2009-05-29: FCST: IA/IL/MO

[Ryan Wichman]

This set-up could be a bit of a surprise event with a weak cold front (really more of a wind shift line) moving across eastern IA to be the main trigger.

Extreme Instability ~ 4000-5000 j/kg of CAPE providing strong updrafts. Not much helicity (~ 100-150) and even less low level shear. Steep lapse rates will create rapid storm growth once initiation begins. CAP looks to remain in place until late afternoon, which will allow the instability to really increase. Right now it looks like a Lamoni to Iowa City to Keokuk triangle looks on target. Not a chase I would drive a long distance for, but having it being in my own backyard it will bear watching.

 

[Scott Weberpal]

This set-up could be a bit of a surprise event with a weak cold front (really more of a wind shift line) moving across eastern IA to be the main trigger.

Extreme Instability ~ 4000-5000 j/kg of CAPE providing strong updrafts. Not much helicity (~ 100-150) and even less low level shear. Steep lapse rates will create rapid storm growth once initiation begins. CAP looks to remain in place until late afternoon, which will allow the instability to really increase. Right now it looks like a Lamoni to Iowa City to Keokuk triangle looks on target. Not a chase I would drive a long distance for, but having it being in my own backyard it will bear watching.

I'm not seeing the high CAPE you mention, the NAM shows a small bullseye of 2500, but largely 2000 cape. Regardless, E. IA could be a bit of a surprise as you were saying. I am a bit worried about the cold front sinking too fast and undercutting storms, but being close to home it may be worth a shot. I'm also wondering how the good moisture is going to get here in time unless the models are incorporating major evapotranspiration. 60 Tds are hundreds of miles to the east and southeast with westerly flow at H85...how is that moisture going to get to the upper-mississippi valley?

 

[Ryan Wichman]

Scott: Not sure if this picture will show up well enough, but I attached the link below. It is probably time sensitive. But it shows a pocket of CAPE ~5200 j/kg in south central Iowa. This is the GFS, the NAM does have a bit lower amount but still on the order of 4000 j/kg.

http://68.226.77.253/models/GFS/CENTRAL/CENTRAL_GFS_ATMOS_CAPE_60HR.gif

http://68.226.77.253/models/eta/central/CENTRAL_ETA212_ATMOS_CAPE_60HR.gif

 

[Scott Weberpal]

The 12z NAM shows roughly 2500

http://www.twisterdata.com/index.ph...nglemap&sounding=n&output=image&archive=false

Given the lack of good deep moisture, it is what I'm going to believe at this point.

 

[Andrew Pritchard]

0Z NAM has raised the bar with a pocket of 4500 j/kg sfc cape, but like Scott I think this is partially due to it's exaggeration and pooling of moisture near the front. 60F td's might be all that we see in southern IA. The current NAM is magically bringing td's into the lower 70's by 7 PM... yeah okay. It's wanting to build in quite a cap as well. It does break the cap and allow for convection, but this also could be in part to the overdone moisture.

I guess we'll see how it plays out, but I'd like to see a little more robust moisture return early on before I start believing all the high intensity output numbers.

Again, with Weberpal on this one - unless the corn is pissing dew points all day, I don't see 70F happening.

 

[jshields]

the omaha wfo sounds pretty interesting for tomorrow if the cap can be broken. the gfs doesn't break the cap, but the nam does.

SEVERE POTENTIAL APPEARS REASONABLE GIVEN THE 45-55 KNOTS OF BULK
SHEAR AND MLCAPE VALUES OVER 2000 J/KG. SURFACE FRONT AND
ISENTROPIC LIFT ABOVE FRONTAL ZONE WILL LIKELY LOW AND MID LEVEL
FORCING ALONG EAST-WEST BOUNDARY WHICH SHOULD PROVIDE LOW LEVEL
HORIZONTAL VORTICITY FOR THE STORMS TO DEVELOP ROTATING UPDRAFTS AND SOMEWHAT LOW CLOUD BASES. ORIENTATION OF THE SHEAR VECTOR TO THE LOW LEVEL FORCING SHOULD RESULT IN INITIALLY DISCRETE SUPERCELLS.

storms could start to fire by 20Z tomorrow afternoon. i'll be watching closely!

 

[Darrin Rasberry]

If there's a cold front-y feature with the attendant relatively unilateral forcing, then either absolutely nothing is going to happen or, with all the drainage from the early-winterish wraparound 1 day long, 2 inch/hr rainathon here yesterday, the entire boundary will be filled with chattery segments and HP's with discreteness lasting (as it always does this time of the season in Iowa) about as long as it takes the towers to fill all the remaining gaps between your storm and the next one. You can get a spinup here from a briefly discrete supe/quasisupe, especially in this CAPE, but when the chorus line all grabs hands and forms a massive linear/quasilinear wall one hour after the first initiation kicked off, your hope reduces to imbedded echoes. Again, that's Iowa at the foothills to June, where the E.T. here probably kicks in a little bit of that frustration I always have with the "superdevelopment." Shoot, the only discrete trophy we've had here (back in early June) was off a theta-E ridge, and even that was rainy and low-vis.

