2015-05-15 EVENT: NM, CO, TX, OK, KS, NE

Jeff Duda

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Already lamenting an ugly forecast for Saturday ("the day"), I'm going to focus on Friday ("the day before the day").

The GFS, FIM, and ECMWF have been consistently in agreement in a pretty deep trough crossing the Rockies early this weekend, spreading some very good flow across the Plains. After a mid-week shortwave passage across the Plains, recovery will be in full swing as moisture advects back in.

The 12Z NAM bulges the DL in KS and has it farther east than the 12Z GFS, but they agree in the DL location and orientation better to the south. However, the moisture gradient in the NAM is much weaker owing to higher moisture content west of the dryline, perhaps finally responding to the immense amount of increased standing water and soil moisture from last week and subsequent latent heat flux across TX. I suspect we will see that change as the land surface model continues to "catch up" to the dramatic change in moisture content across the southern Plains.

Moisture issues aside, both models have very weak to no capping, with h7 temps of only 6-8 C. While h85 temps over far W TX and NM suggest a decent EML will form, the GFS never advects that over the moist sector. The NAM does in a limited area of OK, which actually provides for strong capping, but that doesn't even matter because...

...in both models, the trough lags the moist sector by a significant distance. With the dryline located so far east, we barely even get 30 kts of h5 flow over the DL by 00Z in either model, although the west-east gradient in wind speed is pretty sharp, so even small changes in the longitudinal placement of the DL could result in much better shear. There is decent flow below h5, so much like this past Wednesday, there could still be tornadoes across KS and OK given the degree of low-level shear and the high CAPE that is forecast. In fact, if the NAM verifies, given the strongly backed low-level flow, good moisture quality, and high CAPE, there could be a pretty significant threat along the DL/WF near the KS-NE border. The GFS is far different in this region, though.

Finally, MPAS. MPAS agrees with the DL placement across the SGP. The biggest difference between MPAS and the operational models is that it has 50+ kt h5 flow overspreading the DL in OK/KS by 00Z, thus providing for more than enough deep shear for things to get interesting. I'll go ahead and wishcast, saying I think the MPAS forecast will verify. If it does, we'll be seeing classic supercells along the dryline in TX, OK, and KS. :)
 
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Just looking at the NAM and GFS on UCAR, the NAM seems to think it will be a 'leftover trough' more SW-NE oriented in TX, OK and into C KS, whereas the GFS has a more DL look and quite more west than the NAM. I like the GFS better. Haven't seen a good, real, DL this year and I hope Fri is it. I noticed little precip blobs the night before or morning of, so hope this change holds.

If it's a surface trough struggling to catch a wave gone by, then I'd think NE and points west of NE would be the play, but if the GFS of more a DL, then I'll take DDC. Though one more model bust like the last system, and I will just go to Wichita Falls on odd days, and Wichita KS on even. Why not?

And yes, Jeff, I agree with your 'day before' theory. Time for gulf and transevap to spread. Time for oil changes and whatnot.
 
Though one more model bust like the last system, and I will just go to Wichita Falls on odd days, and Wichita KS on even. Why not?

Best advice so far this season.

After a quick glimpse of the 12z NAM I-70 in KS and north into NE looks pretty promising. 4000+ CAPE in central KS. As Jeff noted, deep layer shear not amazing, but the 45 kt LLJ, low LCLs and great CAPE could help overcome some of that. At the very least there is great veering with height :)

Two soundings:
Central KS
12_NAM_084_38.88,-96.93_skewt_ML.gif


South Central NE
12_NAM_084_40.98,-97.91_skewt_ML.gif
 
I like the SW KS area for Friday, perhaps into the panhandles as well.

The 500mb jet arrives by 0z, just as the 850s begin to really ramp up, and they are favorably oriented as well. Speed and directional shear look good, with a sharpening dryline not forecast to move a whole lot. Given the strengthening 500mb flow storms should be able to move off that fairly easily and remain discrete. The weak capping as Jeff mentioned is of some concern, but with the main pieces of energy not arriving until that night, Im hoping that would limit early morning junk that could wreak havoc with the day like we saw with the last trough. The GFS has things drying out at 12z but initiation could be early. The question then becomes will there be enough CAPE?

