6/9/06 FCST: NE / IA / IL / MO

[Brian Stertz]

Previous forecast:
I am liking the setup for Friday more and more with each run. The deep layer shear is impressive at/above 50 knots BL-6km shear and 30+ knots 0-1km shear on the 12z ETA. Instability and strong shear looks supportive of intense supercells and tornadoes, particularly across C/E Iowa in the vicinity of the boundary. As with much of the IA severe wx events of this spring, these have been centered on the Iowa City/Cedar Rapids/Davenport corridor. This particular area looks like it may get hammered again Friday.[/b]

Update of tonight's 00z ETA/WRF...shows that the more significant severe wx theat has shifted further northwest than previously forecasted. Both models were indicating a corridor of good instability and deep layer shear across W. Iowa. The BL-6km shear while not as impressive as the 12z run, but was still showing 40+ kts. across much of W/C Iowa with 2000-2500 j/kg ML CAPEs focused between Sioux City IA and Omaha NE. It looks like the probable window for tornadic supercells over extreme E.Nebraska/W.Iowa will be open after 5pm and probably start shutting down shortly after sunset when a MCS cluster or several clusters organize becoming elevated with time.

 

[nickgrillo]

Shoot, both the deep-layer kinematic and low-level thermodynamic structures around the region make FRI look sweet -- east of I-35 -- with >500 0-3km SRH and 40-50kts 0-6km shear, which is relatively perfect shear given the CAPE progged. The NAM progs a pool of ~70F tds along and east of I-35 (with a bit of pooling closer to I-380). Given the degree of moist surface inflow -- the 12z NAM indicates 2500-3500j/kg of SBCAPE. Strong deep-layered ascent in the warm frontal zone should provide enough lift for the initation of deep convection by the mid-afternoon -- although the NAM does want to hold onto CINH across central/eastern IA (admist initiating precip). Nonetheless, should we get surface-based thunderstorms to initiate near the warm front -- the degree of low-level shear would be able to significantly enhance updraft rotation (and would support strong supercells and tornadoes) with vertical pressure perturbation gradients further augmenting updraft intensity as storms ingest the rich low-level vorticity.

If I can be decently sure that surface-based storms will initiate, then I'd leave early Friday morning for somewhere in central or eastern IA. We'll see...

 

[Brian Stertz]

Thur. morning's 12z ETA is really locking into a NE Nebraska and NW Iowa supercell event for Friday night. If there can be good instability in place on the boundary near Sioux City, there is plenty of BL-6km shear to support strong and potentially damaging tornadoes Friday evening. The significant supercell action may start on the Nebraska side but get tougher as it moves into that favorable sheared environment over NW Iowa. Chase target would be within 30 miles of Sioux City if trends continue.

[/b]

Forecast update:
Tonight's 00z ETA has shifted slightly further north with the sweet spot over extreme SE South Dakota into NW Iowa. The forecasted instability has backed off some which is not a real problem; but strong deep layer shear and 0-3km shear trends have been pretty persistent, with 0-6km bulk shears of 40-50 kts. I think the corridor (while somewhat smaller than originally thought) should be supportive of a few supercells and tornadoes between Sioux Falls and Sioux City. I agree a big MCS cluster will probably gel towards sunset as the wave moves into the strong shear environment and storms become more elevated. There will be a continued severe wx/supercell threat across extreme EC Nebraska and WC Iowa on the west and southwest flank of this MCS cluster dropping southeast towards C/E Iowa Friday night. It still looks chase worthy to me as supercells should be plodding along at 20 mph, and not a real strong threat of giant hail with these supercells...kind of akin to the Wisconsin supercells Tuesday.

 

[Adam Atkins]

This morning's 12z ETA is really locking into a NE Nebraska and NW Iowa supercell event for Friday night. If there can be good instability in place on the boundary near Sioux City, there is plenty of BL-6km shear to support strong and potentially damaging tornadoes Friday evening. The significant supercell action may start on the Nebraska side but get tougher as it moves into that favorable sheared environment over NW Iowa. Chase target would be within 30 miles of Sioux City if trends continue.

Go get 'em Jeff and Mike !! I will be eastbound tomorrow on I-70 to Buckeye land for my brother in law's graduation from vet school at Ohio State Univ.
[/b]

I see a large cluster of storms developing near the low/boundary due to strong convergence and high PW values. The lack of any good mid and upper level flow will prevent any good venting aloft and leads me to believe these supercells will be short lived if any do form and outflow from individual storms will interact to produce a large MCS. I would chase if it was local but it's too far for a slim window of a chance. 2006 has seen a consistent model trend in decreasing the mid and upper level flow as an event comes closer and closer, so don't get excited about a possible strenghtening of the shortwave and deepening of the surface low.

