NWS forecasters have the ability to make modifications to the three RGB components (Red, Green, Blue) in AWIPS. These modifications can be made easily by right-clicking (and hold) on the loaded RGB in the product legend, and selecting the “Composite Options” (Fig 1). Slider bars and numerical values for the max and min values, along with the Gamma, for each of the Red, Green, and Blue composition of the RGB will appear. The forecaster can adjust the numbers using the sliders or by editing the numbers, which will adjust the appearance of the RGB on the fly. It is recommended these adjustments be made on the fly as opposed to saving a procedure as desired thresholds will change depending on the time of day, season, and situation. The goal of the adjustments is to enhance the usefulness of the RGB by drawing out relevant features in certain situations.

There are occasions where making these modifications will extend or improve the usefulness of the RGB, particularly for RGBs which utilize VIS or NIR channels, and therefore depend on sunlight. The Day Cloud Phase Distinction (DCPD) RGB is useful for tracking cumulus cloud evolution from early water cloud (cyan), to convective initiation and glaciation (bright green), to mature convection (yellow). Because this process often takes place midday during the summer, the relatively low max thresholds of the reflectance/albedo components (79% for the 0.64 um vis green component, 59% for the 1.61 um snow/ice band blue component), are often exceeded, causing those components to wash out and provide no texture detail. Therefore, it is beneficial to increase the thresholds for those VIS/NIR fields to ensure saturation of the field will not occur. The exact thresholds to set will depend on the situation, including time of day, time of year, location, etc. These changes should only take around 15 seconds to make, and may need to be updated periodically during the day as the sun angle changes. A video from the 2019 Satellite Applications Workshop exemplifies the process of making adjustments to the DCPD RGB is such a situation.

A convective initiation example from 9 August 2019 over Colorado compares the DCPD RGB before (Fig 2) and after (Fig 3) a change to the RGB recipe was made. The forecaster was monitoring the scene for signs of imminent convective initiation, and made the changes to the RGB on the fly. A comparison of the values for the default RGB and modified version, along with the corresponding imagery at 2014Z, is included in Figure 4.

It is also beneficial to modify the DCPD RGB during low light situations, just after sunrise and before sunset, in an effort to extend its usefulness. During these times of day, features will be more difficult to discern due to low albedo from the green (0.64 um) and blue (1.6 um) components relative to the ranges set in the RGB. Therefore, adjusting the upper threshold downward for both will make cloud features apparent slightly earlier in the morning and later in the evening. If doing this in the morning, the user will need to be sure to return thresholds upward as albedo increases.

A common situation where such low light adjustments are helpful is for low cloud monitoring around sunrise or sunset. For example, early morning low clouds in the San Luis Valley of CO/NM on 12 August 2019 were difficult to visualize in the RGB with default settings (Fig 5). Adjusting the upper bounds downward for the VIS and NIR components, the fog (aqua colors) becomes brighter and easier to diagnose (Fig 6). The adjustments and direct comparison are shown in Figure 7.

Alternatively during low light situations, the Gamma values for the reflectance components in an RGB can be lowered in an effort to put more weight on them. An example of valley fog over PA/NY compares decreasing the Max values with decreasing the Gamma values for the VIS (green) and NIR (blue) components in the DCPD RGB (Fig 8). Both adjustment strategies similarly make the fog more apparent when compared to the default settings. Some combination of Max/Gamma reduction would also work to draw out the low level cloud features.

The VIS/IR Sandwich RGB similarly benefits from adjustments during low-light situations. Lowering the max threshold and gamma for each of the three components equally results in a brighter picture with clearer detail in convective cloud tops. An example of the changes made for a likely severe storm in southern Colorado during the evening of 13 September 2019 is shown in Figure 9. Features such as cumulus clouds, overshooting tops, and above anvil cirrus plumes are more easily and quickly detected in the modified/brighter imagery.

Forecasters across the northern US plains have found utility in the Day Snow-Fog RGB for identifying and tracking areas of blowing snow. This RGB includes two VIS/NIR components, and therefore its use can be extended further into the morning and evening by making simple adjustments (Fig 10). Carl Jones (FGF) and Andrew Ansorge (DMX) discussed this task at the 2019 Satellite Applications Workshop, and specifically mention the value of making on-the-fly adjustments to the RGB in an effort to make features clearer during low light situations.

Alternatively, there are also situations where these modifications may change the meaning of the colors in the RGB, and could lead to misinterpretation of important features. Additionally, making adjustments to an RGB and then sharing it with others who are not aware of the specific adjustments and interpretation of the new scheme can be dangerous. Therefore, forecasters making adjustments should be sure to understand how their changes are influencing the meaning of the final RGB, and should probably not save and share modified AWIPS RGBs with others. Rather, the process or practice of modifying an RGB to help in specific situations should be shared.

