04/09/11 FCST: OK, KS, NE, MO, IA, MN

[Chip Redmond]

The 18z NAM is slowly coming out now. Looks very similar to the previous few runs with the only exception of the 500mb vort max a bit weaker and nearly non-existant around 00z and the result of that is that no precipitation is being developed in that timeframe. I am really starting to get concerned with temperature/dewpoint spreads as highs will reach near 90, if not eclipse the mark and dewpoints look like they will barely make it into the mid-60s. The cap is still supposed to erode from 21-00z with instability well into the 2500+ range. Helicity values along the dryline are still fairly good from 250-300 in the lower 1km. Hopefully the models stay like this and don't change much. I am a big fan of the set up and the ~20kts storm movement!

 

[Logan Karsten]

What looked excellent last night, now seems to look very conditional to me now. Today's NAM runs have a very strong cap in place through the evening with the upper-level forcing further displaced to the west, making convective initiation even more difficult now. Chip pointed out the fact that there will be very high temp/dp spreads along the dryline, making LCLs very high. I still have optimism, but it's starting to fade as things are shifting in the models. Shear, helicity, and thermodynamics all still look very good. It's just timing of upper level features that's the problem.

 

[Jeff Duda]

With each new run of the GFS and NAM I am becoming more and more concerned with the potential for initiation on this day. The NAM started pretty far west with it and continues to slow it down. The GFS is also slowing it down, although not quite as much as the NAM. With upper level forcing so far to the west of the surface features, overcoming the 50 - 100+ J/kg of CIN looks to be very difficult. Granted, a dryline would probably have stronger upward forcing than a cold front, so I don't see initiation as impossible, but it's looking less and less likely with each coming run, especially since the dryline isn't moving west as fast as the upper level wave is. Obviously I no longer think Saturday will be the big day, as I now think it will be Sunday. However, if something can go up in this and LCL heights aren't too bad, a discrete tornadic supercell looks very possible with the decent CAPE (at least 2000 J/kg) and shear (up to 250 0-3 km SRH). As the LLJ kicks in around 00Z, the low level shear will really jump up, as most forecast soundings are indicating. The big question is how much more the models move the wave westward.

 

[Skip Talbot]

The models have been consistently showing a dryline bulge in southeast NE/southwest IA that seems to surge mostly in the early afternoon before starting to retreat by 0z. Ideally, I'd love to see storms initiate with the enhanced lift this bulge provides and then move northeast toward the favorable low LCLs and low level directional shear along the warm front. If a storm can initiate here it would have the best chance of producing. Unfortunately the capping looks a little stronger along the warmfront. Initiation might be more likely to occur along the dryline where capping is less and the initiating boundary is closer to the upper level support. As others have noted, however, dewpoint depressions of 20-30 degrees across much of the dryline will result in high LCLs. 1km SRH is a little less favorable down the dryline as compared to the warmfront, but the effective srh still looked decent for a tornadic supercells.

I'll be sitting at the Wendy's on the Iowa side of Nebraska City if I think the cap can break there. Otherwise its off to the dryline in KS where a shot at a tornado might be a little less probable.

 

[jshields]

a few of the local wfo's have been talking about a small piece of a mid level wave possibly being ejected out saturday afternoon across nebraska, and this is what they are focusing on for possible initiation as overall the main piece of energy is way too far west. i've noticed the dryline bulge especially on the nam run after run too, so hopefully that will help things up closer to the warm front/dry line intersection late saturday afternoon or early evening. the LLJ will be kicking in too of course, but hopefully that's before it's totally dark. there is some evidence of a small weak piece of energy on the nam in southeast nebraska at 0Z:

http://www.twisterdata.com/index.php?prog=forecast&model=NAM&grid=221&model_yyyy=2011&model_mm=04&model_dd=07&model_init_hh=18&fhour=54&parameter=VVELD&level=700&unit=MB&maximize=n&mode=singlemap&sounding=n&output=image&view=large&archive=false

http://www.twisterdata.com/index.php?prog=forecast&model=NAM&grid=221&model_yyyy=2011&model_mm=04&model_dd=07&model_init_hh=18&fhour=54&parameter=RELV&level=700&unit=MB&maximize=n&mode=singlemap&sounding=n&output=image&view=large&archive=false

 

[Andrew Stoller]

I agree, capping is once again becoming a big concern and the 18z NAM is no longer breaking out any convection, even well after dark, as CIN builds in big time. But the 18Z GFS is a thing of beauty, breaking out convection all along the dryline! Usually it's the other way around and the GFS shows nothing when the NAM does. Arghhhhhh. It's like watching a pot of water boil...

