08/07/09 FCST: NE, SD, ND, MN, IA, WI, IL

[Bill Schintler]

Storm Chasing Targets for Friday, August 7
Spencer, IA (east).
Bullhead, SD (west)

Timing and storm mode (east):
Storms should fire after 6 PM CDT, with all modes of severe weather likely. A transition into a derecho is possible during the evening, with widespread wind damage possible as the system tracks through IA.

Timing and storm mode (west):
Storms will develop after 7 PM CDT.

Synopsis:
Temperatures were again mild in the Upper-Midwest with high pressure centered over eastern IA into IL and sunshine filtered from CI blow off from numerous convective complexes over NEB and SD, which resulted from WAA. Looking ahead, the closed upper-level low currently over the PAC NW will slowly edge southeastward. One or more disturbances will eject from the trough base, round the top of the northern Plaines ridge, and then translate SE into the Upper-Mississippi valley. By Friday morning, a SFC boundary should be aligned parallel to and north of I-90 in SD and MN. At the lower-levels, southerly flow will transport higher dewpoints into the western Dakotas; with the LLJ then veering towards the NE into MN over the next 24-48 hours. This will bring a gradual moistening of the BL over the northern Plains and Upper-Midwest through day-2. Meanwhile, high-pressure over MN into IA will shift eastward as a lee low-pressure organizes over the western NEB panhandle into western SD, with a WF extending northeast of this feature, extending into CNTRL MN.

Discussion – western Dakotas and Nebraska:
Tonight, storms will continue track E through NEB. Several elevated convective complexes will be ongoing during the first part of day-2. Outflow boundaries in the wake of these storms will provide the focus for surface-based convection later in the day. Moderately strong capping will be in place with an inversion around 750 mb owing to the EML. This cap will weaken after 22Z as the exit region of a compact shortwave overspreads the area, with initiation by 00Z over northwestern SD in a zone of easterly flow north of the SFC boundary. Steep mid-level lapse rates coupled with generous LLVL moisture will result in a very unstable AMS. Moderate shear parameters will contribute towards storm organization, with deep later shear AOA 50 kts associated with the aforementioned shortwave. Modest low-level directional shear will slowly increase after 03Z as the BL decouples and a LLJ backs back towards the west.

Discussion – Upper-Midwest - Tonight:
Two convective complexes should affect the area as a 50-60kt LLJ feeds an extremely high PW AMS into the area. One elevated convective complex is ongoing in association with the H85 WF in SRN MN and NRN IA. This activity should increase in coverage and intensity overnight as the LLJ veers into the area along with steepening of mid-level lapse rates along the IA/MN border. Corfidi vectors takes this activity E and later SE into ERN IA, especially areas N of US-30. A second bulls-eye for convective genesis is southwestern IA after 3 AM along the SFC WF. This activity may eventually merge with the complex further to the north over eastern IA. Heavy rainfall will be the primary hazard with this activity with large amounts of moisture feeding into it.

Tomorrow:
Guidance initialization is better then average for this time of year with the UKMET and ECMWF similar with forcing trends, with the GFS also seeming to have a decent handle on these fields, while the NAM is the worse and does not depict the ongoing MCS activity in southwestern MN.

A large convective complex should be ongoing in the NERN half of IA during the AM hours, with some degree of back building during the morning hours. This activity shifts east through mid-afternoon, with attention turning towards northern IA for renewed severe development Friday evening. Clearing will slowly take place from W to E, with strong instability developing over IA. Capping will also be strong with H7 temperatures of 12-13C nosing into the area. Initiation should take place along the NRN edge of the CAP along the IA/MN border after 00Z, aided by assent from another shortwave. An outflow boundary from earlier convection should also increase convergence. Instability and shear parameters will support all modes of severe weather. SFC-3km SRH will increase to 300m2/s2 as a veering LLJ strengthens to 50 kts, while deep-layer shear increases to 50 kts aided by the mid-level jet streak. This activity should track through northeastern IA in areas N of US-20, and later into WI and NRN IL.

