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

[Jeff Duda]

As the next major trough crosses the US, there could be another stretch of consecutive severe weather outbreaks. Saturday is looking like a possible big one. The GFS indicates 80 - 90 kts SW flow over the central Plains and into the upper Midwest at 00Z, accelerating even more afterward. There is about 50 kts SW flow at 700 mb, and 30-45 kts SSW at 850 mb. It looks like a great capping scenario, with moderately high 700 mb temps before 00Z, then cooling rapidly as the upper-level cooler temps move in. This could hold down convection during the early parts of the day, thus eliminating early overnight convection or early day crapvection that might otherwise kill the instability. There appears to be plenty of deep moisture with surface Tds > 60 pretty much throughout the region. There looks to be some pooling of higher Tds near the warm front in S MN, but that might be overdone. Unlike with last weekend's set up, surface winds look to be S or SSE, so shear is fantastic. Looks to be 1000 - 2000 J/kg SBCAPE throughout the region as well. Plenty of surface forcing with a dryline in place across C NE/C KS/C OK. There could be severe weather in a large area this day. The only fly in the ointment is storm speeds. Storms will have rocket packs on them, so keeping with one particular storm would be rather difficult, if not impossible. Perhaps you could just let a cell pass by, then jump south/east to the next one down the line.

This could be a big event. I've been watching it for several days now on the GFS, but just this morning it sped up the system to a Saturday event, whereas previously it looked like a Sunday event. Not how the GFS typically evolves. Things could obviously change greatly between now and then.

ADD: The 00Z ECMWF has this system a bit slower than the GFS, with the sfc low sitting out in W NE/NE CO by Saturday evening.

 

[Skip Talbot]

I agree, Jeff. Saturday is looking like a major plains/midwest event with that impressive 90 knot 500 mb flow starting to eject over a very large, moderately unstable warm sector. Everything appears to be in place for supercells down a large length of the dryline (and maybe even the warm front) from MN to AR (and possibly points to the east).

My first major concern is that Saturday is still a moving target. The GFS has not settled at all on the surface features' placement. Yesterday morning the triple point was in central KS, last night it was in eastern NE, this morning its NW IA. The GFS is actually speeding up with each run, not slowing down like it usually does. We saw this with last weekend's setup too though. The 12z NAM out to 84 hours (Friday evening) is still slightly faster than the GFS, so I imagine will see this trend continue a bit more.

Another potential issue I see, which may or may not disappear when the models settle on a solution are the largely unidirectional 500 mb and 850 mb winds. Given the speed shear, we'll probably see excessive storm speeds instead of the more manageable speeds you see when those 500/850 crossovers are larger and storms move against the mean flow. The lack of veering in these levels might also put the storm mode into question, with cells splitting or forming mini lines. However, there should be some area of better directional shear somewhere in that large warm sector, maybe on the fringes of that mid level flow, or further east on the warm front.

Those issues aside, this is probably the best looking setup I've seen the models plot all year.

 

[Brandon Redmond]

Probably should add Illinois to the mention as well.

Obviously a lot of uncertainties that still have to be worked out, but the 12z GFS showing some pretty significant EHI and CAPE values Saturday Evening.

 

[Andrew Stoller]

I totally agree Skip about the GFS speeding the system up; wouldn't be surprised at all if Friday becomes the day for the central plains and the system is all the way into the northeast by Saturday evening.

In any event, this setup looks pretty decent; the trough is starting to show a negative tilt as it digs into the central US, the surface low is down into the low 990 mbs, the moisture return looks like it will be excellent as there are no strong cold fronts coming through this week to wipe out the gulf. We'll have 3 days of fairly unimpededed moisture return.

It's hard to get excited though just yet this far out as the GFS has been waffly the last few systems and things never panned out like what they looked 4 days out.

 

[Greg Blumberg]

Feeling pretty good about this setup from the wind profile and upper level forcing standpoint. Both the SREF and the GEFS are showing some pretty good signs of setting up the 500 mb shortwave trough in a good position so far. Thankfully there's not a whole lot of spread in the GEFS and SREF members when it comes to this 500 mb trough. In fact, the SREF has diffluence east of the 500 mb shortwave trough axis and looks to be preparing to take on a negative tilt as the jet it has at the base of the trough builds its way on the east side of the trough axis.

