2012-03-26 FCST: SD/NE/ND

[Chip Redmond]

ECMWF and GFS agreeing in a almost negatively tilted (more neutral on most recent runs) trough digging into the west coast and ejecting over the Rockies on Monday the 27th with cyclogenisis occurring in Western MT/WY.

The ridge over the eastern half of the US will slowly break down with the progression of a few shortwaves over the weekend helping to deamplify the ridge ahead of the trough. A 500mb jet of ~90kts will propagate around the crest of the low in the day of the 27th allowing for cooler temperatures, steepening lapse rates, and aiding in upper level divergence. SW winds at the 500mb level overlayed with a good low level jet at 850mb of ~30knts will allow for very good turning with height and speed shear, yielding impressive hodographs (I attached one from just east of the MO River in northern SD). Dewpoints of 60F will soar northward into SD and even up to the ND border. A warm front will set up along the boarder, eastward with a dryline dropping south through the western portions of SD, WY, KS. This would place a handsome triple point in the northern portion of SD. If moisture can surge that far north, instability won't be an issue that is for sure. Temperatures will be warm, but not overly warm keeping LCL's low and manageable/breakable cap. There seems to be some hints at early initiation with precip occurring around 18z, but that is still a small detail.

Obviously there are still timing issues with this system as it has sped up from the 0z run, but definitely hints of upper level pattern support for some action early in the week for the North Plains! Definitely a rare occurance for March, but for this odd year of odd patterns, nothing is surprising to me these days. A few other things I would like to see and have concerns for at the moment is the 500mb jet to speed up maybe a bit more and perhaps the dryline to have a better gradient, though these details are small concerns at the time.

If things weren't to change at all, would target a region near north-central SD around 20z.


Chip

 

[dmckemy]

I agree with ya Chip, this could be a good severe weather setup for the Northern Plains come Monday. The 12z runs from today still have that tongue of moisture (and +60 degree dewpoints) coming up through SD, though the 12z GFS run has more of a cap (as reflected in the soundings) over central/eastern South Dakota compared to the 00z run. If things play out like the 12z GFS has going, I'm thinking storm initiation could occur in NW South Dakota and southern North Dakota, pretty much right along the warm front. The storms might not fire off until 00z though, so there will be short window to catch the storms with daylight. Shear is definitely favorable for supercells, and hodographs look good... Wish the LCL heights were a bit lower where convection does fire off, but I think we could squeeze a few tornadoes out with this set-up.

 

[Andy Jackson]

The 18z GFS does have some promise as far as vertical wind shear and associated surface low pressure. As far as the thermodynamics are concerned a good moisture return is still in question, the cap also seems to be stout and could limit the setup for more elevated convection if any. The N. Plains in March is an iffy situation but it could be a fun chase if the models are consistent or better (still a ways out). Right now the best bet would be from Norfolk, NE northward to around the I-90 corridor and beyond.

 

[Jeff Duda]

I'm seeing a possibly more potent threat area across SW NE, W KS, and the OK/TX PHs. A well defined dryline will be present, although moisture will not be of great quality unless you're in the southern portion of that threat area. However, since it's near the shortwave trough axis, there will be lots of forcing and 50 kt SWlys above a generally 40-50 kt Sly LLJ and SEly winds at the surface. Instability is a little on the low side given the low surface dewpoints. It's also interesting to note the general lack of a well-mixed PBL in the GFS, as moisture zooms off to very low values instead of remaining constant over the lowest kilometer or so. However, the hydrolapse is really high, so entrainment of warm dry air won't be as big of a problem.

This is still 100 hours out, so changes in the timing and placement could still occur. Will have to revisit the scenario once it falls within the NAM's time limits.

 

[John Wills]

I would have to agree with the southern play. At least south of the Dakotas. Earlier today I was liking eastern NB/Western IA... but with some changes in the models, I'm a bigger fan of the TX/OK panhandle. However, this is the problem this far out. Wait and see what happens in model world over the weekend. Things could get interesting though.

 

[Jared Orr]

Chip--I think you meant to write 3/26, as that is Monday.

Wow. What a tough decision. With such an expansive dryline, one has to pick carefully which sector to target. I really like a lot of things about the triple point, and the NAM is showing a more positive CAPE outlook than I expected for NE. I'm going to have to watch this one carefully with time. But I'm for sure chasing. Spring break, baby!

 

[Skip Talbot]

Either through changes in timing with the models or confusion about dates and zulu time, most in this thread appear to be talking about the 26th and not the 27th. I'm going to change the thread title to the 26th, but let me know if you were indeed posting in regards to the 27th and I will split your post off to a new thread.

