10/22/04 FCST: Plains/midwest

Mike Hollingshead

For discussions about the BIG day Friday WILL be. Jinx. I really like the looks of tonights GFS for c NE and to the ene and south from c NE. SFC dews will hopefully be better up near the sfc low and west to the sfc trof. Looks like it has some potential to be big. 80knts at 500mb will assure some rather high storm speeds given this unfolds as is. Very nice dry punch aloft over the boundaries. Thoughts.............
 
Seems rather silly posting on a day still so far out, but why not. Would tend to agree with Mike that the location of the surface moisture boundary is probably projected too far east - as this is a typical model bias to over mix the dryline to the east. So, forecast is for low-mid 60 dewpoints - which is pretty good for mid October. GFS has a strong jet streak rolling around base of trough on Friday, leading to cyclogenesis in the northern plains and will aid in backing of surface winds to SEerly, with surface low tracking from sw NE to sw MN from 12-00Z. Winds south of warm front expected to be SSW, and deep layer shear orientation is also expected to be from the SW, so little turning of the winds in the warm sector and hence along the dry line, which should rapidly then evolve into a convective line instead of discrete cells. So, might have to work the warm front instead, but cell motions will not favor long residence along the boundary, so most warm front convection should become elevated quickly. So, that basically leaves trying to play the triple point - which is not forecast to have favorable instability - but maybe due to over mixing of moisture. So, the early look I'd call this event marginal - but a lot can and most likely will change in this forecast.

Glen
 
Well, my 48 hour abstinance from the models didn't work out too well, and I am now looking at the 12Z data from the ETA...

Using ONLY the ETA (wishcasting basically), the best threat would appear to be from eastern OK into southern WI, with southern WI/IL/IA/MO area having a pretty unusual looking setup. Dewpoints actually climb into the upper 60's across this region, which is very rare for October - ETA also forecast CAPE of close to 3000J/KG (using forecast soundings with a parcel of 74F/64F temp/dewpoint averaged around the region), with close to 400m2/s2 of helicity!

As Glen said, this will change with time... But it is just interesting that this is even a forecast possibility, as we head towards the end of October!
 
Hope the eta's trend isn't going to continue with the veering sfc stuff(more positive tilt nature to the system.....trof is sort of "flat"). The good thing about looking at things so far out is when they don't look terribly good they have room to improve, I guess. This is the wrong direction though.

The Friday morning "lows" the eta is spitting out the last two runs is rather interesting...lol. 75?
 
Yeah, the ETA forecast may not be very reliable (typically has less skill than the AVN until forecasting within 36 hours). Note the double warm front structure forecast, with persistent convection from 12Z onward north of the southernmost boundary - which usually doesn't materialize as forecast in my experience. But if it did, that would offer a more favorable scenario for tornadic potential than is present in last night's GFS. Mike understandably may not care for the eastward shift - but it strongly favors a better system by improving the deep layer directional shear - leading to more favorable storm motions. Since it is closer to my location - I hope the solutions come closer to the extended ETA.

Glen
 
Hope the eta's trend isn't going to continue with the veering sfc stuff(more positive tilt nature to the system.....trof is sort of "flat"). The good thing about looking at things so far out is when they don't look terribly good they have room to improve, I guess. This is the wrong direction though.

The Friday morning "lows" the eta is spitting out the last two runs is rather interesting...lol. 75?

I think it all depends on what area you plan on chasing... For us Midwesterners/Ohio Valley folks, the ETA would be an awesome setup (I haven't checked the GFS yet)... Of course this is the Plains thread though, so I will keep my mouth shut, LOL
 
There I fixed it, talk away...lol. I think GFS coming around to a real system and back west.

Edit:

LOL, yeah GFS knows what it's doing....go away eta!!! I'll take the 12z gfs for sure. That is pretty dang impressive to me. Early target remains the same based off new gfs. I wonder which model is more reliable with the strength/orientation of this system.
 
There I fixed it, talk away...lol. I think GFS coming around to a real system and back west.

Edit:

LOL, yeah GFS knows what it's doing....go away eta!!! I'll take the 12z gfs for sure. That is pretty dang impressive to me. Early target remains the same based off new gfs. I wonder which model is more reliable with the strength/orientation of this system.

Thanks for adding the midwest, LOL...

Anyway, I am changing my wishcast to the GFS as well (just took a look), though I would still settle with an ETA solution. I think once this system actually comes on shore (probably around the 00Z timeframe) and gets sampled by the RAOB sites, things may start getting a little clearer. It is also possible that the system will slow even further, and we will be changing threads again, especially if a deeper solution verifies...
 
It seems like these the character of these transitional season systems is even harder to guess without real onshore sampling than mid-Spring or winter storms. I'm pretty happy, too, about the trend on the GFS and feel confident at least that the system will move in more slowly than progged, for whatever that's worth.

Taking a blend of the extended outputs so far in the last two series of runs, I'm leaning toward a central to southern Kansas area. The reasons are that moisture and instability progs have been consistent there, and the 12Z ETA showed 250 m2/s2 co-located near 1500 j/kg MLCAPE (to the south). Because I think the system will slow and because I think the sfc winds will back more than progged, I expect these values would verify or come in even better.

