05/06/05 FCST: KS/NE/CO

[Ben Prusia]

SPC has a slight risk out right now for parts of western Kansas and for a majority of Nebraska during the day on Friday, the Day 3 outlook mentions of possible supercells.

The new 12Z run of the NAM shows a bullseye CAPE value of up to 3500 J/Kg and a LI of -6 near Oberlin, KS running up to McCook, NE. Dewpoints into the lower 60's can be found along and east of this line. However, 500mb winds are low, 30-40 knots, but I think still enough to sustain thunderstorm/supercell thunderstorm development.

Although the major show is progged for the weekend, I feel this day could have some potential, as well. Graphic for this forecast can be found on the "My Chase Forecasts" link on my signature line.

 

[Mike Johnston]

Ben, I was just looking at this and considering starting a thread but you beat me to the punch. As I was beginning to despair over Saturday's somewhat deteriorating outlook, noticed the negative tilt beginning to show on surface low deepening in WY - showing SE surface winds and 30-40kt SSE 850mb flow across Central and N KS and much of NE. Although 500mb velocities just beginning to nose in by Fri, nice little shot of warm moist air w/ directional shear may be sufficient.

 

[Kevin Polston]

Friday potential in Nebraska

I am also glad to see some people talking about Friday. Looking at the 12Z NAM model it does suggest that south central and perhaps southwest Nebraska look pretty good environment wise for supercells/tornadoes. Of course that was still 60 hours out......but as of right now I think it looks promising. The only thing I would be questioning is where the initiation would be. It appears there is a mid level shortwave trough that moves through the area which is forecast to generate some uvv between 18Z and 00Z. It was hard to define a synoptic scale boundary in the "target area" so I am not sure if storms will develop further west (near the dryline) and then have to move into the better environment or if the trough passage will generate enough lift to get storms going ahead of the dryline. It was also interesting to note at 18Z at 250mb there is a bullseye of divergence across central and southern Nebraska......

It will be interesting to see how this pans out over the next 2 days.

 

[Mikey Gribble]

I am thinking Friday might be the sleeper with this system. Some people may overlook it because of moisture concerns and because Saturday will probably be a chase day too. CAPE of 3000JK/G and deep layer shear at 35-40kts will support supercells. The NAM is showing SSE surface winds in the warm sector and backed winds along a warm front that extends SE from the surface low into central Kansas. Storms coming off the dryline in NW Kansas will encounter 0-3km helicity around 300m2/s2. With storm motion being ENE, any storms in NW Kansas and far SW Nebraska might interact with the warm front. I am planning on heading out Friday morning for NW Kansas if this holds together.

 

[Mike Johnston]

Yes, target for initiation is hard to pin down at this point. The SPC's outlook speaks of chance for a warm front developing. In looking over the regular graphical NWS forecast maps, the closest surface lifting feature I could identify was the hint of a wind shift line (SE to SW) draped across western NE associated with the trough of low pressure centered up to the NW around WY. Not much in the way of temperature differentials across the area, and dewpoint gradients oriented along area of traditional dry line.

 

[Michael P. Morris]

Based on the 12Z NAM, it looks like the warm front drapes west to east just north of the ND-SD border, but is not as well defined on the 60 hr (i.e. 00Z Saturday) prog. A very tight dryline will probably be the focus of convection in this case, so I would want to be in the Oberlin/Colby/Goodland area, depending on the develpment of the dryline. The instability is the highest in the area I mentioned, but the best helicity is progged in Nebraska. Stronger capping appears to be in place over much of central and eastern Nebraska as well. That being said, this event is still 2 days away. I await the 00z models

 

[DanHinch]

I am watching this one closely as well, as I am officially done with school tomorrow and ready to hit the Great Plains. Like some of the rest of you I feel that so far Friday has generally been overlooked with the potential later this weekend and I also share some of the same concerns. There is quite a bit of model discrepancy between the NAM and GFS. Only time will tell which will be favored come Friday but when it comes to "wish casting" I like the placement and dynamics of the NAM solution better, but my main concern is lack of true surface features. This is really a challenging forecast at this point with the surface low forecasted to be so far North in Wyoming, this is not a surface set up that I really favor, but I am going to go out on a limb and say that the model seems to be picking up on somewhat of a double warm front structure. The main wf clearly extends from the low across Northern South Dakota, with "possibly" a secondary wf that extends more Southerly from the low draped across Northern Kansas. I say this because there is a noticeable yet ever so slight wind shift in this region from South behind the front to Southeast ahead, as well as a definite temperature gradient of 5-10 degrees in the same region. Like I said before I think I am really stretching this one considering its still 54 hours out (all my data is from 18z runs by the way). The CAPE and helicity are juxtaposed nicely in the region of this "possible" wf and also in West Kansas along the dryline lending to EHI values greater than 5. This is if the NAM verifies, if the GFS verifies than things could be quite different and most likely focused further North. Now we just wait...good luck to those already out and those getting ready to start your chase vacations. See ya out there.
Dan

 

[Bill Tabor]

Via 12z NAM: Looks like dryline and mild short wave interaction will be the cause of precip breakout. Dewpoints whereas possibly not too deep will probably be sufficient. Everything else is go except upper level winds which are weak for storm venting may cause short lived updrafts and collapse for wind damage / microburst and possibly assist HP mode although lower environment dewpoint may moderate that. 500mb winds are not real strong but enough for supercells. Low level shear will be favorable for tornadoes in parts of western KS and NE.