Edit: I will say that there is spread and uncertainty on the heights to the west of the 500 mb trough axis in the members of both ensembles, which is unfortunate because jet placement on the west side of the trough axis will be critical to the evolution of the upper level dynamics.

GEFS:
http://www.kwixs.net/ncep/ncep_models.html?m=gefs&h=102&r=18&ma=50d&t=m

SREF:
http://www.kwixs.net/ncep/ncep_models.html?m=sref&h=87&r=21&ma=50h&t=s

I am rather concerned that we will be working with modified Canadian air for this weekend setup. The recent cold front really has worked its way pretty far south into the Gulf and I can see from IR satellite at least, that the cold front (or outflow boundary from the convection), has reached the Yucatán Peninsula. This being so, I am very skeptical about whether or not we will be able to get a quality moisture return four days later. I hope I am wrong.

Time sensitive:
http://cimss.ssec.wisc.edu/tropic/real-time/tpw2/natl/main.html

You can really see the cold front coming through and wiping out the Gulf air around 4-5-11 03 UTC in the above link. Ouch.

 

[Michael Riedl]

I am rather concerned that we will be working with modified Canadian air for this weekend setup. The recent cold front really has worked its way pretty far south into the Gulf and I can see from IR satellite at least, that the cold front (or outflow boundary from the convection), has reached the Yucatán Peninsula. This being so, I am very skeptical about whether or not we will be able to get a quality moisture return four days later. I hope I am wrong.

Time sensitive:
http://cimss.ssec.wisc.edu/tropic/real-time/tpw2/natl/main.html

You can really see the cold front coming through and wiping out the Gulf air around 4-5-11 03 UTC in the above link. Ouch.

That satellite image is pretty bad however I believe we can get quality moisture back in time. Tonights GFS show winds shifting back to the south in the Gulf and the Plains by 12z this morning. Almost 60 hours of moisture advection should be able to get the job done and then some starting Friday, and to a lesser extent Thursday.

Just my thoughts.

 

[Tim Supinie]

I am rather concerned that we will be working with modified Canadian air for this weekend setup. The recent cold front really has worked its way pretty far south into the Gulf and I can see from IR satellite at least, that the cold front (or outflow boundary from the convection), has reached the Yucatán Peninsula. This being so, I am very skeptical about whether or not we will be able to get a quality moisture return four days later. I hope I am wrong.

From what the 06/00Z GFS is showing, we should be getting some pretty nice moisture trajectories, which is something we haven't seen so far this year. More specifically, though they appear to be pulling moisture from regions that front won't get to (the central Caribbean). I'm optimistic about moisture not being a problem. Maybe not as much as what the GFS is showing (though that would be pretty nice). Also, via the latest (06/0643 UTC) surface obs, there is southerly/southeasterly flow from central Nebraska to the western Gulf. Dewpoints in the western Gulf generally aren't stellar yet, but there's obviously plenty of time to change that.

One thing that I'm noticing from the current obs vs. what the 06/00Z GFS is showing is that the flow behind the cold front in central Nebraska is a lot stronger than what the GFS is showing (20-30 kts observed vs. 10 kts forecasted). This makes me think the cold front will penetrate a bit farther south than what it's showing (GFS has it stall out in Kansas, the 06/00Z NAM does have the front penetrating to near the Red River). Regardless, once the boundary is lifted northward, the moisture should be fine.

Fast forwarding a bit to Thursday, both the GFS and NAM have a sharpening dryline in the central/southern plains. The difference between the two is that the NAM has the dryline farther east in central KS/OK/TX than the GFS, which has a nice western KS/OK chase on Friday (4/8) and then pretty much the same area on Saturday (4/9). As much as I'd like this kind of two-day chase, my money is on the NAM for this one, as the mixing behind the dryline is probably more realistic in the NAM than the GFS. So my bet is an eastern KS/OK event on Friday, and about the same but expanded farther north and east on Saturday (the dryline retreats westward overnight just to tease us). Plus, as it's been very dry in the southern plains, it won't take much to mix the dryline eastward. I hope I'm wrong (as my chase radius isn't very big and is highly biased to the west), but we'll see.

