5/29/2010 FCST: WY/NE/CO

[Jason Persoff]

Yes, it's early to start this thread, but here's my take and it's my first full day to chase.

As for Saturday, 5/29, however, there is a lot of evidence coming into focus of a negatively tilted trough over the C Plains states with winds increasingly looking like they'll be SSW @ 40-50 kt over NE/KS/E CO. 700mb temps are brutally warm over most of the region except over the NE PH and E WY. I am pleased to see the LLJ appears strong with 850mb winds out of the S/SE at 30kt! GFS hints at low pressure over the CO front range will result in upslope flow along the CO front range.

Preliminary target (120 hr out) will be CO/NE/WY borders where convective precip appears likely in a favorable upslope regime. Moisture return will reasonable for the high plains with Tds in the 50s. Saturday is the kind of chase day I've been praying would happen. Now let's see what 120 hours does to my forecast...

Sunday, 5/30, shows little in the way of favorable situations. The H5 winds will provide a right entrance region to a jet streak, but the sfc is a mess with high pressure dominating the sfc and scoured out moisture looks like it will be quite dry. I don't see anything that is particularly tempting there.

Memorial Day, 5/31, Monday, shows some warm, moist air traveling northward into central KS up to the NE border. H5 winds will be largely zonal and reasonable at 40+ kts. Currently GFS suggests a possible low pressure system (weak) setting up over SE CO. The GOM appears pretty stingy, but there may be a meager LLJ. Nothing so far excites me about Monday and the area is pretty diffuse all things considered with a lot of things out of synch.

Post Memorial Day, things look bleak for the remainder of the week. But we're still far enough out not to despair too much (yet).

 

[Eric Bucsela]

I've been eyeing Saturday's potential too - as apparently has SPC. At this point (and it is early), western NEB, eastern CO seem good. Looking at the 12Z GFS, biggest downsides I see for committing to a trip to the Plains are meager moisture/instability, as well as lack of obvious chase opportunities the following days.

Trying hard to be objective after my regrets about not chasing last weekend. Every setup has potential meteorological pluses and minuses. But it's impossible for us humans (including Vortex2) not to *slightly* skew how we weight these factors based non-wx-related considerations (eg time, money, distance, previous/future chases, other plans, etc).

Anyway, I'll keep watching this one closely and see the NAM's take on it tomorrow.

 

[Jason Persoff]

We'll need to start a Friday 5/28 thread here soon too, surprisingly.

This morning the GFS has matured the system into a very deep trough with less negative tilt for Saturday. At the upper atmosphere: the H5+ winds will be provide a right rear entrance region favoring lift over the SW NE PH back toward the WY/CO front ranges by 00z. 700mb temps will be ideal in the -5 to 0 degree range. An a fairly potent 850mb LLJ (35-40 kts out of the SSE) will converge in this same area of interest by 00z.

At the sfc, a weak low pressure system will form over NE CO (990mb) but sufficient to allow significantly strong backing, upslope winds over the E CO, W NE PH, E WY areas by early evening. The sfc winds will pool warm, moist air up the NE PH (with Tds somewhere in the low 60s).

The downside is projected sfc temps will be in the 80-90 range suggesting very wide T/Td spreads and therefore high based storms. But, the upside is this is the Front Range, where little else is needed to create visually stunning (even nontornadic) storms. Precip will form over the elevated terrain of WY/CO and move generally NNE (based on the 500mb winds). Right-moving storms could be visually stunning.

Other than the T/Td spreads, I see nothing that won't make this a worthwhile chase day (I add here that I'm a structure geek as much as a tornadographer). Compared to the whole of 2009, this looks like manna from heaven.

My target remains WY/NE borders. Awaiting NAM agreement, but I'm happy.

 

[Jason Persoff]

12z runs showing a slightly different forecast....First, Friday is back on the "outs" with the models showing nothing really happening Friday...

Back on topic...Saturday's forecast per NAM shows the trough much further west than the GFS (the latter being more bullish, ejecting a short wave over NE PH). Unfortunately, this suggests the models are not in high concordance and we'll have to wait another 12-24 hrs for this to come into better focus.