Expect to see wet storms tomorrow. That's the Corn Belt at this time of year, especially if we don't get at least some bone-biscuit dry air like most of Nebraska does with those drylines. Tornado chances appear for just a bit, as usual for Iowa's rapid and expansive development and the overstuffed buffet of choices that results (and the low window of time that follows for enacting a choice). Until it goes into the inevitable shelf cloud when the light wind support just can't take nummorez with a state full of storms, you'll have a small window with a lot of guesses. Pick one, and beyond that, with the limited time it's just pray and stay. But stay for the shelf cloud - Iowa does manage to carve out the best ones in the country.

 

[Bill Schintler]

Chase Target for Friday, May 29

Chase target:
Woodland, IA (south-central part of the state, near the MO border)

Timing and storm mode:
Storms will form along the IA/MO border, building westward to the target area through 4 PM CDT with multicell storms the dominant mode, although a few supercells will be possible. Storm motion will be towards the southeast at 20 mph with most storms remaining below severe limits. I caution to add, however, that this could be a low-expectation sleeper event.

Synopsis:
Little change noted in the synoptic pattern, with the Great Lakes ULVL trough dominating the pattern with -30mb/12hr H5 height falls noted over the ERN Great Lakes along with a deepening and strengthening trend. Deep moisture is below seasonal norms over most of the Plains and Upper-Midwest, with 60F SFC dewpoints limited to SRN TX. Locally, weak ridging was noted over NWRN IA and SWRN MN; while an upstream low was centered over South Dakota in association with forcing at the exit region of a 100kt H3 speed max.

Discussion:
The WRN CONUS ridge will amplify during the period, resulting in increasingly cyclonic ULVL flow over the Upper-Midwest. This will drive the aforementioned South Dakota low towards the area, with WAA increasing over IA and MO as a WRLY H85 LLJ develops. Models indicate a narrow axis of instability developing immediately S of the W/E-oriented SFC boundary near the IA/MO border. MLCAPE’s will increase to 1000-1500J/kg as SFC dewpoints increase to around 60 F beneath moderate mid-level lapse rates of 7-7.5 C/km.

Models vary widely with LLVL moisture and degreee of heating, with the 12Z NAM suggesting by far the highest dewpoints and resulting instability. The 12Z GFS and UKMET suggested less moisture and were further S with the axis of convergence and QPF, focusing along the IA/MO border. The latest ECMWF indicates very little QPF through Friday evening. The 00Z runs flip-flopped; with the GFS now most bullish with regard to moisture, QPF, and instability. The NAM is now the most reasonable given low 00Z dewpoints as noted on upstream OAX, TOP, ABR, and BIS soundings. Taking a closer look at moisture, it’s not clear how SFC dewpoints of around 60F will verify given 03Z, 05/29/09 observations in IA indicating around 55F in ERN IA but 35-40F in NWRN IA with flow out of the NW right above the BL.

On Friday, lift and shear will increase as the H7 shortwave races northeast across IA, along the SRN periphery of the amplifying ULVL trough, through 00Z, 05/30/09. Deep-layer shear vectors of 30-40 kts, nearly parallel to the boundary orientation, may favor discrete development. Prior to convective initiation, broken CI will overspread WRN IA as elevated convection over ERN NEB approaches, signaling the arrival of the wave. SFC-based convection will initially form along a line from Mt. Pleasant and Ottumwa in IA, and then back-build west to about I-35.

- Bill
11:10 PM CDT, 05/28/09

PS: I'll be looking at data throughout the day - anyone please feel free to PM me for nowcast support.

 

[jshields]

i'm pretty iffy on tomorrow's chances. like bill said moisture return is questionable. the dewpt right now here in omaha is up to 52 but their doesn't seem to be any source that will bring that up at all. the new day 1 outlook has introduced a slight risk, but they are expecting mainly multicellular storms.

 

[Kevin Walters]

If there's a cold front-y feature with the attendant relatively unilateral forcing, then either absolutely nothing is going to happen or, with all the drainage from the early-winterish wraparound 1 day long, 2 inch/hr rainathon here yesterday, the entire boundary will be filled with chattery segments and HP's with discreteness lasting (as it always does this time of the season in Iowa) about as long as it takes the towers to fill all the remaining gaps between your storm and the next one. You can get a spinup here from a briefly discrete supe/quasisupe, especially in this CAPE, but when the chorus line all grabs hands and forms a massive linear/quasilinear wall one hour after the first initiation kicked off, your hope reduces to imbedded echoes. Again, that's Iowa at the foothills to June, where the E.T. here probably kicks in a little bit of that frustration I always have with the "superdevelopment." Shoot, the only discrete trophy we've had here (back in early June) was off a theta-E ridge, and even that was rainy and low-vis.

Expect to see wet storms tomorrow. That's the Corn Belt at this time of year, especially if we don't get at least some bone-biscuit dry air like most of Nebraska does with those drylines. Tornado chances appear for just a bit, as usual for Iowa's rapid and expansive development and the overstuffed buffet of choices that results (and the low window of time that follows for enacting a choice). Until it goes into the inevitable shelf cloud when the light wind support just can't take nummorez with a state full of storms, you'll have a small window with a lot of guesses. Pick one, and beyond that, with the limited time it's just pray and stay. But stay for the shelf cloud - Iowa does manage to carve out the best ones in the country.

In my experience late May and June have provided me the opportunity to photograph many spectacular classic supercells in Iowa. In fact June is often the best month. Cold fronts are not favored in Iowa or anywhere else for discrete storms. As you get into July storms tend to become more "wet" and morph into raging squall lines. Although I have observed classic supercells in July. As for evapotranspiration, it does not become a real factor until July when crops start to mature.