Further north closer to where the primary surface low is supposed to get organized, moisture is more of a concern, if things advect better than forecast, I would certainly consider a further north target closer to the KS/NE border.

Either way this looks like a setup that warrants a chase. Ive seen those 0z arriving jet streaks do marvelous things this time of year.
 
Since this setup on Friday and Saturday is likely the only one on the plains that I'll get to chase this year, I've been following this pretty close (probably too close this far out). And I've been looking at Nebraska for a few days now, since it appears to be the most obvious choice, going strictly off surface features. I don't feel as good about that as I did a few days ago. Getting the moisture and upper levels to play ball has been a challenge. The stuff just doesn't appear to get there in time for daylight. So, similar to Adam, I'm looking more and more southwest Kansas, or maybe even south-central. The last GFS version gave me some pause with respect to this, since it dropped the dry line way east, making south-central look better. If a small surface low were to materialize in the models out by someplace like Liberal, that would seal the deal.
 
As one of the resident "Nebraska Guys", I'm saddened that I'm working Friday, as this looks like a really nice potential day. Anywhere from Beatrice to North Platte looks fantastic with NAM 4km's 5000! capes and 300+ 0-3 SRH in some areas. Assuming AM precip clear out ahead of the shortwave, Friday is going to be crazy. If I were you guys I'd just sit in Kearney and wait for the Cu-Fields to start forming and head that direction. The NAM sounding for North Platte of 424 0-3k SRH with a nice round hodo for 00z is impressive, but KS is looking good too. At least we've got Saturday to look forward to...
 
Well, with a new day of model output comes new information. The 12Z NAM and 12Z GFS basically remove everyone except N KS and NE from decent contention for severe weather. Looks like the dryline is forecast to be rather poorly delineated with a very weak gradient not located with a wind shift across TX, OK, and into S KS. It also looks like mid-level lapse rates will be somewhat poor owing to 500 mb temps of around -10 C. Combine a little capping with weak shear, and it doesn't look good for the southern portion of this region.

Nebraska on the other hand. There's going to be some pretty good directional shear in the low levels along the WF across Nebraska, with some pretty decent moisture along and north of it as well. Looks like there will be adequate deep shear for supercells up there, too. The 12Z 4 km NAM agrees in general, although it pushes the triple point up far enough that the WF is close to the NE-SD border and leaves a warm sector primed for big storms. The 00Z MPAS also agrees with this scenario, and spits out a string of storms with good UH along an arcing dryline across NE.

The 00Z MPAS does try to keep the window open for areas farther to the south, as it maintains little to no CIN with 40-50 kts of deep shear across parts of S KS and N OK. I guess the question is whether there will be enough convergence to trigger storms.
 
Nebraska has my attention for Friday....

I really liked the 00z suite of models last night as they were speeding up the trough advancement into the central US, spreading 50+ kt flow over Nebraska. However, today's 12z suite has slowed the trough back down again. I have some concern on the level of deep layer shear right now, but surface features look fantastic. Good CAPE and strongly backed surface winds.
 
Friday looks like the better day IMO, but there are some issues. The main one being instability. It seems as though the NAMs are overdoing CAPE (as they usually do), especially the 4km. With the warm front moving through, we might be looking at cloud cover for most of the day. GFS and SREF are pointing more towards CAPE values in the 2000 range, which is just enough to get the job done. Another concern is venting. NAM is forecasting anvil level winds on the order of 40 kts. Not too impressive, but again, should be just enough. Just hoping we don't end up with monster HPs that usually seem to be the case in Nebraska triple point setups. Otherwise, those hodographs are beautiful in central Nebraska and shear should be sufficient for some supercells. Also, the arcing dryline combined with ESE surface winds to the east of it should make for plenty of convergence to fire them up.