 

[Jeff Duda]

The 00Z NAM is in. Thankfully it doesn't move the wave any farther west that I can see. While it still doesn't look great, I am encouraged. There remains decent shear and good instability along the dryline in NC KS, plus the model breaks out precip between 00 and 03Z (thus initiation after 00Z). The lid strength index is between 0 and +1 in a small area of NC/NE KS at 00Z, and decent backed surface and 850 mb winds remain along far N KS into S NE. I'm not too sure about the dryline bulge others have been mentioning. I don't see it. I do see a large depression in dewpoints poking into SE NE, and I'm not sure why, but given the lack of wind shift, I'm thinking it looks more like a secondary warm front. Perhaps some sub-synoptic scale surface low will materialize there. I do see hints of a weak pocket of vorticity at 500 mb moving out into W KS at 00Z, so perhaps there may still be some convection after all.

I also took a look at some NAM forecast soundings and saw daylight. I've been looking at a target along I-70 between Salina and Topeka for 3 days now, and the Salina, Concordia, and Hutchinson soundings all show pretty decent parameters: > 150 0-1 km SRH, > 250 0-3 km SRH, ~50 kts 0-6 km bulk shear, LCL heights around 1300 m, about 35 J/kg or less CIN. These are the best soundings I've seen in the last 2 days. Hopefully this improvement continues.

 

[jshields]

the 12Z models are in and are basically the same, which means they still show the same differences. The GFS breaks out storms, has a weaker cap up across eastern NE and northeast KS due to a nice "hole" of cooler midlevel temps, and it has better 0-1 km SRH across most of the warm sector. The NAM however has a stronger cap until you get right along the dryline, still doesn't break out any storms, has higher LCL's, and MUCH weaker helicity until you get near the warm front or in nw missouri which is well capped. the warm front it now has across northeast nebraska and northern iowa. I'd be worried about tornado potential with the NAM if the storms are going to fire along the dryline only as LCL's are near 2000 meters. i'm going out chasing bc it is relatively close and those $3.70/ gallon gas prices are killing me, and will probably head to southeast nebraska, but this one definitely isn't a guarantee!! i'm hoping i don't see blue skies all day

EDIT: wow SPC has really shifted the outlook area north and east putting the 30% risk across northern iowa, minnesota and into wisconsin, and having the hatched area across most of iowa. has only 5% chance along the dryline too. kind of surprising considering what the 12Z models show

 

[Dan Rupnow]

Pleasantly surprised by the 12Z NAM, as it gives hope to a possible sleeper target of the IA/MO/IL tri state area. Insane instability of 3500 to 4000 j/kg, along with a LI around -10C is forecast in this area. Dews are forecast to be around 70F, but I believe that is probably slightly over forecasted, with dews of 65F more likely, which would bring CAPE values down a touch, but nonetheless there should be plenty of instability in this area. Shear looks pretty good, although I would like to see a little stronger low level velocities. However, there is plenty of veering with height, thus yeilding sufficient 0-1km SRH of 100-200m2/s2. Obviously LCL's should not be a problem with surface temps of 80F coupled with dews of 65F-70F. The thing I like about the 12Z NAM is it cooled H7 temps in this area as opposed to earlier runs, and also breaks out precip in this area around 00Z too. Also, looking at H5 and H7 winds, there is what appears to be a little wave/disturbance overhead which, along with convergence from the warm front, should hopefully be enough to initiate storms in this highly unstable, sufficiently sheared environment.

 

[Matt Hunt]

I really don't pay much attention to the NAM's simulated radar. I haven't seen any cases where it has been accurate. Maybe someone else has? Last Sunday it never showed any convection over IA, and storms erupted around 6pm I think. I have to question the little spot it's showing in IL. All of the parameters look good, but I just don't see a trigger for surface based storms in this area. Actually, I think right now I'm taking more of a pessimistic approach with this setup and thinking it's going to be a bust. I don't really see convergence anywhere, and the GFS now has the Low way out in N/E CO at 00z. Am I missing something?

 

[jshields]

the nam seems to be the least accurate of the forecast reflectivity models, the wrf usually does a good job, and the hrrr is of course the most accurate. tomorrow afternoon there is supposed to be a little wave that kicks out ahead of the main low across nebraska and western iowa. also the LLJ kicks in in the evening. so convergence would occur along that wave and also along the warmfront/LLJ intersection. i would be surprised if anything goes up that far east into eastern iowa and illinois bc there really isn't anything to set something off out that far. i also wouldn't be surprised if nothing goes up until overnight north of the warm front, and those would be elevated hailers. the 18Z NAM did come in less bullish on the cap tomorrow, so we'll see.

I really don't pay much attention to the NAM's simulated radar. I haven't seen any cases where it has been accurate. Maybe someone else has? Last Sunday it never showed any convection over IA, and storms erupted around 6pm I think. I have to question the little spot it's showing in IL. All of the parameters look good, but I just don't see a trigger for surface based storms in this area. Actually, I think right now I'm taking more of a pessimistic approach with this setup and thinking it's going to be a bust. I don't really see convergence anywhere, and the GFS now has the Low way out in N/E CO at 00z. Am I missing something?