There are problems with deep layer shear this far south, it's true, but the region is in the rear exit of the jet on the last two ETAs and GFS operationals. Deep layer shear is certainly marginal and the warm front is far, far away, but I don't think lift is an issue. The dryline/sfc trough should suffice to initiate convection from the triple point all the way down to northwest Texas--all models have consistently shown precip trailing well to the southwest.

I admit some regional bias, as always, but not because Iowa or Nebraska is farther from me. I live in Indiana and it's about the same distance to Harper County as to O'Neill. What I'm nervous about is instability that far north that quickly in October, when insolation is so marginal anyway. I really like the 70F isodrosotherm peeking into southern Kansas at 18Z on the 12Z ETA, even if the veered LLJ erodes it back into Texas by 0Z. I am hoping this issue will be mitigated by a slower solution and (even more wishcastingly) a more neutrally or negatively tilted trough.

Still way early, but more fun than watching the 5400m line and thinking about how much salt I have on hand.
 
I'd make a chase into Kansas if Amos is right! Actually, taking a quick glance, I, too, am impressed with the late-season setup. Obviously timing will be an issue as I am smack in the middle of the semester.

I'll wait for the next couple days to pass by to get a better idea of what's going on. AFD here in Denver have talked about this system making a good change in the weather about Friday, but they are thinking slowing will occur as well, so they haven't figured it into their forecasts. Why am I mentioning Denver's AFD here? Well, normally they get concerned here with weather changes and the terrain effects on weather as these kcik through; and when it goes through, it usually pushes its way into the Plains. However, they mentioned in this morning's AFD that the big low will be well south of Denver, which if I followed this correct, would send the triple point right through Kansas on its way eastward.

Again, waiting for things to get sorted out before making judgements, but do have the weekend on standby just in case.
 
Another day another trend... First off, for the northern plains Thursday Night- Strong lift and good moisture moving in and not to forget MUCAPE of 1500/ This is supported by ETA and GFS. And now it looks like the GFS has taken its que from the movie back to the future and amplified the system. Instability for the Northern Plains could reach a peak of 2000/ on friday with the main axis located near the great Lakes Wisconsin, into Iowa area. ETA seems more bullish with the instability with GFS going for about 3/5 of ETA's forecast. Will have to see if the models keep slowing and moving SW the location of the system.... Looking good though.
 
18Z ETA coming in, and it's pretty much stickin' to it's 12Z run. One difference I can see is that the 850mb low is weaker as it moves into the Plains/Midwest... But it's still as positively tilted as can be. The trough at 200-300mb is deeper and slower, which would be a slight trend towards the GFS...

What we really need, is the ETAs' forecast thermodynamics with the GFSs' forecast synoptics... Now thats really wishcasting it, LOL... (i.e., the ETA is putting 850mb temps in excess of 20C over northern IL!)
 
I only looked at OK, and from what I see as of Tues evening it looks like you'll need a rocket strapped to your a** to keep up. Unidirectional, 50-70kt mean flow h85 through h3. Looks a lot like a 4-6-01 or a 3-4-04. I somehow manage to miss all severe weather on days like this.

I'll hold off on any real opinions until this thing is 48 hours out, but if it verifies anything close to what it's showing now, I'll predict a High Risk with a strong emphasis on damaging winds. One thing I've noticed through the years - a linear windfield with a 60-70kt average through h5 with a breakable cap, including any metropolitan area, equals a High Risk. If the event verifies but further east (away from DFW/OKC), I'll say Moderate.
 
Ahh, the wonders of long term forecasting.

The long wave trough currently situated across the western United States certainly does look to be moving out into the plains and upper midwest later this week, with emphasis on the Friday into early Saturday timeframe at this time when one considers all available model solutions. These solutions, of course, do leave quite a bit to be desired, which should be expected this far out, as everyone knows. But for some specifics... where the 12z runs of the major models are concerned: the ETA model maintains a positive tilt to the upper trough as it enters the plains Friday, with this nature resulting in quicker surface and upper air features when one compares the solution to the other models. Moreover...its tendency to move the dryline to the east so quickly may be overzealous considering its inability to resolve low level moisture profiles at such a range, when there is likely to be additional backing in the low levels in response to the increasing height falls, which could conceivably slow the dry lines' movement.

Considering, then, that the tendency for upper level troughs of this nature to slow and deepen is readily accepted, one leans toward the GFS model solution for the forecast of synoptic scale features. It should be pointed out that a major reason the ETA and GFS differ on their handling of the trough is the differences that exist in the models' solutions for the strength and amplitude of downstream ridging, an element directly tied to their handling of the upper low expected to develop off the coast of New England. It's here that the GFS gets my nod, which means that the overall pattern would slow and that the trough would have a more negative tilt as it emerges. Perhaps of greater importance when one is comparing the ETA and GFS today is the fact that the ETA suffered from a lack of RAOB data from the continental U.S. being ingested into the model due to problems with data flow to the supercomputer running the model. This in itself lends credence to a solution other than that of the ETA.

Of course, it does look like there will be plentiful moisture and instability available for active convection when the system does exit the Rockies, so its likely that someone....somewhere...is going to see some active storms late this week. The details will have to be hammered out as we move forward in time.
 
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.
 
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.
 
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.
 
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!!
 
... 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"
 
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.
 
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
 
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)...
 
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)...
 
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