10/22/04 FCST: Plains/midwest

[Robert Dewey]

WOW! ETA really exploded with this system between 72-84FH, with a sub 990mb low! GFS only goin out to 42 as of now, but it already appears stronger...

ETA sure does have a nice looking moisture profile, considering its October. Moisture wasn't even that good for the October 24, 2001 event, which featured a few F3's (If I remember correctly Td's at my house only climbed to 55-58F, with some pretty strong TORs just to the southwest in OH)... Don't see why it would be any different in the Plains/upper Midwest.

 

[Amos Magliocco]

Examined 0Z operational GFS and ETA and they are in excellent agreement both aloft and at the surface. Unfortunately, this system is evolving into a powerhouse, a dynamic trough shearing quickly into a shortwave with powerful winds and mainly unidirectional shear above 925 mb. Something I notced right away on the ETA was how the best helicity values are displaced from instability, particularly in Iowa where the warm front lifts quickly.

This looks like a damaging squall line and high wind event right now, maybe with emedded supercells and a tornado here or there. Definitely a moderate risk in the making, but discrete storms don't look to be the mode. I hope the morning's runs look much different, but right now I'm relaxing any chase plans.

 

[Jeff Snyder]

I'm pretty pleased with the recent change / trend in the ETA runs... The latest 0z run shows a more neutrally-tilted trough with a more westward placement of the surface low. This means the relatively high Tds are farther west compared to earlier runs... UCAR/NCAR and CoD only have the 12z and 18z GFS runs up, but the ETA continues to forecast the surface low southeast of the GFS position. Absolute winds speeds, in any case, will remain very strong, indicating, at the very least, a good chance for damaging winds. Despite decent moisture, especially given the time of year, surface instability is only on the "moderate" side, with ETA-forecast SBCAPEs running 2000-2500 along the dryline/front from IA southward to northcentral TX. The ETA is showing the best low-shear shifting into western MO by 0z, courtesy of the very intense southerly LLJ over the same location. The strength of the DPVA and the general NE-SW orientation of the dryline makes me think squall line, but this could very well change in future runs...

OUN mentioned, in their AFD this afternoon, the likelihood of the dryline being in or near western OK by friday afternoon... I agree with this forecast, as the models have continued to trend farther and farther west (slower) with the system... In addition, expect the higher Tds to "back" into the dryline by afternoon, as the models tend to "smooth out" the dryline.

 

[Amos Magliocco]

NCEP has GFS out to 72 hours attm.

Amos

 

[Jim Bishop]

Tornado Outbreak

I'm not sure why the phrase unidirectional shear has been used for this setup. Based soley on the 0000 UTC Wednesday ETA forecast for the eastern half of Oklahoma:

850mb: 40-50 knots SSW
700mb: 55-70 knots SW
500mb: 65-80 knots just north of due SW. So slightly WSW.

Given the 25+ knots of speed shear between 850mb and 500mb (not to mention the 40 knots 0-6km shear!), this setup is incredible for tornadic supercells. End of Story.

Now I do think an intense squall line will form along the axis of strong vorticity advection in Kansas. But further south in Oklahoma where there will be minial vorticity advection, I'm sure if the model verified there would be supercells. There will be strong dynamic forcing from the very strong 300mb jet, but it doesn't look to be conducive to linear development.

If the 850mb winds veer any then a squall line will be more likely.

How I wish I could chase this Friday!!

 

[Robert Dewey]

... This means the relatively high Tds are farther west compared to earlier runs... UCAR/NCAR and CoD only have the 12z and 18z GFS runs up, but the ETA continues to forecast the surface low southeast of the GFS position. ...

GFS has 64-68F Td's from eastern OK northward into western IA and extreme eastern NE at 66hrs... Spreading into IL at 78-84hrs...

EDIT: Surprisingly, the ETA moisture axis is very close to the GFS in terms of "how far west"

 

[Amos Magliocco]

I wish I could work up some enthusiasm for this. I'm definitely ready to chase. But these runs tonight are so much faster, and the surface low is actually far more northeast of prior runs, (nearly in MN by 0Z Saturday) that it looks unlikely discrete storms can form with moderate instability in such powerful kinematics.

Oklahoma's deep layer shear looks better than terrible, but I wonder about the timing for initiation down there considering sunset at 0Z and what will be a rapid loss of heating. 850 is veered and surface is southernly east of 35. That could be an area where the model gets it wrong--I hope so.

I hope I'm wrong. I'd love to jump in the truck Thursday and hightail it for Norman.

 

[Glen Romine]

Re: Tornado Outbreak

I'm not sure why the phrase unidirectional shear has been used for this setup. Based soley on the 0000 UTC Wednesday ETA forecast for the eastern half of Oklahoma:

If the 850mb winds veer any then a squall line will be more likely.

How I wish I could chase this Friday!!