I won't reiterate the issues in Kansas that have already been mentioned, besides that that surging dryline will probably undercut any storms that do manage to form along it.

We will probably hang out somewhere along I-80 between Kearney and North Platte and see what happens.
 
Another concern is venting. NAM is forecasting anvil level winds on the order of 40 kts. Not too impressive, but again, should be just enough. Just hoping we don't end up with monster HPs that usually seem to be the case in Nebraska triple point setups. Otherwise, those hodographs are beautiful in central Nebraska and shear should be sufficient for some supercells. Also, the arcing dryline combined with ESE surface winds to the east of it should make for plenty of convergence to fire them up.

On the venting... don't see much of an issue, especially if along the warm front/triple point. The mid-level winds don't back - which has contributed to a greater amount of HPs this year. Therefore, I am trending to play up north.

If this system trends (like most have this year) a tad further west and south, I think the system will be even more ideal with some upslope contributions. It will help overcome any moisture problems (if there actually are some) that far north and should assist in a more easterly component east of the dryline, enhancing convergence. A fairly large warm sector should develop easily with this passing shortwave mid-week so moisture shouldn't be an issue, but like previous systems - increased moisture may make a tight gradient dryline difficult. Mid-levels lapse rates still a tad weak and the trough is keeping coolest air 700mb still pretty far west on Friday (still fairly neutral) which could aid in some capping issues. Honestly hope more storms go further south along the dryline to spread the masses out.... which still seems not out the window completely. Low level shear is there and someone could be treated to a nice isolated supercell. Thus, not completely on the NE bandwagon yet. Regardless, not to many negatives with the system still in dreamland.

I'll be out!

Chip
 
The main one being instability. It seems as though the NAMs are overdoing CAPE (as they usually do), especially the 4km. With the warm front moving through, we might be looking at cloud cover for most of the day. GFS and SREF are pointing more towards CAPE values in the 2000 range, which is just enough to get the job done.

Normally I would agree with this, but the GFS/Euro/etc. were too low with instability this past weekend for the areas that were not affected by morning convection. Areas that had SBCAPE forecast in the 2000-2500 J/kg range were hitting up to 4000 J/kg from what I saw, so this raises some suspicion in the instability forecasts from the lower side as well. I'd tend to think something roughly in the middle of the two is the good way to go for now, which leaves plenty of instability for robust supercells assuming other conditional elements work themselves out.

Also, the dryline isn't really going to undercut those storms if they form further south. That is more of a problem with cold fronts due to CAD to the lee of the Rockies (see April 17th, 2013).
 
Also, the dryline isn't really going to undercut those storms if they form further south. That is more of a problem with cold fronts due to CAD to the lee of the Rockies (see April 17th, 2013).

Maybe it won't "undercut" them, but it sure can make them dry out. Just from intuition, with such a diffuse dryline they could pop up in the area of the dryline with low 50 dews then struggle to move into better moisture with how quick the dryline is moving. I'm not a master on the mechanics/thermodynamics of a dryline, but more often than not with setups like this (excluding NE) storms seem to pop up and then wither away because they can't get into any decent moisture. One example was last Wednesday in Kansas where a storm fired along the dryline that wasn't even moving that quickly and we visually watched it dry up and wither away because it couldn't get into the better moisture. Maybe this time around, with motion vectors more to the east than last Wednesday, it might not be as much of an issue, but as of right now I don't like the way Kansas looks.
 
On the venting... don't see much of an issue, especially if along the warm front/triple point. The mid-level winds don't back - which has contributed to a greater amount of HPs this year. Therefore, I am trending to play up north.
Chip

Just double checked that and I think I was looking at the wrong day for that, but I can't go back and edit now. Thanks for pointing that out.
 
I think the biggest problem with the Nebraska portion of the setup is shear in the mid-levels. Deep layer shear is okay, and low-level shear is great, but shear between about 800 mb and 500 mb is pretty weak. Might not make that big of a difference, but I've seen setups where storms are messy or tend to go linear quickly because of that.
 