 

[Jeff Snyder]

I'm not really sure what to think of tomorrow... It does look like the areas south of the front across southeastern NE will be characterized by hot temperatures and moderate dewpoints, leading to very high LCLs (>1500m). Along and just north of the front, the situation will be more manageable, as long as destabilization occurs. Looking at the Sioux City forecast sounding valid for 0z tomorrow evening, low-level shear should be very strong with strong hodograph curvature in the lowlevel. Midlevel flow is again progged to be ~25kts off this morning's NAM, but that will be good enough north of the front as strong directional shear helps yield sufficient deeplayer shear. 25kts at 500mb has been okay for a few events in the past few weeks, but those were largely with NW 500mb flow and S sfc winds... Here's the KSUX forecast sounding for 21z... Both appear to be in convective parameterization (with the look of the thermo and moisture profiles), so it's tough to say what the environmental profile will look like...


So, despite hot temps south of the front, very warm 850mb temps are progged to signal a relatively strong cap in the warm sector. With high LCLs and relatively weak shear, this would lead me to target those areas along and north of the front. I'm certainly on the fence... If I lived with 3-4 hours of the target, I'd definately jump on it. As it is, SUX is about 600 miles from OUN, and I'm not sure how much I want to drive that for a slight-better-than-marginal setup... If midlevel flow strengthens more than expected, or the target area shifts southward more, I'll jump on it.

 

[joel ewing]

Sitting around in the catbird's seat at my cousin's ranch outside O'Neill Nebraska. After 2 weeks....8000 miles, 8 states and nothing to show for it, I am ready for my luck to switch. Still looks iffy for tomorrow, but who knows? FYI.......there is such a wicked drought around here that even if it doesn't nader.....the heavy rain will be so much more appreciated around here than you can ever know. Pray for rain.....AFTER the tornadic window closes.

 

[afischer]

If warm frontal convergence is able to force convective initiation near the progged sweet spot of Sioux City, I agree some HP supercell structures are a distinct possibility given moderate-strong CAPE and significant vertical veering profiles hopefully contributing to ~40kts of 6km shear. The environment north of the warm front will be relatively stable, so the tornadic window may be narrow unless a storm can turn right and track ESE along the warm front. Maybe something will go farther west nearer the thermal axis/surface low, mature, and then intensify as it crosses the front. I'm off tomorrow, will be interesting to see how it pans out.

 

[Ben McMillan]

Preliminary chase target- Onawa, Iowa

Time of Departure- leave Ames by 1:00 pm

Esitmated Arrival- time 3:30 pm

Discussion-

A stationary front will become increasingly defined during the daytime hours
Friday across western Iowa as a surface low pressure system and lifts from
southwest to northeast across central Nebraska. Located in the Columbus area
by 7pm, this feature is forecast to increase convergence along the boundary
which models suggest will cause storm initiation along it from west to east
around the 6 pm timeframe. Due to a strong cap, decently offset mid and upper
level winds, and strong backed easterly surface winds supercell storm
structure will most likely evolve in any updrafts that sustain themselves.

Forecast CAPE values will range from 2500-3500 J/KG in the target area by
7pm. This, along with very impressive low level shear, (200-300 SRH 0-1KM)
will yield a higher probability of low level rotation development, and
possibly tornadoes later in the evening.

Concerns- LCLs, CAP, 500mb winds

 

[Gabe Garfield]

Personally, I have a feeling tomorrow could hold some good action for chasers. Though the overall magnitude of the winds in the upper troposphere will be weak, the deep layer shear should be sufficient for the supercell mode. If we do not see an immediate transition to MCS, there should be a decent bet for tornadoes along and north of the warm front (given the magnitude of the low-level shear). Also, I've noticed a tendency in the past for good tornado days in spite of weak mid-level winds provided there was strong enough instability and strong low-level shear. Such might be the case tomorrow.

LCLs are a little worrisome given the temps in the mid 90s south of the warm front. However, if sufficient instability can develop on the north side of the front, I think we might be in business. Additionally, I'm somewhat concerned that the progged Tds will not show up as advertised. The juice still remains in TX/LA and points southward, so we're going to need a nice LLJ plus some evapotranspiration to get the moisture in the target area.

Also, as always, we will have to be cognizant of wind profilers tomorrow as the operational models sometimes underforecast upper tropospheric flow. Tomorrow is one of those days where it could be the difference between mediocre severe and a chaser's dream.

Gabe

 

[Bill Schintler]

Chase target:
Yankton, SD.

timing:
Storms will initially form near the SFC low in Rock, Loup, and Holt counties of north-central Nebraska at 3 PM CDT, with a tornadic supercell storm moving into the target at 4:30 PM CDT as storms develop to the east of the surface low along the warm front.