 

[Mike Johnston]

Following yesterday's initial little flury of excitement about this forecast, I suppose an update based on the 12z NAM is in order - especially as we welcome an influx of storm vacationers into the region this weekend. Let's review our setup, shall we?

Disappointment, of course, with the lack of an apparent surface mechanism. The negative tilt of the surface low over WY has moderated; as a result the emergence of wind shift line across S NE has disappeared, although we do have SSE surface winds still forecast over western portion of target area. This leaves us looking to the dryline for help, and indeed there is some hint of a bulge over NW KS by 18z, coincident with SB Cape of 2,500 to 3,500 j/kg forecast to develop by 00z. Sufficient instability shows along E/W axis straddling KS/NE border, with steadily decreasing LCL and LFC heights as one progresses eastward, along with pocket of 300+ m2s2 SRH over N Central KS / S Central NE. Convective inhibition parameters appear workable.

In summary, convective initiation is the main question mark for Friday. If we can get initiation in the western portion of the area, where 500mb flow is nosing in, early enough in the afternoon so that storms can advance eastward before instability subsides with sunset, then we have a pretty decent chase scenario. As of now, I would rate it a 40/60 chance.

 

[Mikey Gribble]

I am pretty happy with the way things are shaping up. Going off the NAM CAPE is at 3000J/KG and 35-40kts shear will exist along the KS/Ne border. It is still showing the weak secondary warm front with backed winds running from Wcentral Nebraska back SE into Ncentral Kansas. I feel pretty much the same way Mike does, if we can get a storm early enough to move East along the border we could be in business.

 

[Ben Prusia]

I am thinking that NE Kansas - SE Nebraska, running into NC Kansas and SC Nebraska could be in business for a few severe thunderstorms to develop. For NE Kansas, the area will be in a good instability environment with SBCAPE of running into the lower to mid 2000's J/Kg. For 500mb winds, NE Kansas is at the eastern fringe of an area of 30-40 knot wind speeds, but running into NC Kansas, there is a blip of some 40+ knot wind speeds. There could be a weak or fringe warm front as far east as the Kansas City area which could be a focus for some thunderstorm development in the area and to the W and NW. Helicity looks good enough for supercell development in a peak area of SC Nebraska, SEward to the Saint Joseph/Kansas City areas. Moisture is also really good in the area, with the NAM progging 60+ degree dewpoints in an area of the highest helicity values, SE to Kansas City, with a peak value of 65+ degree dewpoints around the Kansas City area.

I think for the best chances of seeing a tornado, the warm front is the best place to play. Lowest LCL will be from the peak of helicity SEward to the Kansas City area, with thunderstorms near Oberlin and McCook being a little more elevated. NWS in Topeka mentions of possible severe on Friday, mainly north of Interstate 70 during the evening hours. I am still monitoring the situation for a possible NE Kansas/SE Nebraska or into SC Nebraska chase. I have a new graphic for this forecast up on "My Chase Forecasts" on the link below.

 

[Bill Tabor]

I was considering making the drive for tomorrow, but after reviewing the 0Z NAM it certainly looks weaker. Now if I was local I'd probably chase it. I chase lots of local setups like that here in TX; however it's different if you have to drive 13 to 15 hours to target.

It seems it has slowed it a bit- moving some features such as 500mb flow and sfc low more to the west. Most lower level winds aren't showing as backed. I'm having a hard time seeing what convergence will cause precipitation to break out and additionally the mid level forcing doesn't seem to be there in strength to help precip break out either. This leaves me questioning how much precip will break out if at all. Perhaps daytime heating will be the vehicle - guess I didn't check that. But regardless the NAM does not show a heck of a lot of convective precip breaking out. Also looking at EHI and other forecast tornado and supercell parameters they have weakened as well. Now granted there is still stuff there. If convection can break out and be surface based in northern parts of central KS and south central NE then there is a chance for a supercell or two and a slight chance also of a tornado or two. If I was SPC I'd probably stick with a slight risk for severe over the larger area in KS and NE and my torn chances would probably be at about 5%.

That's a chaseable amount if you are local, but it looks like it is weak enough to keep me home. Now to look at Saturday. Perhaps holding back some of Friday's potential will make Saturday a better day. I'll do it if it is strong enough.

