06/27/05 FCST: Central/No. Plains

[Mike Peregrine]

Originally it sounded as though largely unidirectional winds would favor a large MCS or squall line this afternoon, but latest forecast runs seem to indicate the possibility of veering winds that are supportive of supercells at least during initial convection invof frontal boundary/triple point that will be located over central Nebraska.

Present surface maps show a frontal boundary draped through central South Dakota and extreme northwestern Nebraska. Radar and water vapor imagery shows clusters and linear storms currently initiating and moving across NW Iowa and along area of convergence in southeast Nebraska. A shortwave impulse ejected from an area of low pressure in western Nebraska later this afternoon through unstable air (MLCAPE>3000) should provide focus for lift across eastern and central Nebraska into western Iowa and promote rapid thunderstorm development in an area of 70 degree dewpoints and lifted indices that should be in the -10 to -12 range.

0z NAM shows >150 m^2/s^ 0-1km SRH across northwest Iowa by 0z, 4.7 0-1km EHI, BL-6km shear generally around 30-35kt, LCL ht of 1800, and LFC >2000. Apart from the high LCL and LFC, other fcst parameters indicate a supportive environment for at least a few tornadoes after initiation, provided storms can initially remain somewhat discreet.

After looking at all this, I'm currently in agreement with SPC in favoring extreme east-central Nebraska into west-central/northwest Iowa as possible target this afternoon, but will need to keep an eye on further RUC runs and soundings during the morning to see if there is better evidence of sfc-based convection, as the situation currently seems to favor elevation to me with those higher LCL and LFCs.

 

[Glen Romine]

RUC guidance is struggling with the morning sounding input that showed marked drying above the boundary layer, where last night's LLJ brought in considerably drier air as well as much steeper elevated lapse rates from off the Mexican Plateau region. This dry air appears over mixed to the surface in the RUC output leading to very low dewpoints across large portions of the central plains - probably too large of an area. As such, instability estimates are probably best derived from other sources today. Forecast surface wind profiles look problematic for tornado potential, particularly below 700 mb, with an 850 mb deformation axis setting up near FSD, and weak surface flow. Morning analysis does show a weak surface feature near Hastings, and an upper level impulse evident in water vapor imagery, so the optimistic low-level flow offered by the NAM model may be reasonable for a possible adequate environment near the Mo River valley. Considerable haze today will make chasing 'visually' a bit of a challenge, as will expected mode of HP storms. But, if the LLJ can back a bit more than expected, a tornado or two certainly looks possible with current available guidance.

Glen

 

[Mike Peregrine]

MD 1501 has been issued for eastern Nebraska and western Iowa -

http://www.spc.noaa.gov/products/md/md1501.html

Scattered elevated convection continues across the Omaha area, which is expected to become rooted in the boundary layer as greater instability develops. Surface winds are southerly at 15 kt ahead of the current convergence, so though high dewpoints and instability may allow for some organized supercells to develop over the next few hours, due to weak shear it doesn't look like there will be much of a tornadic threat at this time. Look for a SVR watch sometime in the next couple of hours. I'll probably just stay home and watch on this one ... waiting for something better later in the week.

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EDIT - WW536 issued at 2:00 p.m. http://www.spc.noaa.gov/products/watch/ww0536.html