5/20/09 FCST:NE/SD

[Robert Edmonds]

Figured I would start a thread for this day, despite not being enthralled with the setup. The NAM is currently showing up to ~2000 J/kg cape and upper level winds of ~40knts over NE and SD. While this sounds good initially, it appears the winds from looking at the forecasted hodographs will not be all that supportive of rotating structures. The LCL height also appears to be high. Will at least be keeping my eye out for this day, appears that the best place for severe weather will be southern SD and northern NE. Would be interested to hear about what others thoughts are for this day...

Edit: The 00Z GFS is not nearly as favorable for the chasing prospects.

 

[Sam Hall]

Hi Robert,

New to forum so bear with me! I agree at the moment with your post. I am liking the wind profiles for this day in the areas suggested better than any other day so far! 72 hr forecast (ETA) showing some nicer dewpoints too in SD. Also some nice heat from early in the day, so hopefully some decent day surface heating may be likely. That and the progged CAPE values hopefully may spawn some activity in and around these areas with a slight dryline west - running southwards through NE, OK, KS and TX. My only concern is the level of CIN early in the day. Though there seems to be a slight hole where it seems more held together until later in the day sort of S/CTRL SD.

I too would be interested to hear others' thoughts. I certainly am hoping for a turnaround, however slight! I am only in this country chasing until June 4th!

 

[David Wolfson]

It looks very linear from central SD northeast into ND, with the possibility of a bow or two that seems to be one signature of this year. My eye is drawn more to the drag-end in the extreme western NE panhandle as the nose of the impulse comes out with some mid-level moderation of the stout cap over moisture that should have been sloshing upslope for several days. The NAM shows some respectable juice advecting in the 21-24Z frames.

Three days out I'd plan on being someplace like Scottsbluff, NE, where magic sometimes happens in these situations. You might just be hustling south from the singular supercell you caught the day before in extreme southeast MT! FWIW.

 

[Jeff Passner]

I'm not sure how good this will be... The basics are there; cold front moving southeast in SD and Nebraska later. There's decent flow at 500 mb flow in SD and a moderate low-level jet at 0000 UTC Thursday. Otherwise, my concern will be the huge, awful upper low in the Gulf of Mexico which will bring strong NW flow into Gulf and might limit how much moisture and how much deep moisture works to SD-Nebraska. There's enough moisture hanging around from the wet winter and recent rains that maybe 55 dew points can meet the front. It seems like the model (WRF-NMM) brings higher moisture to the front, but that's from convective feedback. This seems like a long shot right now for tornadoes. The convection looks linear, the surface winds might veer off the boundary to the SW, there's no strong vort max to enhance lifting, the area is well south of the main jet max, the 700-mb temps are a little warm, and to add some insult to the situation, the WRF-NMM shows high-level moisture (cirrus) advecting all the way up from Mexico to the area of interest. Still, if I was out there, I'd chase it. You just never know what will happen, especially since it's 72-hours away.

 

[Derek Weston]

In just looking at the NAM... relatively narrow band of instability right up near the CF and away from best low level shear further S and E. Even then... veered 850s aren't too appetizing. Agree, looks mostly linear, but should decent CAPE coincide with favorable low level shear... maybe a few rotating storms.

5/19 looks more interesting, fwiw. Warm 700s a big issue... but looks to be some nice CAPE into N/C SD along with good turning w/ height. Of course, 500 flow is a bit anemic, but 30kts might be just enough -- already happened down in KS... Could be kind of fun if something does pop... could be photogenic.

 

[Sam Hall]

I agree. I am still favouring S/C SD east of the [sort of] dryline... that worked well for us last year mind you what didn't work well last year!

With the 18z eta stating temps to be in and above the 90s I am hoping there may be enough heating through the day to pop through any inhibiting cap that may exist, though wind profiles are better now with their turning from west at 500 mb to more southerly at 850 mb, their strengths may be limiting to organised cells perhaps - not entirely sure.

I'd have really wanted the CAPE, helicity and LI to be best in the same areas as the other good ingredients LOL but that's all better further north according to the eta - you can't have it all!

30 hr NAM giving -4 or so LI for S/C SD so I may suggest to my brother that we head a bit further east tomorrow (we are in the Super 8 at Spearfish SD atm) and see if we have North/South options just in case.

I am still learning in these types of situations, so I will be interested to see what does happen tomorrow and learn from the day. I am wondering if SPC will say anything soon in their next update. I'd have thought they may say something with the winds being better, but I suppose the main lack of instability and moisture is the most debilitating factor in this setup?!

 

[Darren Addy]

South Dakota forecast discussions see some CAPE (1500) in the vicinity of the James River Valley for Wed. evening and weak wave could spark some storms, but a severe lack of moisture (TDs in the 50s combined with high temps) would make for very high-based storms with wind being the main threat. (They also mention that the cold front as having a stronger punch than the models anticipated.)

I'd be thrilled just to see some thunderheads and lightning, at this point. What a weird Spring.

 

[Jason Persoff]

So, I've decided to stay in leisurely mode, and won't be attempting MT or SD today (5/19) as I'm in Lincoln, NE, and that's way too much of a stretch. So my focus will be on tomorrow.

Current target is North Platte, NE.

At the surface, the cold front will meander SE-ward and will be in a NE/SW orientation from CO up to E SD. A dry punch appears pretty well demarcated in W KS with Td's in advance of the intrusion a mere 45-50 (though that's more than enough in the CO/WY/NE area). Sfc winds will be a touch problematic as they'll be out of the SSW, and risk running more or less parallel to the front. But, if the forecast low in NE CO can tighten up a bit, we may see some backing winds in response. T/Td spreads are on the order of 30-40 deg if the highs verify, meaning very high based storms. These still can be photographic treats, and may offer good, highly visible lightning shots. And in this regime, that's a lot.