So, to summarize:
Good things: 1) Moisture will most likely be doable, possibly even great. 2) Signals of backing surface winds in the NAM and GFS are encouraging. 3) Both the GFS and NAM suggest that capping won't be an issue. Though this may change if both are over-forecasting the moisture return.
Bad things: 1) Chase territory may be less than optimal. 2) Forecast for Saturday depends heavily on what happens on Friday. (Where does convection form? Does it form at all? Where are the outflow boundaries?)

That's my two cents.

 

[jshields]

i am fairly impressed with saturday's chances, especially if the GFS verifies. it has shown saturday as being the best day the last couple of runs. i'm not too worried about moisture return as this is still 4 days away. some of the indices are looking amazing, especially across eastern iowa, western illinois. however i worry that this will be too far away from the greatest forcing and also the cap is stronger as you head to the border. most of western and central iowa has virtually no cap and cape over 2000. bulk shear is 50 kts plus with some areas over 60 kts. i worry a tad that with an elongated low pressure system that is showing up on the gfs there isn't a lot of backing winds, so shear seems to be more speed vs directional. also storm speeds could be fast with not a lot of turning between 850-500.
the new 0Z euro has come in and is way different than the GFS. it doesn't really kick anything out til sunday. at 0Z monday the main low on the euro is still out in kansas vs the GFS shows the storm all the way to upper michigan! the canadian at 0Z monday has the main low in northeast nebraska. still lots of changes coming i'm sure.

 

[Jeff Duda]

I am rather concerned that we will be working with modified Canadian air for this weekend setup. The recent cold front really has worked its way pretty far south into the Gulf and I can see from IR satellite at least, that the cold front (or outflow boundary from the convection), has reached the Yucatán Peninsula. This being so, I am very skeptical about whether or not we will be able to get a quality moisture return four days later. I hope I am wrong.

I hope you are, too. The American models are all indicating that the big ridge over the southeast US will be moving well offshore (eastward) by as early as tonight, and the Gulf pretty much opens wide after that up through 850 mb. The Thursday shortwave does not appear to halt the northward advection of moisture, only delay it.

The event is now in range of the NAM. It looks to roughly agree with the ECMWF yesterday in putting the sfc low out to the west, but with the dryline well out ahead of it (there is a really narrow string of moisture that trails back to the west right along the warm/occluded front to the low. However it doesn't have as great of moisture throughout the warm sector, rather in a smaller region near the warm front. Due to the slowing, the better shear is a tad displaced from the best instability, but not insurmountably.

 

[Chip Redmond]

I hope you are, too. The American models are all indicating that the big ridge over the southeast US will be moving well offshore (eastward) by as early as tonight, and the Gulf pretty much opens wide after that up through 850 mb. The Thursday shortwave does not appear to halt the northward advection of moisture, only delay it.

The event is now in range of the NAM. It looks to roughly agree with the ECMWF yesterday in putting the sfc low out to the west, but with the dryline well out ahead of it (there is a really narrow string of moisture that trails back to the west right along the warm/occluded front to the low. However it doesn't have as great of moisture throughout the warm sector, rather in a smaller region near the warm front. Due to the slowing, the better shear is a tad displaced from the best instability, but not insurmountably.

With that kind of detail of moisture lining up with shear probably will fix itself over time (hopefully). Moisture seems to be recovering well and never really gets cut off from the Gulf with southerly flow on both GFS and NAM. I am not quite sure what I think about forcing at the moment. The system seems to have slowed down a fairly good deal and it looks like Saturday could end up like a pure dryline play. CIN is progged to erode through the afternoon but there seems to be a lack of forcing. The cold front is nearly non-existant with such an immature low pressure, which is good for discrete storms. However, the lack of any vort max and also the seemingly late arrival of the sub-tropical jet streak make me me wonder if there is anything to actually get storms to fire on Saturday. It seems we would be waiting on a dryline push to get initiation started. Excellent directional shear and good speed shear with sfc-700 bulk shear values from 45-50knts and storms that develop will definitely will have the ability to go supercellular, but something just seems to be missing at this point. Definitely going to watch this setup and how it evolves this week.

