04/13/06 FCST: Upper Midwest

[nickgrillo]

The latest 0z NAM and GFS are relatively remarkable in terms of tornadic supercell potential, particularly in eastern IA by the early-mid afternoon... In fact, both models show >65 Tds across the region (perhaps a bit overdone, but still) with NAM developing extremely large sbCAPE (e.g. 3000-4000j/kg) -- supported by strong insolation/moist advection -- by the early afternoon admist particularly impressive 0-6km shear profiles for supercells and more than 150m2/s2 SRH accumulating in 0-1km layer (with veering flow in the boundary layer increasing low-level curvature in forecast hodographs for the region)... The NAM forecast soundings for the region show dry adiabatic thermal lapse rates from the sfc on up to 2km on some of the soundings. However, the one negative factor I do not like is the Td depressions progged, especially with the NAM... Most of the NAM forecast soundings from the region show MLLCLs in the ~1400m range, which isn't particularly pleasing -- but still "doable", I suppose. Low-level WAA/surface convergence near the warm front will serve as large-scale ascent to lift surface parcels to their LFC by the early-mid afternoon (likely a bit later in the afternoon further south into the warm sector) as CIN erodes through sufficient heating/moistening of the boundary layer -- with deep, moist convective initiation closer to the warm front possibley initiating as early as 1-2pm (with CIN eroding sufficiently for the release of instability, per NAM forecast soundings near the warm front in northern IA) -- then likely building southward across eastern IA, and progressing east-southeastward through the evening (possibley congealing into a severe squall line by the late evening, and then traversing through northern and central IL/IN-- supported by low-level WAA/moist advection with the strengthening LLJ by nightfall, maintaining at least 1000j/kg sbCAPE through 06z). All in all, if we can

1) Realize 62-65 Tds (and a low-level moist layer of reasonable depth) to augment both boundary layer CAPE and maintain low LCLs across the region, and get
2) Strong enough convergence in the lower levels for the release of instability by the mid-late afternoon further south in the warm sector in eastern IA -- and maintain the relatively large SR helicity in the 0-1km layer... Then the potential for tornadoes will be pretty decent across eastern IA and perhaps further eastward into IL by later in the evening...

The average storm speeds shouldn't be too terrible in the afternoon -- perhaps around 35-40mph. I will make my decision tomorrow night... If I decide to go, then I'd probably leave early THUR morning for somewhere in IA (hopefully as far east as possible LOL)...

 

[Kevin Polston]

Hey Nick......I think after looking at the 12Z ETA and going off the 00Z GFS I kind of like the area of northeast IA/southwest WI/and possibly the southeast corner of MN. Looks like thermodynmaic parameters appear strong in this area.....IF the models are correct in the moisture return. Given how the models handling of low level moisture have been less than stellar so far this year....that would seem to me to be the big question mark at this time. But if that is not a problem then yes...I think the area I mentioned looks like a good possibility for tomorrow afternoon/early evening.

 

[Kevin Polston]

I did take a look at some forecast soundings from the area valid at 00Z tomorrow....and it appears that maybe eastern and northeastern Iowa might be the best location. In southwest Wisconsin and southeast Minnesota...it appears the instability is elevated somewhat...with LFC's/LCL's correspondingly higher.

 

[Bill Schintler]

1) Realize 62-65 Tds (and a low-level moist layer of reasonable depth) to augment both boundary layer CAPE[/b]

While I haven't seen any model data based on the new 00Z data, one thing I’m having trouble with is the actual amount of BL moisture return that will occur by tomorrow afternoon. The 36hr NAM, which advertises 65F dewpoints into eastern IA and 60F dewpoints into SERN MN; has a +15F Td bias along I-80 for 00Z today (00Z IOW and AWG Tds were in the mid-30’s, vs. the 50F for the 12 hour NAM). Tonight and tomorrow, the LLJ cranks up and advects a 10C (H85) airmass into ERN IA. So, tell me, in the absence of significant evapotranspiration and considering the degree of mixing that will take place with very warm H85 temps tomorrow afternoon, how the heck are 60F dewpoints going to make it into northeastern IA?

- bill

 

[Scott Lincoln]

Taking a look at the ETA/NAM bufkit soundings for tomorrow, they look kinda marginal for it being a good severe weather day.... and thats IF the moisture its predicting does actually verify.

LCLs and LFCs are fairly close together, but kinda out of the range for anything that significant. They are in the range where the best you'll get is mostly made up of brief tornado spin ups.
Directional wind shear could be better.... its there, but only for a few hours is it marginally good. Most of the afternoon you only have about 45 degrees of turn. Towards 4:00pm when the cap is the weakest, the turning amounts to about 70-80 degrees.
There is definitely some good speed shear, and enough directional shear to see at least some isolated supercells in whatever forms, but i dont anticipate them being anything too big on the tornadic scale.

 

[J West]

It doesn't look any better today. Moisture is down south, in KS/MO, and it looks like it's gonna stay that way until later afternoon/evening. Too little, too late to get decent surface based storms going.

It appears a few elevated storms are the order of the day.

 

[Bill Schintler]

Chase target:
10 miles east of Austin, MN; along I-90.

Timing:
Storm initiation 5 PM CDT.

Comments:
A storm will develop just west of I-35 at the IA/MN border at 5 PM CDT, and track EWRD towards the target area as it begins to rotate. This storm should exhibit impressive structure and produce a funnel cloud or brief tornado before additional cells form through 7 PM. This complex will evolve into an MCS later in the evening.

Discussion:
Inspection of H7 chart and WV loops reveals a number of impulses embedded within broad ULVL ridge, with the S/WV of relevance for today’s WX lifting towards the ERN Dakotas. At the SFC, impressive moisture return is underway as a WF is developing along a CID to AXA line in IA; and the previous WF in MO, noted at 15Z from FAM to AIZ, is washing out. The only visible evidence of moisture return on the visible satellite is a CU field in MO.

Later today, the developing WF will lift NEWRD; reaching a RST/DEH/SQI line though 6 PM. Additionally, a SFC trough or DL will be located along a FOD/MCW/RST line during the same period. It is the triple point of these two features that will be the primary focus for convection this afternoon. It appears as though the potential for the best hodograph curves exists along and immediately NE of the WF; where (0-3 km) SRH’s should easily exceed 500m^2/s^2 while deep-layer shear will be around 60 knts. In this area LCL and LFC parameters will be in the neighborhood of 600m to 800m. A concern today is thermodynamics and moisture return. SFC dewpoints in the upper-50’s F; along with H85 dewpoints of around 8C and mid-level lapse rates of –8.5 C/km will support MLCAPEs of 500-1000 J/kg.

- bill

 

[Scott Lincoln]

Although i'm still worried about some moisture return and capping issues, i do see there being potential if storms are able to form today. LCLs are not the best but the directional shear is looking better in the central to east-central Iowa area. Forcing still looks kinda weak along the southern part of the trough (or dryline as the SPC thinks it will become)... but there is some potential. I may go chasing if stuff forms local, at least to see some good supercellular features without a major anticipation of great tornadoes.

EDIT: I guess if you look at the RUC Bufkit profiles for the waterloo area.... its not nearly as good in the LCL, LFC, and directional shear area. Still looks like some decent hail possible in any storms that form, but maybe they wont end up getting surface based enough to do anything fun.