06/17/07 FCST: ND/SD/MN/NE

[Justin Turcotte]

Forecast models are in quite the agreement three days out. Convection is likely to be ongoing in ND/MN early in the period as lead shortwave feasts on the LLJ. A few severe hail reports are possible with the AM storms. Hopefully this wave doesn't mess up the low-level flow too much for the afternoon/evening set-up when robust west to southwest mid/upper flow regime will be in place. Low pressure forecast to be centered in eastern ND with warm front possibly enhanced by outflow flanking east through the Red River Valley into MN. Secondary low possible in the NE panhandle or extreme southwest SD and trough between the two. Instability shouldn't be an issue as dews easily reach the upper 60's and low 70's, perhaps locally higher as soil moisture is rediculously high in portions of the region. Both NAM and GFS have no trouble spewing out LI values exceeding -8. Cap isn't terribly strong but perhaps just strong enough to hold back convection late enough to realize the 3000-4000 J/kg MLCAPE advertised in some forecast soundings during the late afternoon. Triple point in eastern ND could offer an earlier show if there is any degree of clearing in that particular area otherwise some supercells are possible along the trough in ND/SD/NE. I'm thinking there is a fair tornado threat even if the environment is only remotely close to the 0-3 km 6+ EHI spewed out by Thursday's 12z NAM and will likely chase if there are no significant changes. I suspect an SPC upgrade to moderate is in the cards when this goes to swody2. Tornado Father's Day gift would be nice.

 

[Bill Schintler]

Chase target for Sunday, June 17

Chase target:
Bowdle, SD (50 miles W of Aberdeen).

Timing:
Storm initiation at 6 PM CDT. Supercells will be likely early in storm evolution, with a gradual transition to a large complex that will travel slowly to the east and southeast. For a good overnight lightning show, an overnight stay in Aberdeen is suggested.

Synopsis:
SFC analysis indicated low pressure centered in CNTRL WY with a WF extending EWRD into ERN WY and along the SD/NEB border. Upstairs, a 100kt H3 streak was rounding base of broad trough over ID while WV loop indicated associated weak ULVL energy spreading over WY and MT, and several areas of convection have developed in response to this forcing. Earlier storms over WRN SD dissipated before noon and appear to have left behind an EWRD moving MCV with attendant pocket of -10C H5 temperatures which appears to be aiding ongoing convection over SRN MN. Further E in the Upper-Midwest, storms have fired in the weakly capped environment and aforementioned MCV over SRN MN while isolated convection also broke out along an OFB along I-80 in IA earlier today.

Discussion – Sunday:
A significant severe event appears likely from late afternoon through midnight. A secondary threat should be heavy rainfall with training convection with forecasted precipitable waters in excess of 1.5 inches and also a 50kt LLJ transporting 17C H85 dewpoints into convective complexes. Most of SD and NEB will remain strongly capped throughout the afternoon as H7 temperatures increase to from 12C to 16C while allowing MLCAPEs increase to 4000J/Kg along and immediately ahead of the front by late afternoon. Further S over NEB, a broken CI shield should overspread the area through the peak heating hours while serving to limit the degree of attainable insolation there. The most likely scenario is that convection will hold off until the CF pushes into CNTRL SD. Convection should then initiate W of Aberdeen in response to SFC forcing along with slight cooling of the mid-levels with the approach of the second in a series of waves, which should be sufficient to erode the cap. Winds may locally back in this area just E of a developing SFC low, while strong 60kt H5 SWRLY flow overspreads the area. Following initiation and into the overnight, strong to severe storms will continue along and behind the front as a 50kt LLJ noses into the area.

- bill

 

[Matthew K. Hartman]

Interesting as well that the 00z NAM this evening (consistent with the 18z run) is convecting later (towards 00z Monday) over southeastern ND and through Central SD, while the older 18z GFS and 12z GEM are both slower and convects things west of the Red River Valley (central ND through Wrn ND).

Much like my co-worker says above, I'm concerned with the initial wave. It appears that it keeps winds more southerly inthe warm sector and limits the backing of the surface winds east and southeast of the track of the sfc low. This is obviously resulting in less impressive bulk and storm-relative helicities on the fcst point soundings.

Still, it looks like SVR will be likely, but am afraid it may be a slightly later event than what we expect. H70 temps are well above 12'C through the afternoon and don't really fall off until the 00 to 03z period, at least over eastern North Dakota. Further south, the cap holds strong, with no mid/upper level cooling occuring until/towards late-evening.

My personal chase status is on stand-by, but will be monitoring things once again in the morning for any changes. If I were heading out, I'd be targeting the Carrington, ND area, where I would be closer to the triple point. And where likely there would be a slightly higher tornadic threat.

 

[Aaron Kennedy]

Today looks real interesting for the Red River Valley. Super-backed winds, high dewpoints (sitting at 68 in GFK right now) and plenty of wind shear. Biggest question is where will initiation occur (assuming it does). Haven't quite nailed down a target yet (maybe it is here?), but I expect something to fire within 2 hours driving distance of GFK if not much shorter.