On May 14, 2017, SPC had a Marginal Risk for Severe Weather out for a region from far SE CO/SW KS south through much of the TX Panhandle. With a broad ridge centered over the central plains and deep trough over the west coast, southwest flow dominated over the Rocky Mountain region. A shortwave embedded in the flow was spreading upper-level cloud cover across the region. At the surface, a stalled northwest to southeast oriented boundary had draped itself across southern Colorado into the Texas Panhandle. This blog post focuses on the threat in SE Colorado, OK Panhandle, SW Kansas.
By early afternoon, dew points were only in the mid 40s to low 50s and EB shear was only around 30 knots per SPC mesoanalysis. However, intense daytime heating had risen surface temperatures to the mid-80s, creating steep low-level lapse rates, and inverted-V profiles with high based instability. Mid-level lapse rates were also impressive given cool temperatures aloft. Given the thermodynamic environment, gusty winds and large hail were possible with and storm that managed to develop.
Morning GOES-16 imagery was useful in identifying the area where convective initiation would be most likely later in the day, well before the development of a single cumulus cloud. The Split-Window Difference (10.33 – 12.3) showed a narrow band of relatively high positive values, indicating locally elevated low-level moisture content (Fig 1a). The Split Window Difference is useful for identifying variations in low-level moisture The 10.33 um channel is the cleanest IR window band, so is seeing closest to the surface. The 12.3 um band is known as a dirty window IR band because it is slightly sensitive to moisture. Given the sensitivity, the 12.3 um channel will sense higher in the atmosphere and appear slightly cooler than the 10.33 um channel. Differencing the two channels reveals spatial variations in low-level moisture since the greatest concentration of atmospheric moisture is located in the lower part of the atmosphere. Locally greater difference values relative to the surrounding area reveals locally high concentrations of low-level moisture and moisture convergence.
Similarly, the low-level water vapor channel (7.34 um) showed relatively cool values along the same area, again indicating high low-level moisture content relative to the rest of the region (Fig 1b). Given the dry atmosphere, we can be confident that this channel was sensing moisture low in the atmosphere.
The narrow region of enhanced low-level moisture as detected by GOES-16 points to likely low-level convergence, and therefore a region of possible future storm development. Surface METAR observations showed convergence in the region, but the GOES-16 imagery provides better precision to the location of greatest moisture pooling/convergence.
Figure 1: 14 May 2017 GOES-16 a) split window difference (upper left), b) 7.34 um low-level water vapor channel (upper right), c) 0.64 um visible channel (lower left), d) 6.19 um upper-level water vapor channel (lower right).
As upper level clouds (indicators of large scale lift) moved over the region, cumulus clouds developed within this strip of elevated low-level moisture and convergence in the early afternoon. Storm initiation took place around 2100 UTC in the OK Panhandle. This storm would move northeast into Kansas, split, and produce severe hail. The evolution of the storm and it’s split is depicted in GOES-16 visible imagery in figure 2.
– Bill Line, NWS
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