A vigorous trough and favorable thermodynamic environment led to the development of numerous severe storms and flash flooding during the day on 20 May through the evening and into the next morning. Storms produced over 25 tornadoes and many large hail and strong wind reports, in addition to flash flooding.
Water vapor imagery from the event showed the trough become negatively tilted as it swept across the Rockies into the plains from the 20th into the 21st (Fig 1). The negative tilt implies differential temperature/moisture advection in the vertical (increasing instability) and increasing wind shear. Combining the water vapor imagery with NWP analyses (in this case, hourly RAP) helps to improve analysis of the overall synoptic picture as features from the model can be connected to features in the imagery. Over time, this can improve ones ability to diagnose features in the imagery alone. Additionally, variances between the imagery and model can be noted and extrapolated into the model forecast. The NWP overlay in this case quantifies the strength of the trough and shows the strong jet rounding its base and advancing over the southern plains.
One-minute satellite imagery from GOES-16 was available to forecasters during this event. Visible imagery from west Texas/Texas Panhandle into western Oklahoma showed rapid thunderstorm evolution, including storm initiation and development of overshooting tops and above anvil cirrus plumes (Fig 2). The quick development of these storm top features was no surprise given the favorable setup, and indicated very strong updrafts and significant severe potential.
One-min IR imagery confirmed the trends and features, with rapid cooling rates implying swift updraft growth, the small cold regions indicating overshooting tops, and downstream warm regions surrounded by colder tops (enhanced V) suggesting the above anvil cirrus plumes (Fig 3).
A 19 hr long IR loop (17Z – 12Z) revealed persistent strong thunderstorm activity from west Texas across much of Oklahoma (Fig 4). Thunderstorms training over the same areas led to numerous reports of flash flooding.
A Mesoscale Convective System (MCS) containing a line of severe thunderstorms rolled across south Texas early in the morning on 03 May 2019. GOES-16 1-min imagery was available over the region to support forecast and warning efforts. One-minute IR imagery indicated a broad region of overshooting tops with gravity waves emanating out away from the updrafts (Fig 1). Gravity waves across the top of a convective system are formed when the updraft interacts with the stable tropopause. A strong updraft may “overshoot” the tropopause into the lower stratosphere, appearing as an overshooting top. In an effort to return to equilibrium, these air parcels go on to oscillate (sink and rise) past that equilibrium level. This air reaching and overshooting the tropopause is forced to oscillate outward and downstream by the mean flow, appearing as “waves” at the storm top.
Transverse banding (buzzsaw looking cloud area) was apparent in the imagery north of the main updraft region. These mid-upper level clouds are an indication of potential aircraft turbulence. There were indeed several aircraft reports of moderate turbulence through these cloud features.
GLM flash extent density overlaid on gray-scale IR imagery from the same period highlights the storm cores (highest lightning rates), as well as lightning flashes extending well away from the updrafts through the anvil region (Fig 2). The average flash area confirms smaller flashes associated with the active/newer updrafts, with longer flashes in the anvil away from the main updrafts.
One-minute visible imagery at sunrise revealed the storm top features in much greater detail, including the very active updraft/overshooting top region, and series of gravity waves across the anvil.
The day cloud phase distinction RB has been discussed as a useful tool for monitoring the cumulus cloud field leading up to and including convective initiation. Pure liquid cumulus clouds appear as cyan in the RGB because they have relatively high reflectance in the 0.64 um (high green) and 1.6 um (high blue) components, but are warm in the IR (low red). As convection begins to initiate and ice develops in the cloud top, the blue component decreases since ice does not reflect as well in the 1.6 um band (low blue), but the green component (0.64 um) stays the same or increases (high green). The red component begins to increase as the temperature of the clouds decreases (mid red). Therefore, for convective initiation, you are left with a transition from cyan to green.
In the 01 May case, you can see a transition from cyan to green in areas along the boundaries, indicating that convective initiation is imminent or occurring. There are quite a few orphan anvils in this case, indicating failed initiation but that the CAP is likely close to breaking. While these features are all apparent in the VIS alone as well, the color detail added to the imagery as a result of the combination of multiple channels makes these features and trends easier to diagnose. The high, 0.5 km, resolution available from the 0.64 um VIS is maintained in the RGB in AWIPS.
Once the convection matures, the color transitions from green to close to yellow since the convection is still highly reflective in the 0.64 um channel (high green), and is also now cold (high red), but still not as reflective in the 1.6 um channel (low green) due to the presence of ice.
Just as convection begins to initiate, attached are the three components to the RGB, plus the RGB, at 1721 UTC
It should be noted that with this case, the RGB was modified slightly. The 0.64 um (green) component had its max increased to 100% (from 78%) to account for the higher reflectance of cumulus clouds and mature convection during the middle of the day. This change is often necessary when using this RGB to monitor for convective initiation, and can be made easily in the “composite options” setting of the product. Without the change, the green component saturates quickly, leading to a loss of storm top detail in the imagery provided by the high resolution 0.64 um channel.
A potent shortwave trough helped to bring severe storms to the southern plains on 30 April 2019 (Fig 1).
The GOES-16 10.3 um – 12.3 um split window channel (SWD) difference, as discussed in previous blog posts (first introduced here), can be utilized to highlight low level moisture features from the clear sky. In this case, dry southwest flow (darker gray) can be visualized mixing out the low level moisture (lighter gray) from west to east across southeast New Mexico and southwest/west Texas (Fig 2). Surface obs confirm the progression of the dry line and gusty southwest winds, with dew points in the mid 50s quickly decreasing to into the 20s. Cumulus clouds were observed developing in the moist air just ahead of and along the boundary, with some areas initiating and developing into severe thunderstorms. In such a case, the SWD can be used as a high resolution source to track evolving gradients in low level moisture (often a dry line), filling gaps between surface observations/analyses. The SWD can also be used to detect lofted dust, the black to brown colors found in Mexico in this animation.
The southernmost thunderstorm developing along the dry line produced hail to 3 inches in diameter, or teacup sized (Fig 3-5). This storm developed an impressive and persistent overshooting top and above anvil cirrus plume. As mentioned in previous posts, the AACP is often associated with severe storms (Bedka, 2018) .
Taking a quick look further north, GOES-16 5-min GLM Flash Extent Density total lightning data shows widespread thunderstorm activity, highlighting the most intense storm cores.