Back east towards IA/MO I think the cap will hold out much longer through the day and perhaps into the evening. Though LI's are solid, below -6, I just don't see any dynamics that would keep me there unless there is some earlier convection traveling along the weak warm front/stationary front.

Chip

 

[jshields]

the 12Z NAM seems to have a more classic warmfront across central iowa with backed easterly winds across the center of the state, and a strong temp gradient. 0-1 SRH is over 300 here. the gfs doesn't show the warm front really at all. the nam however shows the cap is much stronger with a LSI of 2-4 across most of the southern half of the state. i'm too worried about something to set anything off with the main storm much further back though. the cap is weaker across central and eastern kansas, so hopefully something can fire along the dryline, but the srh isn't as great down there.

http://wxcaster4.com/nam/CONUS_ETA212_ATMOS_LSI_84HR.gif

http://wxcaster4.com/nam/CONUS_ETA_0-1KM_SRH_84HR.gif

 

[Logan Karsten]

With the 00Z NAM now in, I must say the amount of instability showing up Saturday evening across the region is very impressive. As mentioned earlier, it looks like a warm front will be draped across southeastern Nebraska and southern Iowa. Of course the biggest problem is the potential for capping issues if 850-700 mb temperatures end up being warmer than the NAM suggests. Outside of that, shear and helicity are impressive as well near the warm front.

 

[Andrew Stoller]

The NAM has been quite consistent now, and the GFS is slowing the system down instead of speeding it up like it looked earlier. The NAM is keeping the surface features further from the upper support, but capping is looking like less of an issue on the 00Z NAM, and it's finally initiating convection along the dryline in east central KS. Deep layer shear vectors are fairly perpendicular to the dryiline. If everything continues coming together, Saturday might produce an isolated discrete supercell or 2 in northeast KS. The NWS Kansas City sure thinks so - read their AFD for Wed. afternoon.

 

[Chris Allington]

A couple of things of note looking at the 00z NAM this evening: #1 I'm fairly certain this is the first time this year I have seen backed 850's rather then shallow moisture with veered 850mb winds. With moisture pooling along the WF across NC/NE KS and into SC IA/ NW MO moisture quality won't be a problem for the first time this year! That being said (EAX AFD hints at this as well) with the NAM showing dewpoints around 70F, the amount of moisture and resulting instability is likely overdone. That being said... looking at 500mb and 700mb VVELD maps, as well as the 700mb temperature progs, I'm not thinking that the cap will be as much of an issue as people are thinking. In fact the NAM this evening intiates convection along I-70 before 00z, in an area of backed winds along the DL.

 

[Mikey Gribble]

The 00Z NAM is great. There might be a few chasers parked near I70 and I35 if that were too verify lol.
As others of mentioned 850mb winds have backed more ahead of the dryline with the 00Z NAM run than we've seen in previous runs. That was one of my two concerns with this setup. That change has really taken the potential up a another level for me with this setup. The warm front doesn't look very good though with 850's continuing to veer there.
My other concern was the cap, which the NAM is breaking out precip along the dryline (as mentioned before) with tonights run. That is encouraging. In addition to that the previous two runs of the GFS have hinted at precip along the dryline and tonights 00Z GFS shows a very breakable cap along the dryline. 850mb winds are veered slightly more with the GFS than the NAM ahead of the dryline, but they are also a little stronger. Both models show thermodynamics and shear profiles being very favorable for tornadic supercells ahead of the dryline in Kansas. The central into north central Kansas portion of the dryline is definitely looking the best at this point.
It's obviously too early to start getting overly optimistic about this setup since a lot can and likely will change between now and then, but the environment the models have been showing ahead of the dryline is something I would expect to see strong tornadoes out of. IMO it's the best tornado setup we've seen so far this season in the plains.