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.
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.
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).
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).
The March 2019 RPM included the addition of a VIS/IR Sandwich RGB to AWIPS-II. To create the VIS/IR sandwich in the past, forecasters were required to overlay a semi-transparent IR over-top VIS. The RGB automates the creation of this image combination in AWIPS. The benefit of the VIS/IR Sandwich RGB is that it combines the high spatial detail apparent in VIS, with the temperature information of the IR.
In a convective scenario, for example, a forecaster can utilize the RGB to monitor a cu field and boundaries as they normally would, since only the visible imagery will be present up to the point of convective initiation. Once the clouds have cooled enough during convective initiation, the IR component will be combined with the visible, allowing a forecaster to continue to visualize the detail in the cloud top via the VIS component, but also now the temperature information/trends from the IR in the developing storm. Post initiation, this combination improves identification of cloud top features such as overshooting tops, above anvil cirrus plumes, and gravity waves. In the RGB, forecasters are unable to sample the IR temperature, so trends must be gleaned from the colors. However, forecasters can underlay the IR channel so that brightness temperatures can be sampled.
Figure 1 includes an example of rapid convective initiation to mature convection over the Texas Panhandle on 17 April 2019. In a little over one hour, the storm initiates and develops an overshooting top and above anvil cirrus plume. The rapid cooling is apparent in the imagery while the high detail of the VIS is maintained. Figure 2 shows the same storm a little later. One-minute imagery was available over the region to help forecasters monitor the evolution of convection. These storms produced large hail and tornadoes.
An example from 18 April 2019 of a large QLCS advancing east across Louisiana and the adjacent Gulf shows the ability of the product to highlight storm top features such as overshooting tops, above anvil cirrus plumes, and storm top gravity waves (Fig 3).
A Mesoscale Convective System (MCS) originating over Texas traversed the Gulf of Mexico during the evening of 07 April into the morning of the 8th. This system developed in association with a shortwave trough digging across the southern US plains and ahead of a east-moving surface cold front. GOES-16 10.3 um 5-min imagery captures the evolution of the MCS well (Fig 1). Periodic overshooting tops, above anvil cirrus plumes, and gravity waves are all apparent at storm top.
GOES-16 GLM Flash Extent Density (5-min accumulation updating every 1-min) indicated an abundance of lightning activity with the cluster of thunderstorms (Fig 2). The field highlights the most intense updrafts (greatest FED), and shows lightning flashes extending well away from the main line of storms.
The GLM Average Flash Area shows relatively small flashes associated with the main updrafts, while the flashes extending outward from the most intense cells are long (Fig 3). This makes sense, since stronger updrafts are typically associated with smaller flashes.
Looking at the GOES-16 derived TPW field, the cold front pushing east behind the MCS is easily discernible. Behind the front, TPW values are around 0.5″, while values ahead of the front in the vicinity of the MCS are around 1.5″.
Finally, 1-min imagery was available over the coastal areas to aid forecasters in tracking thunderstorms associated with the system. 1-min visible imagery at sunset reveals active thunderstorms, the development of new thunderstorms through the cirrus shield, and a number of gravity waves emanating away from the updrafts.
An early April eastern US coast cyclone exhibited rapid intensification as it advanced north-northeast off the coast of North Carolina during the evening of 02 April 2019.
A 24-hr GOES-East water vapor loop showed a shortwave trough quickly advance across the southeast US during the previous evening and interact with a low situated off the Georgia coast by the afternoon. The features organized into a single much larger storm that accelerated to the north-northeast just off the east US coast. Rapid strengthening of the storm is evident as dry air expands on the southwest descending side of the low, and cool air expands and clouds develop on the northeast ascending side of the low, wrapping around the center of the low in a counterclockwise manner. Convection is also obvious near the center of the low, as well as along a cold front extending south. MSAS MSLP analysis contours overlaid on the water vapor imagery help depict the organization and deepening of the surface low.
1-min imagery was available over the storm to aid forecasters in analyzing its evolution, particularly convective elements during the day. Figure 2 combines 120 minutes of 1-min visible imagery with 1-min Flash Extent Density (FED) 5-min accumulation. The imagery reveals persistent convective activity on the northwest side of the low. Towards sunset, convection develops quickly southeast of this area of convection, as evidenced by a rapid increase in FED. 1-min IR imagery also shows rapid cooling of cloud tops in this area, indicating updraft acceleration, with consistently cold tops in the broad area of convection to the northwest.
Bill Line, NWS
Adding to Bill’s post, my colleagues and I at the NWS Ocean Prediction Center were looking at this impressive storm through the Air Mass RGB and 1-min visible imagery as we issued hurricane-force wind warnings for the Offshore Zones.
The above animation shows the evolution of a meso-low on the southwest flank of a supercell-like thunderstorm, which becomes the new surface low pressure. Vorticity increases and the pressure falls (~13 mb in 3 hours!).
A snapshot around 2014 UTC on 04/02/19 shows the main thunderstorm updraft with surface low pressure nearly coincident. The arrows point to the cold conveyor belt (north of low), the dry, descending air scouring out low-level clouds on the south side of the convection (see Air Mass imagery below), and a wavy coastal front that will aid in the development of the bent-back front. It appears in the animation that some extreme winds are pushing out the southwest flank of the supercell with a haze near the cloud edge. Could this be precipitation being pushed out similar to a rear-flank downdraft? Or is it sea spray? Regardless, at the time of this image, winds were estimated over 65 kt (75 mph) and possible gusts approaching 90 kt (105 mph)!
Finally, these two images showcase the Air Mass RGB utility in a rapidly intensifying extratropical cyclone. The GLM is highlighting the lightning flash density near the supercell mentioned above and the convection that is lined up along a pre-frontal trough and eventually a fast moving cold front. The dry, descending air (red coloring) is indicative of stratospheric air that is descending rapidly and transporting potential vorticity (PV) towards the developing coastal storm. Note how the upper-level feature moving through the southeast U.S. loses organization as the energy jumps to the coast storm and associated strong convection.
We will try to update this blog post as necessary with new findings, but as of 11 pm EDT (0300 UTC), winds are subsiding on the Outer Banks of NC after gusts of 60-70 mph were reported. This leads me to believe that stronger gusts existed with the nearly closed, eye-like feature observed on radar tonight.