The NAM is showing a surprisingly nice combination of shear and instability up in the SD bandlands on Monday. SBCAPE to 2000 J/Kg, 40-50 knots of deep layer shear, and 200+ 1km SRH looks like a very favorable combination for supercells. What ruins this setup, however is the cap. Lid strength index never drops below 3.

http://68.226.77.253/models/eta/central/CENTRAL_ETA212_ATMOS_LSI_84HR.gif

That's way too strong of a cap for surface based supercells. The only storms we'll probably see with that lid will be elevated junk north of the warm front or well to the west in drier air where the shortwave trough is ejecting. The GFS has been showing a very stout cap the past several runs as well with CINH over 100-150. Also ruining the tornado potential are the relatively low dewpoints across the South Dakota target of 50-55 and lack of low level instability:

http://68.226.77.253/models/eta/central/CENTRAL_ETA212_0-3KM_CAPE_84HR.gif

Further down the dryline, instability and deep layer shear is more modest. Isolated storms may be possible down here, but no area in particular is jumping out at me. If the cap index plots are correct, this looks like a non event, but you don't want to ignore those shear and instability combinations.

 

[Chip Redmond]

Did I say it was the 27th? I can't even remember. I definitely meant Monday, sorry for any confusion.

The entire setup seems to have shifted south a bit over the past few days, not surprising. A few differences at this time that are surprising to me is that NAM is less aggressive with the Td but has slightly stronger surface to 850mb winds out of the south that would aid in moisture transport northward into SD/NE. With such a negatively tilited trough, CAA is fairly week at this time period in the mid-levels helping to make the cap pretty large. With highs now progged to/above 80F, the LCL is quite high. However, it appears that the NAM at least is hinting at cloud cover through part of the day Monday and this could lead to temps not reaching their expected highs, but that would enlarge the cap as well.

http://weather.rap.ucar.edu/model/eta72hr_0_clouds.gif

At this time, not to terribly concerned. It will be a close chase, so I will likely head out. Sunday doesn't warrant an additional forecast thread, but a few isolated elevated thunderstorms are possible in the same region that would have ample shear to attempt to become supercellular.

Chip

 

[Brett Nickeson]

Pretty much agree entirely with Skip. Instability, shear, and corresponding EHI's look fantastic across southwest and south-central SD, but the nuke-proof cap, moist upper levels, and a relatively large T/Td spread will likely make SD a bust.

I am a bit intrigued by the last couple GFS runs which places the surface low further south into the eastern NE panhandle along with a tongue of 1500 CAPE and a much weaker cap. Td's will struggle to reach 50 or above, but if the surface low verifies that far south, southwest Nebraska could be a reasonable play with the capping over SD. We'll see if the NAM starts trending the surface low further south, otherwise I'll probably pass on taking a chase day Monday.

 

[Chip Redmond]

Well, appears that the models have actually now shifted northward with the system as of 12z. All that will do is spread out the moisture that will be in place and prohibit it from pooling. Temperature spreads at the surface have improved slightly with NAM and the LCL's have dropped, but mid-level WAA is going to just enlarge the cap through the day. With ridiculous hodographs and shear in SD it's very frustrating because of the massive cap. It appears that forcing along the cold front will be enough to initiate thunderstorms across the western part of the state by 0z. The squall line will be flying with motions near 50kts possible that will definitely warrant a high wind threat. Although the likelihood of a isolated storm ahead is smaller than the chance of a 13 seed making it to the elite 8, it will be interesting if any left over boundaries remain from the overnight convection that could help break the cap.

Either way, will hopefully get a few intercepts with storms Sunday and Monday and will be out just in case something can initiate and actually root in the boundary layer.

Chip

 

[Tim Supinie]

I've been lurking for long enough that I think it's about time to post. I pretty much agree with all that's been said here. Since it's been mentioned, tomorrow (Sunday), the entire plains are going to be under a shortwave ridge, so good luck finding any storms to chase.

Now for Monday:
IMO, places from about the KS/NE border on south are out because of the very warm 850 mb temperatures (15-20 C) already in place. Those areas will also get little, if any, help from the trough forcing upward motion (and therefore cooling).

As for the moisture, we'll see how much actually gets up to NE by "go time" on Monday. We still have surface high pressure over northeast TX on the 15Z analysis, which provides weak surface flow across the southern plains and less-than-optimal trajectories for moisture return, at least in the near-term. In the slightly longer term, the biggest roadblock for quality moisture return appears to be a stubborn 850 mb ridge over the southern plains. The NAM, at least, keeps this ridge around until Sunday night (which I have no problems with, given the upper-atmosphere ridging that will be going on as well). This means we'll basically have Sunday night to get the boundary-layer nocturnal wind maximum to kick in and bring deep moisture all the way up to Nebraska, and that looks marginal at best, given the remnants of low-level ridging that are progged to be over AR/LA.

So, what does that mean for this setup? Capping and moisture-quality issues. IMO, initiation is very questionable. While the GFS does have the cap eroding in a few places along the dry line (particularly southwest Kansas), I think it's over-doing the moisture advection this afternoon (the surface flow is too strong in southeast TX, which leads to the coverage of 60 F and 65 F dewpoints being too large), and is likely to continue to do so in the future.

I wasn't considering chasing this setup, but if I were you, I'd save your gas money for April and May. Even a short jaunt is incredibly expensive when gas is $40 or more a tank (case in point: Norman to Wellington, TX, to Vernon, TX, and back last Sunday cost $120 in gas in a small car).