 

[Jeff Duda]

I really don't pay much attention to the NAM's simulated radar. I haven't seen any cases where it has been accurate. Maybe someone else has? Last Sunday it never showed any convection over IA, and storms erupted around 6pm I think.

the nam seems to be the least accurate of the forecast reflectivity models, the wrf usually does a good job, and the hrrr is of course the most accurate.

Simulated reflecitivity (in models) is usually based off of microphysical variables like cloud water, cloud ice, graupel, and rain number concentrations which are usually a feature of double moment microphysics schemes, or in the case of single moment schemes, parameterizations based off of just the mixing ratios of the aforementioned water species. It's a well known fact in numerical modeling that the larger the grid spacing, the bigger the grid box and the harder it is for one of those boxes to become sufficiently saturated (i.e., high enough RH wrt whatever species) to get the microphysics to kick in and create particles. Many researchers would probably argue that even at grid spacings of 3 or 4 km, it is still very hard to get full saturation in a grid box, but grid boxes on those scales are certainly easier to become saturated than those at the scale of the NAM, which has a grid spacing of 12 km. That's why the simulated reflectivity field from the NAM never has values above about 40 dBZ. It's just that hard to get the microphysics to produce a lot. That's why it uses a convective parameterization scheme (the Kain-Fritsch). I wouldn't advise anyone ever look at the NAM's simulated reflectivity for anything more than to see where the model thinks convection may be occurring. The intensity and organizational mode will be nearly impossible to decipher from it.

i would be surprised if anything goes up that far east into eastern iowa and illinois bc there really isn't anything to set something off out that far. i also wouldn't be surprised if nothing goes up until overnight north of the warm front, and those would be elevated hailers. the 18Z NAM did come in less bullish on the cap tomorrow, so we'll see.

I don't really see convergence anywhere, and the GFS now has the Low way out in N/E CO at 00z. Am I missing something?

There is a warm front snaking eastward from the low in NE CO/SW NE all the way through NE and into S IA. It really seems to tail off and lose character once you get much east of the Mississippi River, however, so yeah, there won't be as much convergence there. I think convection is more likely to form farther to the west along the warm front, or along the dryline even. I should mention, however, that the warm front does run near the southern tip of Lake Michigan. With the thermodynamic gradient from the cooler lake waters, the warm front could be locally stronger right in that area and perhaps be enough to fire off a storm. Low-level shear there is pretty good, too, but upper level winds are lacking a bit there, so deep layer shear would be limited.

 

[Matt Hunt]

That first part is way over my head, but the 2nd to last sentence is what matters, ha! That was my thinking with the warm front, there just isn't a very steep gradient there, and the winds don't converge along the front either. I think if I were going to chase tomorrow I'd planning on sitting somewhere near Omaha and adjusting from there. The good thing is that's in range of either the dryline or the warm front.

 

[Logan Karsten]

with the 00Z NAM in tomorrow, it seems things are falling in line with the GFS solution of a farther north solution. The warm front looks to drapped from Sioux city down through central IA by tomorrow evening. Surface winds are backed very nicely along and across the warm front. In addition, there is abundant rich moisture, instability, and low LCLs along and near the warm front. In addition, there is a nice smooth veering of winds from the surface on up to 500 MB and decent shear as well. I like this solution better because, now the instability is farther north, which makes breaking the cap somewhat easier now. The NAM seems to be breaking out convection near Sioux City and points west. It seems the best region tomorrow will fall in a box from Sioux Falls north to Sioux City and east to I-35 in IA.

 

[Chip Redmond]

NAM is loving that 500mb vort max and spitting out initiation around the Sioux City area as mentioned above. The cap is somewhat stronger than previous runs farther down the dryline and keeping erosion at a minimum just along it. The dewpoint spread latitudely (a word?) is much more condensed and is showing a sharp dryline. This could help forcing along the boundary but I am still hesitant with such a small amount of eroding CIN. Shear is still favorable and helicity values have improved south along the dryline. With that said, I am still wanting to be in KS and/or just along the NE border but am starting to feel sucked by the initiation along the triple point where curved hodo's are much more appreciative. Will just have to sit and wait for the remaining models and the next few runs...

Edit: GFS has the dryline farther south, not that it matters a whole lot as it has the best instability in KS right along it. Hodo's for the region up and down the line are much better. It has the cap eroding but not completely. This run seems more favorable for dryline action farther south but also has precipitation firing in NE about the same time as the NAM. It looks like this will be a case of sit in the middle of the north and south areas and see which goes first with the best shear. A long waiting game.

Chip