The NAM shows that the best CAPE will be well east of my ongoing target of WY/NE borders...I'm hoping that we'll see 1500 J/kg or higher move more westward over the next day or so. The sfc forecast is similar between the two models, and there is little change to suggest upslope flow will be in abundance. The other encouraging issue is that the LLJ also appears to be concordant showing 30+ kt 850mb winds screaming out of the SSE.

So, my target hasn't changed, but I am now thinking Friday will be used exclusively for positioning for Saturday. Continuing to monitor and will update tomorrow.

 

[Eric Bucsela]

Thanks for your analysis, Jason. Here's my $0.02.

Yep, big difference in the NAM and GFS positioning of the trough in the 84 hr forecast. I recall for May 10, the NAM was also much slower 84 hr out, and the trough verified closer to the GFS solution. On the other hand, for the shortwave of May 24, the differences at 84 hr were smaller, with the NAM being only slightly slower in rotating the wave, and also a bit further east with the whole system. I think the truth was a combo of NAM/GFS (but maybe slightly closer to the NAM solution).

Problem is, I'm not sure which setup is more analagous to this one. Safest bet is probably a compromise between the two models. Very roughly speaking, this would have the area of strongest H5 winds passing just west of the NEB PH at 00z.

I agree the surface features and 850 winds are similar in both models. If you take them literally, then looks like the sweet spot is from parts of the NEB PH east and north into S Cntrl SD. One thing I like is that the past few runs of GFS have been increasing the Tds and hence CAPE over the target area - now > 2500 J/kg in central NEB. Let's hope this continues!

I've probably been way too specific here given the obvious uncertainties, but I still think it looks like a great setup!

 

[jamesmseitz]

The issues that I have with this solution is that there seem to be very little Bulk Shear in NE. There seems to be more in SD but the NAM has Non directional Shear. With little to No Cap these systems tend to be Linear in Mode. GFS is better with the shear so really hoping the GFS is right on this one. SREF has is now too but this one does not look the greatest either.

All that said, this system is worth watching at least if the timing of the trailing low verifies. That would bring backing winds into play. Wait and see mode for me

 

[Jason Persoff]

Finally back on the Plains. I arrived at DIA and now need to prepare for the upcoming week of chasing.

GFS vs. NAM--12z Runs
It's hard to believe how dissimilar the two models are at the moment just +60 hr out...I was hoping they'd be more in synch for Sat, but not so much yet. The GFS is very insistent on keeping the trough over the mtns, while the NAM virtually moves it over the northern plains states. Needless to say, I'm hoping for the latter solution but feel it's a bit too bullish and so will accept that both are showing a possible jet with embedded streak over the NE PH area. Both seem to hint at 40-50kt winds in this area out of the SSW.

The further north you go (into MT), the less capped the atmosphere appears with very cool winds aloft. However, in the NE PH region, temps are modestly capped, corresponding with a large bullseye of precip throughout the day peaking by 00z. 30kt SSE LLJ seems consistent on both models.

SFC
Backing winds in response to a modest low pressure system should be located somewhere on the Front Range leading to upslope into WY. Dryline (? triple point) appears increasingly likely over I-76 corridor in CO with the NAM and the GFS differing slightly in location, intensity, and sharpness; both predict ample low level moisture with Tds in the 55+ range. Minimal cloud cover, intense daytime heating into the 80s, and favorable sfc confergence should get things at least started in the early evening.

Soundings
Forecast soundings for Scottsbluff suggest high-based storm modes with large T/Td spreads.

Initial Target +90 Hrs: Scottsbluff, NE.

 

[Mikey Gribble]

I just updated the forecast on my blog so I'll copy and paste that here. I didn't look closely at this setup until today, so I have little confidence in my forecast right now. Take it for what its worth.
I also posted a map on my blog if you're interested. Here is the link
http://loadedgunchasing.com/blog1/

here is the post from my blog...

