9/18/05: FCST: Central Plains

[nickgrillo]

There is a chance for a few severe thunderstorms across KS on Sunday, assuming that we break the cap - which is possible, with the latest model runs showing precip breaking out across the southeastern half of KS. Favorable veering profiles and extreme instability suggest a threat for isolated supercells - associated with very rich boundary layer moisture, with widespread >70 Tds across central/eastern KS. Therefore, assuming we get enough covergance along the dryline/trough... A few isolated supercells will be quite possible. I haven't looked at anything much at the moment, but SUN was brought to my intention in another thread... I'll make a more detailed FCST later.

 

[Mikey Gribble]

I just took a quick look at the models so I could be wrong, but IMO Sunday isn't looking like it is going to work out. There is a fairly strong cap and the precipitation showing up on the NAM looks like it is from overnight convection. I would love to get out chasing on Sunday and it certainly looks like there is some potential there. Deep layer shear is AOA 50kts, directional shear is good once you get ENE from the surface low, and the levels of CAPE being forecast would support very strong updrafts. I am planning on taking a close look at the 00Z runs tonight.

 

[nickgrillo]

Unmodified 0Z MHK forecast sounding shows a parcel of 88F/70F which yields CAPE of 4196j/kg. Now... There is only a VERY slight trace of CINH on the MHK sounding on 0Z - with the NAM blowing out some precip around the same timeframe in northeast KS which is also where the best wind fields are. A very moist and extremely unstable boundary layer airmass, combined with strong veering profiles through the layer (which is yielding both strong deeplayer and low-level shear) indicates that atmosphere will support supercells if convection does manage to develop. It's quite possible that convection could take til late evening to develop, but the latest MHK/EMP forecast soundings make me think that it SHOULD develop either way.

Overall, the threat is indeed there - we just need to get rid of the cap...

 

[Michael P. Morris]

Both major models are in good agreement that significant instability will be in place over Kansas and Nebraska, but the forecast hinges on two important but small scale features - a vigorous 700 mb shortwave and the development of a low level jet in the afternoon.

The first feature appears in good concert on the NAM and GFS, but does not appear on the NGM. Both models are in agreement that this will be a rather strong shortwave forcing a strong spike in VV fields north of I-70 in Kansas and SE of Lincoln. The low level jet varies from model to model with the NAM being the most agressive at 35 kt from SSW. This is likely attributed to the NAM's agressive movement of the low pressure center past I35 in the mid afternoon, as well as deepening of its attendant baroclinic zone via warm advection at 850 mb

Furthermore, the returns in the 6 hr precip ending 12Z Sunday intrigue me, as there may very well be an east-west oriented outflow boundary (roughly parallel to I70) setting up across the region for storms to interact with.

In all, I would probably target Topeka based on tonight's run, as I would want to remain close to the hypothetical outflow boundary.

 

[nickgrillo]

Taking a look at BIE forecast soundings... It now doesn't surprise me too much to see the SPC highlight further N/NE too. Overall, wind profiles look pretty good across eastern NE - with BIE showing strong directional shear through the column. From strong (>70kts) westerlies at 250mb to strong backing southeasterlies at the surface layer. Combined with strong to extreme instability and ample deep moisture, I'd say it's plenty sufficiant enough to get severe thunderstorms. Favorable deep-layer and low-level shear will support supercell structures when storms first form... Then, I'd expect for storms to line up - producing a hefty squall line as it moves into IA perhaps...

Instability is pretty strong through the rest of the overnight hours... With NAM soundings bringing in >2500j/kg of CAPEs and >300 SRH across IA into the early morning hours. It's gonna be a very interesting situation to see evolve, nonetheless...

GEMPAK-generated graphic: http://midwestchase.com/09_18_05-1.jpg

 

[Mikey Gribble]

It does look like we will see some decent storms up in eastern Nebraska where the cap is weaker and a short wave should trigger convection, but this is away from the better instability and helicity. The only hope I see for Kansas right now is if convection on the nose of the LLJ can lay down an OFB that doesn't get mixed out during the day. At least we got something to look at.

 

[Scott Olson]

Im at least releatively interested in Eastern South Dakota tommorow. Favoring the GFS/UKMET timing as NAM seems a little to fast against induced SE flow. Biggest problem is not a whole lot of moisture being avaliable but at least moderate instability is indicated by both the NAM and GFS (3000 j/kg I-29 NAM at 21z which would be more around 0z using GFS/UK solution. 1000-1500 J/KG per GFS. Strong Divergence is located in SE SD by the GFS combined with good upward vertical motion and a slowly moving front should help to intiate convection most likely near or just west of the James River valley. NAM forecast for 21z yields 50-60kts of Vertical shear for this region which is more than enough for supercells. Looks like Watertown or just south-west of might be a good target for starters near the warm front.