In case you missed the discussion earlier, the focus has been on the convection expected further north, over IA for example. Up there, the deep layer shear is quite unidirectional out of the boundary layer. As for eastern OK, yes, the upper winds do become more westerly than further north, but surface winds are expected to be from the SSW, so again, very limited directional turning of the winds. Further, the shear vector is more or less aligned with the surface boundary orientation. recall met 101 - the surface boundary acts as the forcing mechanism to "lift" the air and generate the storms - and the deep shear steers the cell motions. So, cells developing along the front, while potentially discrete early, are likely to run intl each other quickly - which is enhanced markedly by the relatively unidirectional shear which strongly favors splitting cells that speeds up the rate that cells will colllide. Add to that the very strong dynamic forcing forecast to the north and a squall line is a sure bet further north. Deep layer shear will be adequate for storm organization well into OK - but mesoscale enhancement is likely to be required to have any significant tornadic threat based on the current forecast - so keep your fingers crossed for radical changes over the next few days relative to the current model guidance.

Glen

 

[Robert Dewey]

Best chances for severe (not necessarily tornadoes) appears to be central and eastern IA around 21Z FRI, if the ETA verified. It has the triple point, 700/500mb jets nosing in from the WSW, 850mb jet streaming in from the SW, and ESE sfc winds... Thermdynamics look pretty impressive - 68F dewpoints with SBCAPE in excess of 2500-3000J/KG - But as Amos said, with such strong forcing and a relatively weak cap, things would go linear pretty quickly (but as always is the case, some supercells will develop within or just ahead of the line)...

 

[Jeff Snyder]

Just a quick note concerning the relationship of the shear vector to storm mode, refer to:

Dial, G.L., and J.P. Racy, 2004: Forecasting Short Term Convective Mode and Evolution for Severe Storms Initiated along Synoptic Boundaries. Preprints, 22nd Conf. Severe Local Storms, Hyannis MA.

The title of the publication is linked, so... It seems that for discrete activity, we really want the mean 2-8km wind vector to be as normal to the boundary as possible. On the other hand, linear modes are much more common when the mean flow is parallel to the boundary. Comparing the 500-700mb wind vector forecasts with the forecast dryline location, it appears that the angle between them appears to be ~30-40 degrees... Yes, the mid-level flow is more westerly in OK, but the dryline is oriented more NE-SW, relative to the KS/MO/IA area, where the dryline is more NNE-SSW. So, despite having more of a westerly component in OK, the mean 2-8km winds in the IA/MO area appear more normal to the dryline than points farther south, and thus appears to favor more discrete actions than points south. That said, I think the intense dynamic forcing (DPVA, surface convergence along the dryline, etc) in IA and nearby will most likely result in squall line / quasi-linear convective mode. I'm still hoping that we'll get some sort subsynoptic low forming south of the main low, as is not uncommon (a smaller low near the panhandles)...

 

[David Draun]

Hey, my 600th post! Well, SPC has it's day three out now. Looks like a broad area of the central US is under a SLGT risk. Wow! I could just see a MDT or HIGH risk bullseye right in the center of it. Problem is that there will be a lot of cloud cover well before the system gets close. This is holding temps down. Chicago's forecast high for Friday has fallen from a Max of 72 to 66. Perhaps not enough heating to get these storm going? Probably will be something though.

 

[Dan Cook]

25% ring right over my area.

*does happy dance*

DVN's hwo and afd mention squat, though.

 

[Amos Magliocco]

12Z has renewed my spirits a little more as well as the SWODY3. I'm being very stubborn in focusing on south central and south eastern Kansas as an area with the right combination of instability, shear, and capping inversion to possibly restrain convection to an isolated mode without waiting until after dark. I'm paying close attention to conditions at 18Z and 21Z moreso than 0Z since it's dark so much earlier in October.

Here's a chart that make *me* do the happy dance: 54 hour CAPE/SRH progs for 18Z Friday as of today's 12Z ETA run.

 

[David Draun]

Nice! Those are some great SWEAT and CAPE values. Lots of helicity too.

Look at that max SRH over N IL and S WI.
And a max CAPE of around 2750 in IA.

Looks like things will be really cookin of Friday.

There will be much more "happy dancing."

I would love it if that higher CAPE was forecast a bit more to the east though.

 

[Glen Romine]

Here's a chart that make *me* do the happy dance: 54 hour CAPE/SRH progs for 18Z Friday as of today's 12Z ETA run.

Looks pretty, but if you look at 54 hour vert motion and rel humidity you get the idea that subsidence and cloud cover are supported over that apparent favorable region in se KS. It is also important to note that the favorable prog of low level helicity is aided by the development of a weak surface ridge from the convection that the model develops in w TX that slides along the Red River overnight and during the day Friday. Without any real forcing mechanism for convection in that region - I can't get myself too excited about it.

Seems there could be clearing in the dry slot south of the cyclone in the subsidence band on the south side of the 500 mb jet axis - which is far enough south to allow heating in the warm sector across IA and could give the narrow favorable instability axis leading the surface low noted here - but looking more and more like the surface wind shift will be extreme from SW to NE - and those seem to rarely produce when cell motions quickly traverse a narrow instability axis.

I'm becoming more interested in the possibility of discrete cells developing along the dryline in western OK though. Still not coming together completely - but shear there will be more than adequate and oriented more favorable to support discrete convection. If the overnight w TX convection fails to behave as forecast by the ETA, this could start to look more like a possibility - but I give it a 30% chance at this stage. This is always tough to judge given the model tendency to overmix the dryline.

Glen