Severe thunderstorms developed across the southern plains on 4/22/2020 in association with a shortwave trough traveling east across the TX PH, OK, and N TX. GOES-East water vapor imagery shows the evolution of the large scale feature and associated thunderstorm development, with RAP analyses quantifying the wave and it’s influence on the surface pressure and surface/mid-level wind fields (Fig 1). The vorticity max becomes increasingly well-defined as it accelerates east across OK, with associated dry descending air through west Texas and moist ascending air over east TX/OK into the southeast US. Additional imagery from some of the notable thunderstorms will be highlighted in this blog post.

Thunderstorms were ongoing across the northern half of OK and initiating across the southern half of Oklahoma during sunrise on the 22nd. Analyzing GOES-East 1-min visible imagery, shadows associated with the low sun angle provided excellent detail about storm initiation and storm top features such as overshooting tops (OTs), above anvil cirrus plumes (AACPs) and anvil gravity waves (Fig 2).

During the early-mid afternoon hours on the 22nd, additional convection developed across southern Oklahoma near the interface of a dryline/cold front/warm front surface triple point. A GOES-East 1-min VIS-IR sandwich combo allows for a more detailed analysis of the thunderstorms than a either single channel alone (Fig 3). This sandwich combo was created using a VIS linear color scale with a semi-transparent IR overlay, and can be found on the STOR VLAB page. The procedure maintains details from the VIS while also including quantitative information from the IR that can be visualized and sampled. Features such as OTs and AACPs, as well as cooling/warning trends, are easily diagnosed in the imagery. Numerous NWS severe thunderstorm and tornado warnings were issued with these storms, and are shown in the animation. Note, while the base of the storms fall within the polygons, the storm tops are oriented north of the polygons due to parallax. These storms did produce tornados, severe hail, and severe wind gusts.

Further south in east Texas, a long-lived severe thunderstorm produced a track of tornado and severe hail/wind reports. This storm had an impressive appearance in GOES-East imagery, with persistent and large OTs, long-lived AACPs, and very cold tops (<-80C at times). A simple procedure in AWIPS combines the aforementioned VIS-IR sandwich combo with IR imagery to allow for a smooth transition between the two products (make the low end of the VIS transparent). A 2-hour animation of GOES-East 1-min imagery centered over the the storm using this procedure is shown in Figure 4.

A longer, 5-min animation shows the full 10-hr duration of the storm, and instead uses the “Daylight Transition” image combination feature available in AWIPS (Fig 5). Feature-following zoom is also used to provide a storm relative view of convective evolution.

Finally, a similar animation but highlighting GLM Flash Extent Density is available in Figure 6. The storm consistently produces an abundance of total lightning, with periodic lightning jumps/dips throughout the evolution.

Bill Line, NESDIS/CIRA

During the evening of April 21, 2020, several supercells developed in the Texas Panhandle into western Oklahoma producing hail resulting in numerous reports. Not only was the size of hail large, the amount of hail that fell was significant as noted by reports of accumulated hail of several inches deep. As storms snailed away from their deposited hail swaths, GOES-16 was there to view these swaths. This post will showcase how forecasters can view freshly deposited hail swaths using multispectral imagery at night.

Figure 1 depicts the Nighttime Microphysics RGB with the “clean” longwave infrared band (Band 13, 10.3 um) overlaid while making any temperatures warmer than -25 C transparent. This allows for cloud layer and phase information to be provided within the Nighttime Microphysics RGB while also monitoring very cold cloud top temperatures associated with severe storms as well as overshooting tops denoting an updraft penetrating into the tropopause. As the convective clouds and overshooting tops pull away from the border of the Texas Panhandle and western Oklahoma, notice one to three subtle, magenta-colored lines oriented northwest to southeast as skies clear (see annotated images here and here). These lines correlate well with highest reflectivities that had passed over previously in time as well as with storm reports of hail. These lines are a result of altered environmental and ground temperatures (in this case cooling) from the copious amounts of hail left behind their parent storms.

While the default composition ranges of the Nighttime Microphysics RGB within AWIPS-2 allow for just subtle signatures within the imagery, there is room for adjustment to enhance this signal. A good place to start is by looking at a 4 panel display of the RGB (Figure 2) being discussed and each of the its components set to a black to white scale with the RGB’s default ranges.

Sample the signal in question within each of the RGB’s components and adjust the range within the components until there is good contrast between the signal and background. Once the component’s ranges are adjusted to your liking, adjust them within the RGB itself. In this case, there was room for improvement mainly within the red component (the Split Window Difference, 10.3-12.3 um) and the blue component (“Clean” IR, Band 13, 10.3 um). However when adjusting the red component, the RGB itself became too saturated in red coloring. Therefore we can tone down the saturation of red by lowering the red component’s gamma. Figure 3 is what the new composition looks like now (click here for actual recipe values).

Do you think the hail swaths show up better in the adjustments as shown in Figure 4? Making an adjustment on the fly works in this case, but might not for all cases. Thus adjustments to RGBs are advised to only be made on a case by case basis, especially with those having a large dependency on infrared temperatures. It is also worth noting that this approach of using multispectral satellite imagery to observe accumulated hail swaths is dependent on cloud-free skies as well as enough hail to actually make an alteration to the ambient environmental and ground temperatures.

So how does this apply to IDSS? Well besides increasing overall situational awareness, noting hail swaths in near real time lends confidence in derived products like MRMS MESH tracks through observations. This could also allow forecasters to help core partners like DOT maintenance crews target areas for hail removal should they request it. Additionally, this could also provide an opportunity for target LSR intel gathering, or even safety messaging as accumulated hail can produce hazardous travel conditions through slick roadways and reduced visibility should hail fog form (which by the way is also detectable in this approach through the use of the 3.9-10.3 um “Fog/Low Stratus” Brightness Temperature Difference as the Nighttime Microphysics RGB’s green component).

Carl Jones
Meteorologist
NWS Grand Forks