Comments:
All forms of severe weather appears likely, including tornadoes. Mean storm motion will be 15mph to the ENE, with right-moving cells moving to the E. By 6 PM, storms will evolve from discrete cells into a large multicell complex. A significant concern for chasing is a lack of bridges over the Missouri River in this area. By choosing the target at Yankton, one has the luxury of being able to relocate to a refined target position on either side of the river as the event progresses.

Discussion:
A compact 40kt H5 max under highly diffluent ULVL flow in left-exit region of an H3 streak, and an attendant elongated SFC system stretching from SERN CO into NERN NEB, will set the stage for an active weather day in the Midwest. SFC low pressure should track from Valentine to a location between Norfolk and Columbus during the afternoon hours, and the target region will be along and just NE of a WF extending E and then SE from the low. In addition, it appears as though an OFB in association with a weakening MCS in SD will result in increased backing of SFC flow. The SRN extent of the associated MCS CI shield should also enhance the thermal gradient along the MO River, resulting in reasonably low clould bases (1000m - 1200m) just N of the river.

Moisture and instability will be juxtaposed with impressive shear parameters. Mid-level lapse rates of 8C/km in the H7/H5 layer and LI's to -9C will contribute to SBCAPEs to 4000J/kg as SFC dewpoints shoot into the low 70's F. Regarding shear, a concern is the (0-3km) SRH's that will be realized because of the vertical stacking of the SFC and H85 lows. The latest NAM stacks them, while older runs of other models (GFS, UKMET, and NGM) offset them by over 100 miles. In any case, even a small offset will result in impressive SRH's in excess of 500m2/s2 to the NE of the WF; while deep layer shear will exceed 50kts. Convective temperatures in the low-90's in NERN NE will be reached by mid-afternoon.

- bill

 

[Edward Ballou]

In total agreement with you Bill (Schintler). I am liking the Yankton area. The cap isn't as strong here. Theta-e advection puts a pocket of moisture right into the Yankton area. There's also a nice little pocket of higher CAPE for this area. Storm motion in this area is progged to be ENE at 20 knots. Excellent low level shear and higher as you move east. The only bad thing about the whole setup is higher LCLs and maybe the middle layer shear. Will definitely be heading out tonight, sadly I can't leave till 4-4:30 b/c of work.

 

[afischer]

LBF sounding this morning looks pretty nasty, suggesting 850mb warming and drying throughout the day in advance of the triple point low. the airmass farther east in the mid-MO river valley isn't looking much better, with aircraft soundings from Sioux Falls and Omaha showing 850mb dewpoints around 6C. difficult to tell for sure how BL moisture will shape up/recover along the length of the warm front. it's possible that backed 850mb flow across far southeast SD will retain/pool better moisture in that area.

if convergence overcomes the 850 cap, i'm also concerned about the 25kt 300mb flow progged by the latest NAM and RUC models. also, given the warmth of the air mass, those who questioned the LCLs yesterday were right in doing so; i don't see much tornadic potential today as ML LCLs may be very sad... maybe a brief spin-up if a storm crosses the front.

will check out the 12Z model runs before making a final call on whether to chase.

 

[Joe Zemek]

Today is looking questionable thanks to hot SWly flow from SFC to 850 threatening to eat the moisture. The RUC is less optimistic than ETA-NAM and NGM, which at least show decent precip over the warm front (it probably wants it elevated). Hopefully the next RUC will have better news on the moisture aspect, and if not, perhaps the strong shear will at least provide good structure in high-based supercells.

Since it's a chase day for me regardless, initial target is Sioux City for data recheck before heading either west on 20 or further up I29.

 

[Bill Schintler]

Chase target:
Yankton, SD.

[/b]

As has already been mentioned, moisture is very limited (and what moisture is there is too shallow to be of any use when considering an averaged parcel). One look at the 12Z OAX sounding, and I knew that today would be very marginal. It's also a bad sign where there is no CU field by mid-day. Given that, I would not expect much more then high-based "garden variety" storms today.

- bill

 

[Tony Laubach]

SFC obs showing what looks like a dry punch pushing into the boundry along the NE/SD border. SW winds almost take up the entire state of Nebraska leaving the SE winds east of the river. I guess the only viable target remaining for the day would be the triple point play and the brief window it'll offer. Spreads in there still remain quite hight, though, so I think tornado chances are probably very low in this area which is a shame considering the helicity in that area. Like Chris, who I spent a good 90 minutes with last night discussing whether or not to come out, elected to stay home more out of logistics than metoeological reasoning. Personally, I thought this was looking much better than its playing out ATTM. I'll keep my eyes pealed.