 

[Bill Schintler]

Chase target for today, May 6

Chase target:
Kadoka, SD; on I-90 80 mi east of Rapid City.

Timing:
Storm initiation 4 PM

Storm type:
Isolated supercell storms with tornadoes and large hail.

Discussion:
14Z SFC analysis showed a surface trough axis along a RAP/PHP/SFD line. Moisture was increasing, with dewpoints generally in the 50-55F range S of I-90 and E of an Ogallala, NE to Wall, SD line. ERLY winds along the trough axis had also pushed a tongue of 50+ dewpoints WWRD to near CDR near the NE/SD/WY border. Visible satellite showed extensive ST in this area which should easily erode from W to E as the day progresses. RAP sounding, just W of the moist axis, indicated very steep lapse rates up to 600mb along with a small curved hodograph. The convective temperature there was around 77F.

Today, general subsidence will be in place until a vort max approaching from the SW helps strengthen convergence along the nearly stationary SFC trough axis. Additionally, mid- and upper-level winds will increase, as the exit region of an 80+ kt, 300mb streak rounds the base of the upper-level trough and approaches the area from the SW, resulting in 40kt SWRLY winds in the target area by 00Z. This will also help to stretch hodographs out a bit. A 30kt SRLY LLJ will strengthen though 00Z. ESERLY winds of around 20mph will continue to transport moisture into the area. By mid-afternoon dewpoints will be 55C while a 65C surface theta-E ridge noses into the target area. MLCAPE’s to 2000 J/kG, the aforementioned hodographs, and LCL levels around 1000m will support a small tornado threat.

- bill

 

[Glen Romine]

I'll expand the region highlighted in the thread subject some, but since there is a forecast thread for this event I'll post to it. Ok, so after a quick review of the morning setup, here is a quick summary as I see it. Weakly positive tilt trough now inland along the west coast, with a modest jet streak in the base of the upper trough. Out ahead, crest of upper level ridge now aligned along the western high plains. Flow is particularly diffluent over CO, and weakly divergent as per analyses. Water vapor imagery shows a few weak impulses embedded in the upper flow pattern, perhaps most notable is a small blip sweeping into northeast NM this morning, which should ride out into central CO later today. 700 mb analysis shows a ridge axis firmly planted just east of the Rockies, weakening some across the NE panhandle northward. 850 mb shows a significant plume of elevated moisture extending as far north as western NE that should lead to increasing boundary layer moisture with time as mixing increases this morning, but weak capping on top of moisture layer below primary cap will lead to reduced moisture by late afternoon as mixing reaches the dryer layer above. As such, low-mid 50 dews may be the best around in region from western KS to sw NE. Surface features include a weak dryline-like feature hugging close to the front range, with a trough extending from around DEN-OGA-SUX. Further south, weak ridge along the Red River to near ABQ with backed sfc flow south and weakly veered flow to the north.

Presence of weak boundaries and convergent surface flow, along with arrival of the previously mentioned weak upper wave, offer a reasonable hope for convective development across extreme ne CO region. Unfortunately, the deep layer shear may be too weak to support organized storms, favoring more of a multi-cell character. Weak ascent further south may also suffice to initiate a few isolated storms, where deep layer shear is improved somewhat. RUC model maintains persistent convergence across east central CO (where showers are ongoing this morning), as well as around the Sterling to just north of GLD area, and a stronger bullseye in eastern NE. Would expect SPC to bring the western edge of the slight risk further south, but maybe only to the NM state line. With very disorganized surface flow and precipitation ongoing right now across CO still, I'd be challenged to pick a specific target, but Burlington CO might be a compromise spot for now.

Glen

 

[Mike Johnston]

Well, I hesitate to even revive this KS/NE forecast as it seems to die and re-incarnate with every 12-hr run. However, there are some renewed signs of surface lifting which keep hope alive. First, an axis of surface moisture convergence is forecast from ~Concordia, KS sewd to ~Ottowa, KS, as we also see some backing of surface winds across E NE and N Central to NE KS. An area of positive temp advection @850mb envelopes this axis, tucked just underneath an area of negative temp advection to the northeast. The makings of a warm front or mesolow developing? Not sure, but a little wave in the surface isobars along this axis is enough of a hint to at least take notice.

As for severe parameters, 0-3km CAPE of 2,000-3,000 j/kg forecast to be in place across the region. Lack of deep layer shear remains a concern. However, I am encouraged by the SPC mesoanalysis which has been showing some effective bulk shear (and effective SRH, for that matter) values pop up around the area, even as the standard 0-6km shear values are non-existent. Hopefully, this trend will continue as the day progresses. Now, anvil-level SR winds of 35kt clearly suggest HP will be the mode of the day should storms actually initiate, along with hail potential parameter up to 1 inch in NE KS.

I feel like I keep going out on a limb with this one, and would appreciate any feedback from those of you who have been following this forecast.