Severe thunderstorms are expected across parts of northeastern Colorado, central/southwestern Nebraska, and northwestern Kansas. The main threats include large to very large hail, severe gusts (some 75 mph or greater), and a few tornadoes.

Locations:

  • Enhanced Risk Area: Northeastern Colorado, central/southwestern Nebraska, northwestern Kansas

Threats:

  • Large to Very Large Hail: Especially with sustained supercells.
  • Severe Wind Gusts: Some 75 mph or greater, more common once cells evolve into clusters.
  • Tornadoes: A few possible, particularly with sustained supercells.

Timing:

  • Storm Initiation: Late afternoon
  • Peak Intensity: Late afternoon into the evening

Meteorological Discussion:

Synoptic Setup:

  • A mid/upper-level trough over the western CONUS will shift eastward, with an associated strong shortwave trough moving from the interior Northwest to Wyoming/Utah.
  • A southern-stream trough will move northeastward, reaching the central High Plains by 12Z tomorrow.
  • A broad fetch of diffluent flow aloft will extend over the Great Plains, with westerlies over Oklahoma and southwesterlies from Colorado to the Upper Midwest.

Surface Analysis:

  • The 11Z surface map shows a low in the Siouxland region with a quasistationary front southwestward to another low over southwestern Kansas.
  • Cumulogenesis is expected along the western boundary today, over eastern Colorado, with additional lows forming along a dryline over the northern Texas/western Oklahoma Panhandles.
  • A warm front from the Siouxland low across southeastern Iowa will move northeastward over northern Illinois today.

Storm Development Regions:

  • Thunderstorm development is expected by late afternoon, initially over higher terrain and in post-frontal convergence/upslope-lift zones near the Palmer Divide and/or Cheyenne Ridge.
  • Some of this activity should move east-northeastward over northeastern Colorado and adjoining portions of Nebraska, with both supercells and upscale-aggregating storm clusters expected.
  • Multiple rounds of convection may cross the region in and near the “Enhanced” area through this evening.

Storm Modes and Evolution:

  • Supercells capable of producing large to very large hail and a few tornadoes are possible.
  • Severe gusts may occur from supercell downdrafts and become more common as cells evolve into clusters.
  • Some significant (65+ kt) gusts are possible.
  • Activity should move/grow into a corridor of favorable instability across and south of the front in Nebraska, with more convection developing near the front tonight.
  • Isolated development is also possible near the dryline over eastern Colorado/western Kansas.

Key Factors:

  • Large-Scale Ascent and Low-Level Lift: Both will increase across the outlook area throughout the afternoon and evening.
  • Boundary Layer Destabilization: Preferentially more at higher altitudes diurnally, with increasing CAPE eastward across Colorado to southwestern Nebraska/northwestern Kansas.
  • Moisture and Airmass Recovery: Process will continue for most of the day, peaking later in the day/evening.
  • Hodographs: Long hodographs will support the hail threat with MLCAPE values commonly around 2000 J/kg and 40-50 kt effective-shear magnitudes.
  • Nocturnal Boundary-Layer Stabilization: Delayed airmass recovery makes probabilities more conditional into eastern Nebraska, Iowa, and southern Minnesota.