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.

Bill Line, NWS

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.

Bill Line, NWS

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.

Bill Line, NWS

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.

Bill Line, NWS

GOES-15 continues to operate from 128W, providing imagery to the east Pacific and western US. GOES-15 imagery continues to be available to NWS forecasters in AWIPS. As a reminder of how far we’ve come from the previous generation of geostationary satellites, below are comparisons of GOES-16 (75W) 1-min imagery and GOES-15 routine imagery for thunderstorm development over west Texas on 27 April 2019. These thunderstorms developed at 100W, roughly in the middle (and north) of the two satellite subpoints. The animations are 1.5 hours long, running from 2030 UTC to 2200 UTC. Details such as cumulus cloud evolution, convective initiation, and storm top features (OTs, AACPs, texture) are all significantly easier to diagnose (and quicker) in the GOES-16 imagery. Be grateful for the GOES-R series!

These thunderstorms went on to produce severe hail and wind.

Bill Line, NWS

A late night tornado passed through the town of San Augustine, TX during the evening of 24 April 2019. GOES-16 GLM Flash Extent Density data showed a significant increase in lightning activity (lightning jump), during the 10-20 minute period prior to the tornado LSR at 0420 UTC on the 25th. Below are examples of the 1-min FED product (Fig 1), as well as the 5-min accumulation FED product (updating every 1-min as well; Fig 2), which provides a smoother view of the lightning trends. The rapid increase in lightning is distinguishable in both products with this case. ABI IR (gray-scale), tornado (red) and severe thunderstorm (yellow) warnings, along with the Tornado LSR (yellow symbol), are also included in both animations. The graphs below show the rapid increase in FED and peak prior to 0420 UTC (Fig 3).

Bill Line, NWS

Severe thunderstorms developed along a cold front across southwest and central Texas during the afternoon/evening of 23 April 2019. GOES-16 1-min imagery was available over the region aid forecasters in monitoring storm development and evolution.

Analysis of GOES-16 water vapor imagery reveals a mid-level low progressing east across northern Mexico, spreading increased southwesterly flow across Texas, increasing vertical wind shear (Fig 1).

A 4-hour long, 1-min animation of the day cloud phase distinction RGB leading up to convective initiation shows its utility in such situations. Recall, the blue to cyan (or similar) colors represent water clouds, while the transition to truer green indicates ice increasing in the cloud top (glaciation, loss of blue component), and the following transition to yellow is caused by the cooling cloud top (increasing red component). The southeast-moving boundary which eventually serves as the focus for convective development is obvious in the imagery as a line of increasingly agitated cu. Since the 1/2 km, 0.64 um visible channel is one component to this RGB, we are still viewing imagery at the highest resolution possible from GOES. Eventually, orphan anvils develop and blow off, and are obvious (more so than in VIS alone) in their rapid transition from ~cyan to green/yellow. These are often indicators that the atmosphere has destabilized, but the CAP is still present but on the verge of breaking. The failed initiation attempts (orphan anvils) become more abundant through the loop leading up to successful initiation by the end of the period.

One of the severe thunderstorms produced hail of at least 2.5″ in diameter southeast of Sweetwater, TX. The intensity of the storm updraft was captured in the 1-min visible imagery, with abundant storm top texture, overshooting tops, and above-anvil cirrus plumes, all apparent (Fig ). In this particular period of time, two plumes extend from the main (southwest) storm cluster, indicating multiple intense updraft cores.

IR imagery confirms a broad overshooting top, with a large cluster of pixels having a brightness temperature of <-67C and as cold as -70C, surrounded by anvil temperatures between -57C and -60C. Nearby soundings indicate the tropopause was indeed around -60C. A downstream warm region with temperatures around -54C completes a thermal couplet. The above anvil cirrus plume is evidenced by relatively warm and nearly uniform temperatures extending well downstream of the overshooting top. An enhanced-V/U (aka cold ring) extends from the overshooting top and around the plume. The overshooting top, thermal couplet, above anvil cirrus plume, and enhanced-V/U storm top signatures have all been noted in the literature as being indicators of particularly strong updrafts/thunderstorms, with the plume often associated with severe storms (Bedka, 2018).

The Sandwich RGB, now available in AWIPS, combines the VIS and IR into one image (Fig 3) for cold brightness temperatures (cold cloud tops), while maintaining VIS for warmer temperatures (surface and low clouds).

Bill Line, NWS