8/11/05 FCST: MN, IA, WI, IL, IN, MI

[Alex Lamers]

I am surprised that there is only a 15% percentage risk of severe weather tomorrow across this region. The overall setup echoes a classic long-duration bow echo or possibly derecho type setup. Winds should be nicely veered with nearly all of the 50 knots of bulk shear concentrated in the 0-2.5 km AGL layer. LLvl baroclinic zone is setup from E SD into the Chicago area and it surges north during the day with the aid of strong southerly winds near the surface. Aloft, strong jet streak plows into the area right along the front.

I matched up a lot of charts and found that despite a few differences, this seems oddly close to the setup for May 30-31, 1998. The low pressure system obviously won't be as dynamic as in the spring and the upper level winds might be slightly weaker but everything else seems like a close fit.

I'm thinking a nice MCS will roll along the baroclinic zone. Going down the "derecho" checklist it seems marginally favorable.

Any thoughts?

 

[nickgrillo]

Yeah, not looking to bad for tomorrow...

I like the southern WI / northern IL area for tomorrow. Rich boundary layer moisture, characterized by dewpoints in the low/mid 70s, along with strong isolation, will yield marginal to strong instability by mid-afternoon, with NAM forecasting <2500J/KG of CAPE across the area (but I think it will be higher...). Very strong wind fields are in place, with backing SE winds at the SFC, south at 20-30kts at 850mb, then strongly veering to the west at 30-40kts at 500mb, and with strong low-level flow and at least modest instability, there should be a chance for at least a few supercells to develop (along with a few severe bowing segments...). Due to the strong low-level flow, this will result in high 0-6km deep-layer shear values across most of the area that I hatched in the graphic below, which could enhance any supercell potential.

I'm not so hot on the 15% risk for Day2, but I'd probably do a 25% hatched for tomorrow.

Virtual starting chase target: Madison.

<img src=http://www.geocities.com/nickgrill11/gempakoutlook8.jpg>

 

[Robert Dewey]

No sense in me re-hashing the parameters that Nick and Alex listed, so I will just continue the discussion.

Shear is fairly impressive, but I think instability is the key to this event. Obviouslly, for progressive derecho development, you would want some extremely unstable air in the genesis region. In this case, I would be looking at IA/MN - they are pretty unstable come tomorrow afternoon, but not anything that I would consider "extreme". Still, southwest WI is right within the expected MCS genesis region, and that's where I think the best shot at supercells will be.

Whether or not anything can sustain itself through the night, in the form of a severe MCS, still remains uncertain, as it's one of those tricky "can the shear compensate for instability" type situations. Usually (as shown in some case studies), once a progressive derecho initiates, it feeds off of unstable air just to the south of a boundary. Shear will usually allow the cold pool to remain tight along the edge of the convection, rather than blow out - but if there isn't enough instability, things can easily get choked off... Definitely not an easy forecast.

 

[Brett Adair]

I agree with everyone on this. SPC will likely up there percentage if not the risk entirely. Some key ingredients are in place including some leftover boudaries from the past several days convective complexes. Given the backing wind profiles especially across SE MN/SRN-CTRL WI, I wouldn't be surprised to see an isolated tornado or two develop as a couple of supercells will likely come at the onset in this region.

 

[Jarrod Cook]

Originally posted by rdewey

Shear is fairly impressive, but I think instability is the key to this event.

I agree. I think the main issue for tomorrow is CAPE/instability. As of now, it looks marginal at best according to models. NWS Chicago mentions this as well. Given the track record for N. IL this year ( :roll: ), I'm not too impressed with this set up yet. I'll cross my fingers though.

 

[Justin Turcotte]

The 18z NAM and GFS have some similarities and differences. Both advertise the H70 speed max in WI during the late afternoon. GFS advertises two robust H70 shortwaves in less amplified trough. The NAM advertises a more amplified trough with less robust shortwaves. Also some timing differences with shortwave sliding through eastern ND during the afternoon. Either model solution seems to aim at southern WI with another target possible in south-central IA. I also wont be suprised if eastern ND/northwest MN gets a good storm provided some moisture can return to the area. I suspect much will hinge on amount of insolation for any target.

 

[Alex Lamers]

Well I'm glad to hear everyone agreeing that S WI is the targeted area. We've been waiting awhile for a GOOD afternoon event instead of getting average leftovers.

The entire scenario continues to become muddier and muddier. Model differences in convective parameterization schemes and other factors have produced wildly different solutions not only from model to model but even from run to run, which is surprising given that we are pretty close to the event. I never liked 18Z runs much myself...they seemed to screw up seemingly clear thinking and tended to diverge more than the other runs. Don't know the cause of this odd coincidence or it could just be a personal preference.

Agreement on heaviest overnight precipitation is actually fairly good with both progging it to be over W/C Iowa. The GFS obviously has a bit of convective noise while the NAM is smoother.

Through the morning hours and into the afternoon the NAM likes to maintain convection in a warm advection/isentropic upglide type pattern on the cool side of the warm front. Despite being lifted into an increasingly sheared environment, this does not seem as favorable for severe weather. Meanwhile the GFS tracks its QPF bullseye (likely an MCS) eastward along the warm front and into S WI...N IL with time in the afternoon. This is certainly more favorable for quasi-linear multicells and a wind damage threat. The problem is which one to choose?

The GFS has certainly been more consistent over the past 3 runs. It appears to have support from the UKMET (although the UKMET is further north with features) as well. Problem is that there is too much up in the air to really tell for sure. What about cloud cover? Will it really matter?

Oh well at least there appears to be a decent chance of rain tomorrow and in a drought I suppose that is all that matters. And like I said, I don't like 18Z runs.

...Alex Lamers...

 

[Bill Schintler]

Chase target for August 11

Chase target:
10 mi E of Webster City, IA (I-35 at US 20).

Timing:
5 PM CDT.

Storm type:
Ongoing line of storms will approach from the west and rapidly intensify as they hit an axis of greater instability in the target area. Primary severe risk will be winds to 60 mph and hail to marble size.

Discussion:
Nocturnal LLJ and one of several mid-level waves has fired a series of MCS’s in IA/NE which tracked into IL in the early morning hours. An unseasonably strong S/WV will lift into NWRN IA this afternoon, firing another round of storms there. Important question is how much instability can be attained in light of widespread convective debris and clouds associated with the incoming wave. Recent satellite trends suggests that enough partial clearing should take place south and east of a Charles City to Carroll line in IA for decent insolation to occur between noon and 5 PM CDT to achieve MLCAPE’s to 2000J/kG. Deep layer shear to 35 kts and 0-3 SRH’s of 200 to 300m^2/s^2 will also support a severe potential.

- bill