Nebraska is not without its shortcomings, especially in the upper winds department. The look of that hasn't changed a whole lot since yesterday. That 500mb booster shot just doesn't make it up there substantially. The lack of surface low proximity is still bothering me for Kansas though. Has all day today. It hasn't materialized, and the kooky dry line isn't cooperative either. It looks to back west later in the evening, but not really cooperate at initiation time during daylight. Maybe somebody will nab a short-lived LP storm down that way? I've tried for those plays before, and they never worked out (hope is not a strategy). The next day appears to fix both of those issues, but that's not what this thread is about. So, despite my earlier hopes to the contrary, the safest bet still looks like Nebraska, where at least we have boundary interaction that's more likely to materialize. For what it's worth, the area forecast discussions tonight from those neighborhoods appear to support this thinking. None of this strikes me as a windmill-slam-dunk by any stretch, but there may be something to photograph up there.
 
Waiting on the next runs... I just love the sand hills...[/sarcasm] the setup looks good. I think this is the day. Nice hodographs, perhaps as mentioned though not enough turning with some runs in the 2-3 km. NAM is consistent with convection by 21Z in the western portion of the state. GFS trends further north toward the SD border.

hmmm.... 0Z though paints a different picture.
 
Well based on the last 3 or 4 runs of the NAM I had Kearney, NE area pegged down... The 0z Nam really changes my target. The sharpest part of the dryline and the best moisture/CAPE/EHI's are now really more in central or north central kansas, also the warmfront and its added shear is now way up north or gone almost entirely compared to previous runs of the NAM Bassically it has retreated the dryline punch and lifted the warmfront further north. 0Z nam breaks out precip in central kansas by 21Z but the mid-level and upper level flow is still quite week. Regardless my main area of interest (target?) is now closer to Salina, KS then Kearney, NE. Also CIN builds in very quicky after peak heating probably because an upper level vortmax (above 500 mb to 250mb) just clears the area after about 18-21z. So there are issues but central NE just doesn't look as good to me anymore.
 
Been watching the NAM for a couple days now and seriously Friday and Saturday are both looking good. Since this thread is about Friday, I'll touch upon what I like.

1. Anvil level winds seem to be (at least in Nebraska, which is where I'm targeting) somewhat offset by storm motion which should lead to less HP, even with somewhat lower flow.
2. NAM advertises 3000-3500 CAPE from North Platte to McCook tomorrow, which I don't know if we'll see that high, but 2000-3000 seems reasonable. That should be enough
3. Moisture seems pretty reasonable and deep, and should have no problem with northward transport today and tomorrow.
4. Deepening low throughout the day

Negatives
1. Very little to no EML. Early show, perhaps a lot of convection with forcing
2. Show may end up being further west than I think and earlier.

With that said, I like Northeast Colorado/Western Nebraska earlier in the day, as early as noon. Then I want to drop back southeast to North Platte and south into Northern Kansas near Colby as the atmosphere heats up and destabilizes. Obviously things change if the WF doesn't race as far north as fast as it looks like or if we have a lot of morning convection up there.
 
I was excited about this setup, and while it still has some potential, my enthusiasm has faded somewhat.

While the NAM continues to depict a nice dryline bulge across KS/NE, it has now shifted it further south and west, making the drive even longer for me. In addition, NAM is backing off some of the rich dews it had in previous runs.

Also, I was hoping the models would speed the trough up a bit to bring 50+ kt flow @ 500 to the target area. However, I am seeing around 40 kts at best.

Hopefully tonight's models will give me some reason to drive out there, but I just don't see it right now. I see a marginal setup that may yield a beautiful supercell, if things can be nudged in the right direction over the next 24 hours.
 
Well today doesn't look too exciting anymore. NW KS/NE looks to be main play. I am interested to see what happens with all of the agitated CU in West Oklahoma up through KS. We may get an isolated storm with possibly some structure shots :)
 
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