- bill
09:45 PM CDT, 08/06/09

 

[Jeff Duda]

As is typical of mid-summer severe setups across (at least) Iowa, the biggest limitation to the supercell/tornado threat is the weak mid-level flow. No model I looked at placed 500 mb winds greater than 40 kts anywhere over the upper Midwest, save for possibly the far northern Plains (ND/MT). 850 and 700 mb flow will be excellent, with a nearly continuous LLJ throughout the day keeping winds at 30-50 kts at those levels and at SSW to SW. However, depending on the model, surface winds, despite being sufficiently high velocity, veered slightly throughout the day. I'm starting to agree with those models seeing as current surface winds in the central plains are already SSW, and the all-day LLJ is veering quite a bit right now, which would influence surface winds by mixing. Since 500 mb winds are WSW at best, this does not put much directional shear into the picture.

I'm also a little worried about 700 mb temps. They are progged to be around 12, which, for this time of year, isn't too bad, but with as cool as it's been, I don't know if that will be more of a limiting factor today or not. With the morning convection that has moved across NE/IA, the cold pool and clouds certainly don't help things, but skies are starting to clear from west to east. There are currently a few OFBs in Iowa, which could help shear, but I'm going to throw out a guess that in the upper Mississippi valley today (IA, S MN, NE etc), the storm mode will be linear.

 

[Jeff Duda]

A look at recent satellite trends shows a low-level stratus layer has formed, with its back edge to the OFB across central IA. This could really shape that boundary with time today.

 

[Scott Weberpal]

I'm not so much worried about the H5 flow, most of the classic, cyclic tornadic supercells I've seen were accompanied by H5 flow between 30 and 40 kts. It looks like a big kicker, especially in Iowa today will be the ill timed shortwave. It looks like an initial wave will move through in the early afternoon leaving most of western Iowa under shortwave ridging with the second wave not coming through until after dark.

If the aformentoined stratus deck serves to help create a strong thermal gradient thus enhancing convergence at the surface, that could be enough to get an isolated storm going. However, getting CI before dark in Iowa appears unlikely at this point.

 

[Mike Hollingshead]

Guessing Sioux Falls is going to be the place to be this evening as thermal axis noses up to the warm front near there(just a bit south of there for not). I only hope that front doesn't start moving north as I'm not sure it's worth driving a lot further for or not. Given the ese boundary to turn a storm right on, very strong low level winds bisecting that boundary, and enough vent aloft, plus strong instability...I think it's worth keeping the cap honest today and being out there. One of these summer setups in/near IA with big parameters(at least low level shear and instability most likely) has to pay off sooner or later right? It feels like a bazillion other similar ones in the same area, the same time of year, that loooove to cap bust.

Here's to the siggy tor index values of 9-10 later and the flat cu and tall muggy corn that come with it.

 

[afischer]

Mike H is sure right, these summertime cap busts love to happen in the ABR-FSD-SUX-SLB corridor... except this one doesn't even have the sfc cyclone adjacent to the target, which would enhance convergence and direct the thermal axis more strongly from the SW.

A well-timed series of 3 recent TAMDAR soundings at Sioux City showed 700mb temps increasing from 12.6 to 13.8C during the last 2 hours (though there could be some minor errors in the observations), and a slowwwwly deepening boundary layer with lowest 100mb ML dewpoints gradually topping 70F. If these temps aloft get any warmer (we're already pushing it), we can obviously forget about initiation in the mid-MO river valley...it just won't happen. If we can keep the 700mb temperatures AOB 13°C and warm/deepen the boundary layer such that the sfc warms well into the low 90s°F right up against the warm front, there's at least a shot of initiation if the boundary remains reasonably convergent. If we lose the stratus and heat/destabilize the cool side of the boundary, I worry the convergence will wash out. Large scale support as with most of these types of cases is pretty much lacking, despite a low-amplitude impulse perhaps approaching the area during the next several hours. I'm watching this one from KC so am probably slightly negatively-biased.

