a local chase for me... I was looking possibly a little further east, but models are most likely going to fast with the front. I was thinking southern MN somwhere... Either way, here I go...

Tonights NAM is complete and my analysis is as follows: lol

Basically the area of interest after this run would be the southcentral MN area... Starting off, the cold front should stretch from northcentral MN south/southwestward into southwest MN and then back more to the west and central NE. 250 winds are strong just behind the front, and 300 divergence is decent across central MN just ahead of the s/w that should be moving through. 500 vort shows a capping inversion in front of the cold front while just along/behind a nice vort max looks to be in place, this is mainly over areas of eastern Dakotas though. Surprisingly enough the NAM only has 700 temps at 10/11 degrees through the area of interest, which could easily be overcome given the degree of instability/lift from the cold front.

Cape values are at or above 4000 j/kg, but helicity values are not to impressive at this point. That may be pointing more towards a multicell storms instead of supercells. Nice Td pooling ahead of the front in central MN with low to mid 70's... LI's are very good as this looks to be quite a strong cold front, this should mean no problem to break any capping that is in place. VGP has a nice area in MN/IA which lends me to believe that the area would be more favorable for any good supercell development. LCL heights are decent at least, but very good LCL heights in place from Mankato northward with them being ~500 m or lower. Essentially only problem is the shear that is lacking from the cold front...

More runs will of course change the focus and strength... Talk with local mets and other NWS indicated that the models may be underestimating the threat, time will tell...

 

[Scott Lincoln]

Not sure that I would call it a strong cold front. The GFS does show a strong temperature contrast on its 0Z run, however it quickly fizzles and almost appears to turn into a warm front and head back north. Or maybe the boundary just fades entirely. The only way to see the front very well visually on the ETA is probably by using the forecasted Theta-E values.
Just because the Lifted Indices are high doesnt mean its going to be a strong front. I'm not even sure that there is a connection there at all.

I see the possibility of storms as long as the front holds out, but probably more like a heavy rain producing squall line. Supercells I suppose are possible, but the winds are not all that favorable at least in my opinion.

Time will tell, of course.

 

[Justin Turcotte]

Largely unidirectional shear parallel to the front advertised by both NAM and GFS at this moment. Of course, there could be an outflow boundary levtover from Saturday night convection in Dakotas that would offer a little more low-level helicity. Following GFS, there may be an area of enhanced environmental shear in southwest Minnesota/northwest IA. Respectable cap ahead of the boundary makes me wonder if the bulk of the action would be post frontal.

 

[Jayson Prentice]

SPC seems to believe that it will be a potentially broad area of severe weather from the Canadian border into Nebraska. Storms look to be largely linear in nature do to the front moving quite fast. But, I believe there is hope with some of the first cells beings a strong cap should be in place just ahead of the front. Helicity is good in the earlly afternoon hours, but by late afternooon when initiation is likely the helicty is no where to be found. Either way, it will be very local for me and I will be out somewhere. Can't pass it up...