The GFS and NAM are in remarkably good agreement with the surface pattern for Saturday placing a surface low over northeast Colorado and a warm front extending northeast from there. A dryline will connect with the frontal boundary over the far northeast corner of Colorado and run south from there over western Kansas.
A large trough currently located over the western US will continue deamplifying and slowly pushing east over the next couple days. By Saturday the trough will have weakened significantly. A couple weak disturbances, jet streaks will be rotating through the eastern portion of the trough on Saturday. By 00Z a 60kt mid level jet streak will be in the vicinity of the frontal boundary over northwest Nebraska and should increase 0-6km shear enough for supecells given the moderate to strong instability that will be in place.
Shear vectors will parallel the frontal boundary, complicating the convective mode forecast to some degree. Still I think semi discrete training supercells should evolve Saturday afternoon. With strongly backing surface winds training storms along the frontal boundary probably won’t have any sort of issues with contaminated inflow.
The strongest deep layer shear lies on the cold side of the frontal boundary and given the very modest wind fields, I have some questions about whether or not there will be adequate updraft downdraft seperation. Typically you like to see strong inflow into the updraft region of a storm. With strong downdrafts in the near vicinity of the updraft bases surging gust fronts could pose a problem for tornado potential. That is especially the case when you consider the weak inflow level winds we’ll probably be dealing with. Assuming there are no issues there, excellent directional shear will offset meager wind fields at all levels and create favorable hodographs for tornadic supercells along the frontal boundary. There is about 180 degrees of turning in the 0-6km layer along the front, which is excellent.
The tornado potential is really tough to judge with this one IMO. You have a weird variety of paramaters (kind of nontypical setup) and somewhat high LCL heights. Pinning down exactly what LCL heights will be along the front is tough, but area forecast soundings and LCL charts from the GFS and NAM make me think we’ll be looking at LCL heights in the 1100-1300m range at 00Z. I could be wrong on that though. We just got a good example of why you shouldn’t under estimate directional shear and/or boundaries the other day though. With that fresh in my mind I am thinking the tornado potential is a little higher with this setup than I might have otherwise thought. I think one or two tornadic supercells producing a few lower end tornadoes near the triple point and farther down the front in north central Nebraska sounds like a good bet.
There are a few paramaters showing up on the models that are fairly good. 1km EHI is between 4-6 depending upon which model you go with. And both 0-1 and 0-3 VGP look pretty good.
My biggest concern is the LCL heights and weak wind fields though. It is really tough to pin down LCL heights with rapidly changing moisture and temperature profiles in the vicinity of the boundary though, so my confidence in LCL heights being problematic is shaky. They would be somewhat problematic out over the warm sector, but with storms riding the front they may not be. Even along the front I think they may be a bit too high for a good tornado threat. I do expect one or two tornadic storms though. Temps along the front should be just over 80 around 00Z and dewpoints will be around 60. That puts us right in the 20 degree spread range.
I haven’t spent a whole lot of time on this setup so take this with a grain of salt. Right now I’m thinking we’ll get a tornadic storm coming off the triple point. Farther down the front where dewpoints increase near the Thedford area is another likely place for a tornadic storm. That is reaching a bit far 3 days out, but whatever. My point is I like two areas. The triple point and farther down the frontal boundary in north central Nebraska. If I chase I’ll probably go with the triple point. Typically I prefer to stay close to the surface low on marginal setups like this one. It just seems like they produce a little more frequently.
I haven’t spent any time looking beyond Saturday, so I will do that this evening and update the forecast later.

 

[nickgrillo]

Glanced at the models and the setup doesn't appear to look too bad. NAM paints a relatively favorable synoptic environment for organized convection -- including tornadic supercells -- particularly across extreme northeast CO and the panhandle of NE. I'm not really digging the orientation of the mid-level trough (both the NAM and GFS has it rather positively-tilted) but the degree of boundary layer directional shear should support organized supercells -- including the possibility of tornadoes -- near the surface low where isallobaric forcing associated with surface cyclogenesis will back the low-level flow (and strengthen boundary layer SRH). Moderate southwesterly 500mb flow will lead to >35kts of 0-6km deep-layer shear (supportive of sustained rotating updrafts) across much of the aforementioned portion of the warm sector. NAM shows convection developing across northeastern CO and the panhandle of NE shortly after 21z -- in response to surface diabatic heating / increasing low-level convergence along / near the surface front. Now, assuming we get dewpoint spreads of less then <15f in this region (to receive favorable LCL heights <1200m AGL) then the possibility of isolated tornadoes should be pretty decent given the rather favorable boundary layer directional shear (albeit weak low-level speed shear).

