The Storm Prediction Center included parts of southern Colorado in a Moderate Risk (four on a scale of five) for Severe Weather in the 1300 UTC Day 1 Convective Outlook on 26 May 2019. This was the first instance since 18 May 2010 that a portion of the Pueblo, CO CWA was included in a 1300 UTC or later SPC Day 1 Moderate Risk for Severe. All severe risks were possible, including significant tornadoes, significant hail, and severe wind gusts. Given the risk, 1-min imagery from GOES-16 was available over the region.

Analysis of GOES-17 water vapor imagery from the 26th revealed an impressive shortwave trough accelerating across the Baja California Peninsula and Gulf of California during the early morning hours, and lift across New Mexico and into west Texas during the afternoon into the evening while developing a negative tilt (Fig 1). A jet streak is analyzed rounding the southeast portion of the shortwave into E Co/W KS by the end of the loop. Convective initiation in an associated region of large scale ascent ahead of the shortwave and under the increasing mid-level flow is apparent during the afternoon near the end of the animation across the high plains. Meanwhile, a broad upper low shifts south across central California.

RAP analysis confirms the above mentioned features with the yellow contour representing 50+ knot 500 mb wind, and white contours represent relatively high values of 500 mb positive vorticity (Fig 2).

Morning Day Cloud Phase Distinction RGB imagery from GOES-16 implied abundant low-level moisture per widespread low stratus clouds (cyan), confirmed by dew points in the mid 50s across most of the eastern Colorado plains (Fig 3). By early afternoon under strong heating, much of the low stratus had eroded across the southeast plains, and mid-level clouds (green-yellow) were spreading across the area in response to the approaching shortwave. The Denver Cyclone mesoscale feature is diagnosed in the low stratus over northeast Colorado. Snow cover (green) is still readily apparent over the Colorado Rockies, a testament to the impressive snow totals seen this season.

Convection developed quickly during the early afternoon over southeast Colorado under strong large scale forcing and weak capping. One of the initial storms produced hail as large as 2.5″ in diameter in Springfield, CO (Baca County). A three-body scatter spike was detected in radar imagery, indicating large hail potential (Fig 4). Baca County in southeast Colorado is in an area of relatively poor radar coverage, with lowest radar tilts shooting 10,000 ft AGL. While this level upward generally provides enough information to make an informed (severe thunderstorm) warning decision, additional info such as that from satellite and lightning is useful in confirming warning decisions, particularly in marginal cases.

One-minute VIS and IR GOES-16 imagery in this case showed an above anvil cirrus plume (AACP) emanating from a persistent overshooting top (OT) early on in the life of the storm (Fig 5-8), confirming severe potential. The storm weakened quickly after 1920 UTC as evidenced by cloud top warming and a sudden loss of the OT and AACP in satellite imagery.

Meanwhile, a new storm developed to the northwest in Bent County, CO. At 2033 UTC, radar imagery showed a tightening velocity couplet and potential trend toward tornado development. Unfortunately after this time, the KPUX radar went down unexpectedly. Radar imagery capturing the storm during the 30 minutes leading up to the outage at 2033 UTC is shown in Fig 9.

With this new storm quickly developing a mature updraft and having its anvil sheared to the northeast, the updraft was exposed to the view of the satellite (from the southeast). As has been observed with a few other cases, the 500 m, 1-min visible imagery captured rotation within this exposed updraft, confirming a mesocyclone/supercell thunderstorm (Fig 10).

After a few minutes of no new radar data (2040 UTC), the warning forecaster decided to issue a tornado warning given: 1) the environment supported tornado development, 2) the last radar scan showed meager rotation just starting to develop, 3) a consistent rotating updraft apparent in visible satellite imagery, 4) cloud tops remained cold in IR satellite imagery, and 5) the storm had quickly developed an AACP apparent in VIS and IR satellite imagery. One-minute VIS and IR satellite imagery during the 30-min period leading up the warning decision at 2040 UTC are shown in Figs 10-11.

KPUX radar would return at 2045 UTC, and the storm went on to produce a confirmed tornado at 2054 UTC and again at 2210 UTC, in addition to numerous instances of large hail. A 2-hr long animation (2010 – 2210 UTC) of the storm is shown in Figs 12-14. The rotating updraft, OT, and AACP are all readily apparent throughout the life of the storm.

Bill Line, NWS