 

[nickgrillo]

A synoptic cold front will be stretched from Wisconsin into the Rockies... With a SFC trough extending from central NE into TX. Very rich boundary layer moisture and widespread insolation will aid to the development of extreme instability across the warm sector in eastern KS and northward into NE/IA providing the chance for rapidly developing convection by late-afternoon. Bowing structures / HP supercells should be the main threat in eastern NE/KS with the threat shifting into a potentially long line of severe thunderstorms as it rapidly pushes into IA/IL/WI/MN overnight.

Latest BIE forecast soundings at 0z MON shows a parcel of 76F/72F which is yielding a whopping 5558j/kg of CAPE. Speed shear is abundant through the 250mb to 500mb layer - with distinct veering throughout... Which yields deep-layer / vertical shear favorable for supercells. I expect the genesis region to be on the IA/MO border by sunset... As the storms transition into a line of storms - likely producing significant hail and wind gusts (>65KNTS) as they rapidly push eastward through the night/morning hours.

I'd have to think SPC would go with a 25% hail/wind potential for SWODY1

 

[Dustin Wilcox]

While I hesitate to get too excited due to this years dissapointments in the E Neb area; I like what I see from the SPC, mdt risk area with hatched hail area and 5% tornado right in my backyard. Currently I see moisture as a possible limiting factor currently 53 degree dewpoint but models hint at a slight increase in low level moisture tommorow and SPC also mentions quick return of moisture in the morning hours. With storms now fireing in E kansas any boundaries left in the SE NEB region might be the area to watch.

 

[nickgrillo]

Originally posted by Dustin Wilcox
While I hesitate to get too excited due to this years dissapointments in the E Neb area; I like what I see from the SPC, mdt risk area with hatched hail area and 5% tornado right in my backyard. Currently I see moisture as a possible limiting factor currently 53 degree dewpoint but models hint at a slight increase in low level moisture tommorow and SPC also mentions quick return of moisture in the morning hours. With storms now fireing in E kansas any boundaries left in the SE NEB region might be the area to watch.

There shouldn't be any problem at all with moisture... With the strong boundary layer moisture (65-75 Tds) across OK/southern KS ( http://www.spc.noaa.gov/exper/mesoanalysis/s2/ttd.gif ) easily advecting northward by tomorrow afternoon into NE/KS/IA given the strong southeasterly flow. NAM/GFS both prog SFC Td's in the 65-75F range across eastern NE into MO...

It's also really good to see the convection in eastern/northeast KS ongoing, as this could lay down some nice OFBs for tomorrow (which would enhance any tornado potential). Not at all surprised to see the MDT risk...

 

[Ryan Pfannkuch]

Hmm, I for one am a little perplexed as to why the MDT risk was dropped on this one, especially so early in the day. Anybody else?

Although the 12z RUC/NAM data shows fairlly low confidence in significant convective initiation before 0z, there seems to be a good chance of considerable strong to SVR storm development over eastern NE/western IA between 0-3z with CAPE values actually increasing into southern IA AFTER 10pm as southerly low-level flow increases into this area. With 0-6km shear values up into the 40-50kt range I would expect at least a decent number of severe hail/wind events over eastern NE/western IA after dark. The obvious question of the day is: Will any supercellular development take place in NE before dark, and the 18z OMA sounding may help determine that likelihood.

 

[afischer]

Kind of a complex setup over NE today IMO... i'll give my 2 cents. I think the 06Z MDT was more betting on significant surface-based development occurring (generally) in the eastern half of NE, then clustering or growing into a severe MCS. The prospects for this evolution seem low based on the RAOBs, latest obs/satellite, and the RUC guidance which looks pretty reasonable. Stratus holding down surface heating... weak convergence along an inverted trof in central NE... a s/w trough "passing by" to the south... it doesn't look real great for significant severe before dark. That's definitely not to say there won't be severe as Ryan discussed... I agree the best bet is farther east/southeast in the form of elevated hailers when lift increases across the 850mb front after dark, east of the 700mb cap. What form this elevated convection will take, and whether it is significant enough to justify a MDT... is a tough call. FWIW... SPC did retain a high-end SLGT risk.

 

[nickgrillo]

Current analysis shows a frontal boundary from sw NE into the western Great Lakes, with a SFC trough extending from central NE into central KS. Strongly backed SFC flow ahead of the trough in the warm sector + plenty of moisture advection from the south (leading to strong SFC-based instability of 2000-3500j/kg already across the region) with SFC Td's 65-72F already in eastern KS. Strong deeplayer shear already exists (40-55kts) which will likely support supercells... Now, the tornado threat is dependant on how much low-level shear we will get (and mostly measly 100-200M2/S2 SRH). Pervasive insolation across the warm sector will lead to cap erosion - with convective temperature likely being reached across the area around 0z.

My thinking is still isolated supercells initiating across eastern NE/KS propogating east-southeastward into a relatively strong squall line afterdark, and continuing through the early morning hours.

 

[afischer]

First convective attempts taking place now over Blaine co. NE... oddly enough, to the cool (westward) side of the inverted trof. I hope if these drift east and mature, someone is out there to see em!

Edit- sorry mods, probably time for a NOW thread