CAPE will be in ample supply...if we were in FL. But in the Plains it will be CIN vs. CAPE in a duel of the air parcels. Assuming we hit the Tc, we should see CIN erode sufficiently and hopefully capped enough to allow only a few cells to poke up (instead of MC clusters). The 700mb temps are summer-warm (at or above 10 deg).
As a disturbance pushes across the region, there is hope of a teeny jet streak at 250mb (W around 50 kt) with a stretch-of-the-imagination entry region at 500mb (W winds only around 20-30 kt there). 800mb winds are out of the SE.

So, using my IF-casting model. If the cap can be overcome by the front and daytime heating, if the parcels can keep it together enough to become organized, if the sfc winds back more, etc., than iffy conditions will spell some beautiful single cell high-based storms. If we're luckier, they'll situate the selves over some good looking shrubbery (sufficient to make the Knights Who Say Ni proud), and we'll get photographic storms.

In this pattern, it's really all I could hope for. It's enough for me to be heading to N Platte today.

 

[Neil Robinson]

I'm new at this so please excuse me if this sounds strange, but I noticed on the 06NAM that there appears to be a bulge of lower dp surface air intruding eastwards across the NE panhandle (00Z Thurs), with a subsequent intrusion westward of albeit modest dps of 55-57F over the top along the NE/SD border.
Would this not create a diffuse E-W dryline boundary just in the area showing reasonable mean CAPE, and so assist any upslope forcing going on?

 

[cdcollura]

Good day all,

Would this not create a diffuse E-W dryline boundary just in the area showing reasonable mean CAPE, and so assist any upslope forcing going on?

The major issue will be the available moisture.

Dewpoints may LOOK good on some models, or early in the day, but are only a very shallow layer due to evapo-transpiration (not Gulf moisture as that is cut off).

This layer should mix with the drier air aloft (above it) later in the day giving a much lower mixed-layer dewpoint (40's or so instead of 50's).

The storms, if any, will be very high based / elevated (bases 15,000 feet MSL or more) aided a bit more by iscentropic forcing near the front.

Yes, there is a hint of a dryline...

 

[David Wolfson]

FWIW, I wouldn't be jumping for joy for sure, but the NAM seems to show the best theta-E piling up south of the cold front sagging south along an e-w axis through Chadron, more or less. The levels above surface do indicate a front, not merely a dryline, with moisture up through 850h, I think. Topography is significant at 4000' up there.

The Douglas, WY, vicinity forecast soundings don't look too bad for initiating something off the 8000+ terrain. With luck this will be able to attach to the boundary (such as it is) and then move south of east.

I'd still be planning to hang in Scottsbluff unless I was coming south, and see how the conditions lie in the morning.

 

[Larry J. Kosch]

Storms Breaking Out!!

Looks like storm initiations has started in western NE and northeast CO. It is clearly shown on VIS SAT. No peeps (or twitters) from SPC about any meso discussions or SWWs. Guess this may turn into a garden variety day for storms!

EDIT: Sorry I was looking at the wrong date on the calendar. This thread is for the 20th, which is today, and the storms I talked about happened on the 19th. My Bad.

 

[Paul Knightley]

Well, after landing in DEN last evening to see a thunderstorm to our south, we are currently en route to Scottsbluff for the night.

As I see it, the post-frontal flow is looking fairly moist and as long as some heating can occur around the front, flow is enough for some updraught persistence/rotation.

 

[Jason Persoff]

12z NAM continues to focus a dry punch along the Sand Hills (along the NE/SD borders) Thurs 00z. I'm inclined to believe the progs given the amazing southerly winds blowing across I-80 today transporting the thin moisture up to this area. Upper level support, naturally, is much further north, but a shortwave should rotate through by tomorrow afternoon/evening. I'm hoping the winds to respond a little more than they're progged at the surface. If we can get any backing with the upslope, tomorrow actually could be a diamond in the rough. I'm counting on it; even a single photogenic storm would be rapturous.

So I've pulled into North Platte and look forward to pulling out pen & paper in the morning. Tomorrow, the chase is on.

 

[Jason Persoff]

I'm new at this so please excuse me if this sounds strange, but I noticed on the 06NAM that there appears to be a bulge of lower dp surface air intruding eastwards across the NE panhandle (00Z Thurs), with a subsequent intrusion westward of albeit modest dps of 55-57F over the top along the NE/SD border.
Would this not create a diffuse E-W dryline boundary just in the area showing reasonable mean CAPE, and so assist any upslope forcing going on?

Heya Neil...welcome to ST!

I'm definitely not an expert on this, but I actually think what you're seeing is a feature called a dry punch which looks like an eastward bulge in the dryline, often with its northerly component being oriented E-W followed by a gradual turn to the south then to merge again with the dryline's usual N-S orientation. Often these punches serve as a focus of severe weather when the synoptic scale is favorable too. The dryline proper is a vertical wall of sharply different airmasses (a moist vs. a dry one, though there's no "absolute" difference in Td's that qualify, I like them to be separated by several 10s of degrees, usually over a very small horizontal area).

I think that (although I definitely turn to the more weather-savvy in the group for confirmation here) when you see a punch, it acts both as a lifting mechanism (like a front), and allows for sfc winds to back into it, particularly long its northeast point.

Now, mind you, this is all general interpretation and exceptions to everything I wrote above exist. In my forecast notes above, I actually think the dry punch is going to be an important player tomorrow. But I may be quite wrong.