 

[Andrew Stoller]

Saturdays setup reminds me somewhat of the Lagrange Wyoming setup of June 5, 2009. There will be a somewhat stalled front from the upper midwest backed into the high plains of WY/ne CO/sw WY where it intersects a surface low. The upper level associated trough isn't textbook, as it wasn't for the Lagrange event. Non the less, the GFS and NAM are coming into better agreement - GFS still being a bit quicker/more north then the NAM. You'll have the orographics of the Cheyenne ridge to aid in initiation as well.

 

[Jason Persoff]

Indeed, this appears to be a narrow-corridor of ingredients that will need to be almost perfectly timed. This am's 00z NAM and GFS have done little to change my forecast. I'm looking for orographic lift and some degree of the dryline to help overcome the inhibition. Additionally, it looks increasingly like there will be a sfc convergence/warm front draped EW over the NE/CO borders that should offer lift as well.

I'm eager to see soundings tomorrow am and more data closer to tomorrow, but KBFF remains where I'm driving to tonight.

 

[Mikey Gribble]

After glancing over this mornings NAM I am becoming pretty concerned about the LCL heights near the triple point. It is very tough to figure out exactly what they'll be since we are talking about a very small area over the far northeast corner of Colorado where temperature and moisture profiles change rapidly. I will post a link to an LCL chart from wxcaster, but these things are usually smoothed so I don't trust them much. Still, it looks like LCL heights may be in the 1500m range if you go off that chart. I'll need to look at some forecast soundings, but that is getting pretty high if it verifies. here is the link to the chart
http://beta.wxcaster.com/models/eta/central/CENTRAL_ETA212_ML_LCL_36HR.gif

As SPC mentioned in their previous forecast the best deep layer shear remains north of the boundary. It is looking like we may be able to eek out 35kts of 0-6km shear right along the frontal boundary by the triple point. I didn't bother to look farther down the front yet since I'm not considering chasing there.
Even if a storm can hug the boundary and realize 35-40kts of deep layer shear, the high LCL heights and weak wind fields at all levels have me pretty pessimistic about the tornado threat. Any tornadoes should be weak and short lived.
I am looking at a 7-8 hour drive so I'm definitely leaning towards not chasing. This is a mediocre setup at best near the triple point and it usually takes a little more promise than that to get me to drive those kinds of distances. If I lived closer I'd chase strictly because its May and there are lots of instances where you get a surprise tornado or two near the surface low with setups like this. I wouldn't be at all surprised to see a couple weak tornadoes near the triple point and maybe even another weak tornado or two farther down the front, but thats about the extent of the tornado threat IMO.
I need to look at the models a little closer, but I'll probably be sitting this one out.

 

[Eric Bucsela]

Another interesting area is the vicinity of Valentine NEB, or maybe a bit west or southwest of there. This is along the front and sort of near the northern or nortwestern side of the CAPE bullseye on both NAM and GFS. The NAM has been consistently (last couple runs at least) showing a minimum in the LCL field there of < 1000m. And both models also suggest a relatively isolated cell could develop in that area before 00z. As mentioned, the zone of overlap for good instability and shear is very narrow and maybe post-frontal (hard for me to read), but if the storm can stay in that region long enough, there may be a respectable torn threat, IMHO. Unfortunately, this is near a large no-road zone, so getting close to the storm may require a lot of luck.

 

[Jason Persoff]

I just ran several forecat soundings which suggest LCLs will be around 1000-1200 which is definitely high based. That said, there is evidence of high CAPE and low level wind fields will allow for decent shear. I'm still pretty bullish on tomorrow and plan on traveling to Scottsbluff today.

 

[Andrew Stoller]

Both the NAM and GFS are forecasting sub 1000 mb surface lows in northeastern CO, so that at least will enhance the directional shear near the triple point. Like Mikey said, bulk shear is forecast to be weak near the triple point setup, but if we can get a little more upper support, there's a decent shot at a nice supercell with modest tornado potential in ne CO near I76 into NE.