 

[L.B. LaForce]

I wish I could get a hold of TAMDAR soundings..heh

Anyways, i was looking at the latest profile for Cherokee, IA in BUFKIT, and those 700mb temps don't look good, not to mention the H5 winds at 25kts.

RUC had 13.9 C
GFS had 13.5 C
NAM had 12.1 C

If something does go, cool, but im pretty confident it was a good idea for me to stay home

 

[Nick Hartley]

We are in Canton, SD. Currently the Warm Front is rather diffuse but just a bit north of Sioux Falls to near Spencer. There is nothing to promising on Visible satelite imagery, as the intial bits of cumulus left at 5pm and now we are just starting to get a drop in the 14c 500mb temps that were measured by Acars at 2pm from Sioux City. Hopefully the combination of convergence, a strong low level jet that is readily increasing during this time and the slow dropping of mid-level temps over the next hour could yield a possibility at intiation. Sioux City is reporting 90/79. Dewpoints are getting crazy as moisture continues to advect an evapotranspirative and recently rained on area. Quite a setup given the instability and decent bulk shear. Spencer would tend to be the more obvious target right now- where LAPS and SPC mesoanalysis shows convergence a little more defined. SE SD, seems a little bit up in the air- with 50 cinh still remaining due to the fact that mid level temps never dropped as indicated. We still have 1-2 hours with continued strong moisture coming up making up for the drop in surface tempatures.

CARPE DIEM!

/Scott Olson & Nick Hartley

 

[Jeff Duda]

yeah...another cap over Iowa...go figure.

I made the decision not to go out after watching 700 mb temps fail to drop and no decent cumulus field form despite the convergence along the OFB. Glad I did. CI FAILURE!

 

[Chris Allington]

This is a perfect example of 2009, check out this little excert off of the Omaha AFD:

HAVE UPDATED THE FORECAST TO REMOVE POPS FOR TONIGHT. 00Z KOAX
SOUNDING INDCD EXTREME POTENTIAL INSTABILITY ACROSS THE AREA WITH
NEAR 5800 J/KG OF CAPE...BUT THIS WAS STILL BE CAPPED BY AROUND 50
J/KG OF CIN. MID LVL SHRTWV TROF SEEN IN AREA VAD/VWP`S IS MOVG
THRU ERN NEB ATTM AND WL BE OUT OF THE AREA BY LATE EVNG. UPPR LVL
RIDGING IS THEN EXPECTED TO BUILD IN WITH SUBSIDENCE ALOFT. THUS
EXPECTING ANY CONVECTIVE DEVELOPMENT TO BE NORTH AND EAST OF OUR
FA. OTHER ELEMENTS OF THE FORECAST REMAIN IN GOOD SHAPE AND NO
CHANGES PLANNED THERE.

NEW ZFP/GRIDS ALREADY SENT.

 

[afischer]

And the 00Z Omaha RAOB comes in with... only 11.6C at 700mb (!!!!) and little to no MLCIN for locations that warmed to near 90F along the MO river valley (assuming near-similar evapotranspiratively-enhanced low-level moisture). Thus, it looks like 800-600mb cooling and deep-layer moistening occurred just downstream of that apparent s/w impulse heading at the area. Pretty tough to know whether this cooling overspread the sfc warm front to the N (Sioux City 23Z TAMDAR still had ~13.5C at 700mb), where low-level convergence was maximized. Haven't been watching closely but based on the mid-level cooling and moistening on the OAX RAOB, I'm surprised the more deeply mixed warm sector didn't pop a high-based storm or two beneath that impulse. Wasn't much of a prefrontal trough in the area which is a negative, but there was some wind convergence w/ stronger sfc flow coming up out of KS and toward the GRI/LNK areas. Anyway, interesting/tough case.