Severe storms developed across the central high plains during the period of 9-11 May 2023 in association with an upper trough digging across the southwest US. Per NWS BOU on May 9: “RAP Mesoanalysis and water vapor satellite show the center of this upper trough just off central California coast this afternoon. Broad southwesterly flow aloft is present ahead of this trough axis, and the flow should strengthen and slowly back to the south-southwest through tomorrow afternoon. This trough should continue to progress eastward, eventually transitioning to a more negatively tilted trough axis by Wednesday evening.” Associated satellite iamgery with RAP 500 mb height is shown in Fig 1, during the period of early May 9 to mid May 10.

Two rounds of severe thunderstorms developed across the northern CO I-25 corridor on the 9th, resulting in large/damaging hail: the first during the late afternoon, and the second overnight. A single procedure that captures these storms in detail is a VIS/IR sandwich that transitions to IR only after sunset (Fig 2). During the day, the sandwich combo clearly reveals rapid storm strengthening per cooling in the IR and increase in texture in the VIS, including the development of OTs and AACPs. Overnight, the loss of texture with the loss of VIS makes storm top analysis slightly more difficult, but OTs and probable AACPs could still be diagnosed from the IR-Window imagery in this case.

VIIRS DNB/NCC Imagery can be leveraged alongside the GOES IR imagery for more detailed cloud analysis at night since it provides visible-like imagery at night. Four VIIRS scans (from NOAA-20, NOAA-21, and S-NPPx2) capture the evolution of thunderstorms and low clouds between 0800 UTC and 0941 UTC (Fig 3). The imagery shows a very active storm top with OTs crossing from Colorado into western Nebraska, with persistent lightning flashes. In the wake of the storm, low clouds are observed expanding across the urban I-25 corridor, which is denoted by the bright city lights.

The low clouds were tracked in space and time using GOES RGB imagery, including Nighttime Microphysics RGB at night and Day Cloud Phase Distinction RGB during the day (Fig 4). The low clouds quickly filled in across the region in the wake of the large thunderstorm complex amidst moist/upslope easterly flow. Low clouds persisted through the morning the following day.

From NWS Cheyenne prior to sunrise on 10 May: “Once these early morning storms have run their course, expect low stratus and perhaps even some patchy fog to develop along the Interstate-80 corridor from Cheyenne to Sidney, and as far north as Scottsbluff and Alliance. The HREF shows moderate to high probabilities of this occurring and model soundings do show a saturated profile just above the surface at Cheyenne and Sidney. Surface and satellite observations back this up with both showing an impressive low stratus deck over most of eastern Colorado. Low stratus looks to break up by late morning/early afternoon, which could have implications on convection in regards to when initiation begins, or (less likely) if it is limited by CIN and a capping inversion.”

After sunrise during the mid-morning hours, NWS BOU analysis included: “Quick update to the grids to account for the incredibly persistent low stratus deck this morning. Day Cloud Phase satellite imagery shows a blanket of low stratus across the plains and I-25 corridor. There are signs of this eroding up in Logan County, but expect this to stay socked into the urban corridor for at least another hour or two before it gradually scatters.”

The upper trough continued east into the Four-Corners area on the 10th, and forced another round of severe thunderstorms across the high plains. Another Sandwich product, this time using Geocolor instead of VIS, depicts the evolution of severe thunderstorms across the central high plains on the 10th (Fig 5).

Further south, thunderstorms developed along a dryline during the afternoon and into he evening. The GOES-East Split Window Difference with a simple grayscale colormap (and IR-Window overlay for cold BTs) receptively highlights the location of the dryline throughout the period, with brighter grays representing greater moisture, primarily in the low-levels (Fig 6). During the afternoon, clouds and deep convection developed along dryline bulges in southwest Texas and in Mexico to the south. Additional storms develop further north along and ahead of the dryline near the NM/TX border. After dark, interpretation of the SWD changes, but a signature of the dryline is observed accelerating east, and additional convection develops along it in southwest Texas near Midland. See this graphic for annotation of the scene.

The upper low continued to drift northeast across eastern Colorado on the 11th into the 12th. Abundant moisture pumping into the region along with forcing associated with the low resulted in a prolonged period of rainfall across the Colorado front range, resulting in 1-day precip totals of 1-4 inches+ from Pueblo to Fort Collins. From NWS PUB on the 11th: “Current water vapor imagery has upper low spinning across eastern Crowley and western Kiowa Counties, with deformation band showers
and storms moving south and west across the southeast mtns and portions of the southern I-25 Corridor with the precipitation associated with mid level trowel and wraparound precipitation across northeastern Colorado into the Front Range at this time.”

And from NWS BOU early on the 12th: “GOES-18 satellite imagery displays an upper level low feeding additional moisture into our region. The center of this low sits along the Colorado/Kansas border and is expected to shift northeast gradually this morning. With the persistence of this system bringing ample amounts of moisture, a Flood Watch remains until 12 PM MDT this afternoon.

The evolution of the low from the 10th through the 12th, including multiple rounds of strong thunderstorms and the persistent feed of deep moisture into the front range, all discussed above, is shown in GOES-East Water Vapor Imagery and GLM FED overlay (Fig 7).

Bill Line, NESDIS/STAR