7/17/10 FCST: ND, SD, MN, IA, NE

[Jeff Duda]

Just noticed the severe potential for Saturday. 40 - 60 kts of flow at 500 mb from the WNW across the region by 00Z. 25 - 40 kts WSW at 700 mb. Upwards of 30 - 35 kt SSWly LLJ over the Missouri River into C MN. Sfc winds seem to be generally S or SSW at around 10 kts, so there will be some good shear (40 - 60 kts both deep layer and effective and > 200 0-3km SRH and > 150 0-1 km SRH, especially in MN). 700 mb temps of course high since it is July, but the 14C contour stays west of the IA-MN border through 00Z and even 12C doesn't appear to cover too much of the area, so the strong cap will stay further to the west. Looks like there will be a warm front extending across C/N MN with the cold front back in the eastern Dakotas swinging southeast throughout the day. This seems to have caught SPC's eye with a 30% hatched region for day 3 and the mention of supercells and tornadoes before the traditional upscale morph to a big MCS.

Could be another big chase day in the northern plains/upper Midwest.

 

[Rob H]

Hopefully the decent cap will keep things together and more discrete than they were on 7/14. I've had horrible luck when forecasting in my own state, so take my target area with a grain of salt. I'm liking the Willmar area for initiation around 3pm-4pm. There's a ton of lift, hodos are decent, the cap seems to be eroding, and plenty of CAPE and shear. There doesn't look to be any precip earlier in the day so it should be a straightforward warm front play waiting for some UA disturbance/surface low to kick things off.

edit: The SPC just went 45% hatched/Moderate risk.

 

[Skip Talbot]

The cap looks really, really bad on this one. It actually starts to fill back in before 0z. The NAM is plotting some narrow corridors with CINH erosion between 21z and 0z but I'm not sure that window will be open long enough to get a good chase out of this one. After 0z its game over as the cap fills back in with a vengeance. The best shot at tornadoes is probably on the MN warm front, but you're getting into the lakes and trees up there. I'd probably target up north and get there early, as I'd expect stuff to transition into a linear/elevated mode by evening. Storms further southwest into SD (if there are any) are probably going to be struggling against warmer midlevels, a stouter cap, and veered surface winds. However, my pessimism stems from the fact that I was just in that same area two days ago and am not going back so soon.

 

[jordan.hamilton]

Was just looking over the 18z NAM run and the 00z hodographs in central Minnesota look really nice if they verify. The corridor for supercellular activity seems to be really narrow, as mentioned by others. 0-1km EHI looks very nice as well. I may have to make an unplanned trip to MN this weekend.

 

[Rob H]

I was under the impression that with NW flow setups, you usually have some nice CAA at the mid levels to assist in breaking the cap (even though this will cause subsidence). The narrow band that models are showing precip breaking out in seem to be due mainly to the cap reforming at 00z as Skip mentioned. It really looks to me like the models are forecasting WAA between 700-800mb and a cool pocket forming at the surface over the Twin Cities. Am I analyzing this correctly, and are there some processes at play that explain this? I'm in the dark on this one in regards to the cap's behavior.

My target area is only an hour away from where I live so I'd go out on much worse setups - but I don't have extreme confidence in this setup yet.

 

[Bill Schintler]

Chase Target for Saturday, July 17

Chase target:
Farming, MN (15 miles west of St. Cloud).

Timing and storm mode:
Storms should develop by 5 PM CDT, with storm motion towards the east at 20 mph. All modes of severe weather seem probable; including supercells with very large hail, and a tornado or two. During the evening and overnight hours, storms will transition into bowing segments with severe wind the primary hazard.

Discussion:
Another in a series of shortwaves embedded within a strong belt of westerlies will amplify over NRN MN on Saturday. At the SFC, a low pressure and an attendant WF will lift NE during the day. Southerly flow will strengthen ahead of the system, with dewpoints increasing into the lower 70s by Saturday afternoon. Strong instability will develop along the eastern fringe of steep mid-level lapse rates in excess of 8C/km. Elevated storms should develop to the north and east of the WF by mid afternoon. Severe hail will be the primary threat with these storms.

Later, surface-based convection should initiate along the WF, just E of the SFC low, along the ERN extent of an advancing CI shield. This is within the favored RRQ of a 90kt H25 jet streak. Capping will persist S and W along the advancing CF due to strong mid-level WAA, limiting SRLY extent of convective development until the evening hours. H7 temperatures will increase to nearly 15C by late afternoon in SWRN MN and points to the south. Convection will eventually fire S along the CF in SWRN MN and then NWRN IA after 03Z as the CF pushes eastward, with a linear mode favored.

SFC-3km SRH will increase to AOA 300m2/s2 after 00Z as an increasingly veered LLJ strengthens to 40kts over backing LLVL flow along and N of the WF. Additionally, deep layer shear will increase between 00Z and 06Z as the compact H5 shortwave overspreads the region. LCLs will lower as the evening progresses, enhancing tornado probabilities. The SPC SREF ensemble indicates a significant tornado index of 3 to 4, suggesting large and violent tornadoes in storms that remain discrete. Storms will then evolve into one or more severe MCS(s), which should track towards the SE as the LLJ strengthens overnight. Storm mode will transition into quasi-linear bow structures with widespread severe wind the primary threat.

Bill Schintler
KD0DJG
www.twistertoursusa.com
9:06 PM CDT, 07/16/10

 

[Jeff Duda]

I have continued to watch the models since the other day. Both the NAM and GFS seem to be singing the same song they've been singing for the past few days. The NAM seems to bring the better flow in later in the period (i.e., after 00Z rather than at 00Z), and also has shown much increased CIN from previous runs even though 700 mb temps don't seem to have changed much from previous runs. The GFS seems much friendlier with the setup and is now firing up precip by 00Z whereas it was dry through then in previous runs. The 00Z GFS seems to suggest the Twin Cities are going to get QPF bombed late Saturday night.

The God model (NCEP 4km WRF) has minimal warm front activity and has early initiation in SE ND by about noon local time with the cold front lighting up in SD around 21 - 22Z.

I think with strong enough forcing by the cold front, it will be pretty likely to see daytime initiation along the front. Both models show decent shear later during the night, so if these storms persist (they will probably not remain surface based long after dark) they will probably rotate. I'm not sure if a SWODY2 MOD 45% hatched is warranted, but surely there will be some severe storms tomorrow in the area.

 

[Justin Turcotte]

Hopefully the SPC didn't jinx my initial target of Breckenridge, MN by placing it in the epicenter of the forecast 10% tornado probability. I've been looking at this set-up for the past two days as I lucked out pulling the straw for the ND/SD/MN DoT forecast grids. The Parallel GFS and GEM suggest an H7 shortwave in western ND this morning will flatten the capping ridge this afternoon in the target area. Most of the 00z 7/17 guidance suggests the triple point will be near the area with just minor placement differences. Forecast CAPE and shear values are fairly disconcerting in the sense the potential exists for a significant tornado during discrete mode of the event. I suspect the discrete mode window will close early in west central MN before upscale growth into a powerful MCS which may sweep across a wide swath of MN. Embedded supercells within the line could still produce an isolated tornado further east. Pwat values of 4-5cm and as much as 7cm in convectively contaminated model soundings suggest initial supercells may take on HP characteristics. Primary uncertainty is whether elevated convection attempts to become surface based or fresh surface based towers develop... or both. In any event, I expect the the ideal chase window to close early and if I head out should be back home in time to take a nap before the graveyard.

 

[Rob H]

Most models are suggesting the start of this event will be in a 50 mile radius around Fairmount, ND between 20z and 21z so I have a fair amount of confidence in what the SPC is suggesting for a risk area. The RUC has CAPE at 6000+ in the target area by 1pm, so there could be some elevated hailers to the south. My plan right now is to get to Alexandria, MN by 19z and wait for the 18z sounding and other data, and hang out close to the warm front while moving towards the surface low. Good luck to everyone else going out!

 

[Aaron Kennedy]

Key will be getting to the convection before it turns into a convective mess. Not sure how long cells will stay discrete. My current target is Christine, ND just after the lunch hour. Initiation looks like it could be pretty early.

 

[Jeff Duda]

Wow, the 12Z HRRR (still waiting on the 13Z since it is the one that has data assimilation around this time of day) shows at least 5 ongoing supercells at 00Z tonight from along the I-29 corridor in E ND between Grand Forks and Fargo through C MN to WC WI just east of the Twin Cities. Overall, the 12Z HRRR shows a rather dangerous scenario possibly unfolding across E ND through C/N MN and W WI later today.

The 13Z HRRR looks less threatening, only having 3 obvious supecells in MN/ND at 23Z.

 

[Aaron Kennedy]

Doubt HRRR is capturing the outflow boundary draped near FAR... will be interesting to see how this impacts the day.

 

[Jeff Duda]

15Z HRRR indicates a long-lived supercell in progress from about 20Z near Montevideo, MN through 00Z just southwest of Mankato...discrete the whole time.

 

[Chad Cowan]

Just glanced over the models to see what I'll be missing today in MN... and to my surprise there appears to be a severe weather threat here in eastern KS later this afternoon, contrary to what the SPC is thinking. In fact, they have all but the fringes of KS in the 'No T-storm' category. I'm sure most of the forecasting attention is focused further north but I think that the setup in northeast KS is at least worthy of a SEE TEXT.

The thermodynamic environment is obviously in place (5000 j/kg of uncapped SBCAPE as of noon), and the wind fields are marginally conducive to storm organization, with 10kts out of the south at the surface and 25kts out of the northwest at 500mb. Seems like a fairly good shot at a big southward diving hailer/ windbag if the OFB in southeast NE and northeast KS initiates, which the 12z RUC and numerous HRRR runs say it will.

 

[George Grant]

Just glanced over the models to see what I'll be missing today in MN... and to my surprise there appears to be a severe weather threat here in eastern KS later this afternoon, contrary to what the SPC is thinking. In fact, they have all but the fringes of KS in the 'No T-storm' category. I'm sure most of the forecasting attention is focused further north but I think that the setup in northeast KS is at least worthy of a SEE TEXT.

I was thinking the same thing sitting here trying to fix two computers at a customer's shop. H5 winds from the nw are trying to hit 35kts in se NE and ne KS, while H25 wnw winds try to top 50 kts at 22Z. Unfortunately, LL winds are weak at maybe 10kts from the south. 700mb temps stay manageable until 23Z. Dewpoints are shooting for upper 70Fs! This is helping CAPE reach 6000j/kg by late afternoon with little or no cap left. Might see some good winds and maybe some hail before these southbound storms move south away from better mid-level lapse rates to the north and west.