Bill Line, NWS

An isolated and long-lived thunderstorm developed along the Raton Mesa during the late afternoon of 13 September 2019 in an environment characterized by weak forcing but favorable SBCAPE (~1500 j/kg) and EBSHEAR (~35 knots). While probability was low that a storm would develop, any storm that did develop had the potential to be strong to severe. Surface southerly to southeasterly upslope flow onto the Raton Mesa and low level convergence with the deepening surface lee trough would be the drivers for initiation. Early day HRRR runs hinted at the potential for very isolated storm development in this area as well.

Clear skies prior to cumulus cloud field development and deep convective initiation (CI) allowed for the analysis of the split window difference early in the day. There were no obvious signs of moisture pooling or boundaries in the area of future CI. However, upon crunching a linear grayscale colortable, a subtle moisture maximum corridor is apparent along the northern edge of the Raton Mesa between KTAD and K4V1, east of KVTP. This would imply relatively high amounts of low level moisture, maybe low-level moisture convergence/pooling, and an opportunity area for CI. Additionally, there appears to be a surge eastward out of La Veta Pass (KVTP) prior to CI, possibly enhancing low level moisture convergence up the northern portion of the Raton Mesa. These signs, along with what the HRRR had been showing, should give a forecaster increasing confidence that this particular area probably has the best chance for CI.

Convergence along the leading edge of this surge is apparent in radar imagery progressing toward the moisture maximum just prior to CI.

GOES-16 5-min Sandwich RGB imagery provided some helpful insight during the evolution of this event. During the early stages, clumping cu were becoming more apparent northwest of KTAD after 20Z, with an orphan anvil developing around 21Z off of the easternmost towering cu (fig 3). This feature indicates a failed convective initiation attempt and that a capping inversion is still present but may be on the verge of breaking. Convection successfully initiated from the same area shortly thereafter.

After convective initiation, the IR component to the sandwich appears and rapid cooling is apparent. Pretty soon after CI, an overshooting top is diagnosed, and an associated above anvil cirrus plume shortly thereafter. It appears as though plume generation likely began between 2220Z and 2230Z, at least 15 minutes prior to Maximum Estimated Size of Hail (MESH) first exceeding 1″. One-minute imagery would have made detection and tracking of this feature easier. Figure 4 zooms in on the area of interest and ends on a time when both the OT and plume are obvious in the sandwich imagery.

GOES-16 sandwich RGB imagery for the full duration of the storm through sunset and with no annotations is included in Figure 5. The storm remained over rural areas so warnings were not verified. MESH algorithm indicated hail larger than 1″ in diameter for much of the time between 2245Z and 0055Z, with a peak over 2″ around 0030Z (end of the animation).

Bill Line, NWS

Tropical Cyclone Dorian become a hurricane on 28 August 2019. By the early morning of the 29th, the Dorian remained a category 1 hurricane as it advanced to the northwest, north of Puerto Rico.

GOES-East IR imagery and Sea Surface Temperature derived product with NHC forecast track overlay show the hurricane expected to continue through very warm waters as it approaches the Florida East Coast (Fig 1). Temperatures are between 29C and 30C along the track, or around 85F.

A 1-min mesoscale sector from GOES-East was available over the system. The high resolution imagery aids forecasters in identifying the center of circulation, as well as in tracking thunderstorm activity. Visible imagery with a semi-transparent GLM Flash Extent Density overlay at sunrise on the 29th clearly showed the center of circulation, with areas of thunderstorms, some strong, embedded in the outer bands (Fig 2).

Zoom in view on center of circulation at sunrise Fig 3).

The strong thunderstorm just northeast of the center of circulation exhibited high flash rates (see Fig 2) as well as persistent overshooting tops and even an above anvil cirrus plume, much like we see with strong thunderstorms over the CONUS (Fig 4).

Bill Line, NWS

Thunderstorms developed along a SW-NE oriented boundary draped across north-central Oklahoma during the early evening hours of 26 Aug 2019. The storms quickly grew upscale into a Mesoscale Convective System (MCS), and tracked south through central Oklahoma during the night, producing strong/damaging wind gusts and large hail.

One-minute visible imagery from GOES-East clearly shows convergence along a boundary, development of towering cu along the boundary, and eventual convective initiation (Fig 1), all in real-time. Storms quickly reached the equilibrium level and developed overshooting tops (OTs) and above anvil cirrus plumes, indicating particularly robust updrafts. Given the time of day around sunset, the shadows cast on the anvil provide additional insight into the vertical extent of the OTs.

GOES-East IR imagery showed rapid cooling of cloud tops as well as the development of overshooting tops (Fig 2). One method for viewing GOES imagery and GLM fields together in one display is to create a GLM contour color table. Since GLM fields are not gridded data in AWIPS, one cannot easily change it to a contoured field. However, the user can create a color table that implements contours at chosen thresholds instead of a constant fill. This display allows one to view VIS or colored IR imagery and GLM FED data in one display, not needing to make the GLM field semi-transparent. The GLM color table in this example includes three thresholds: 1 Flash/5-min (blue), 25 Flashes/5-min (Cyan), 100 Flashes/5-min (Yellow). For the most part, these solid color contours stand out and do not interfere with the IR colors.

Bill Line, NWS

Very deep convection developed over the western Mexico near the Gulf of California during the late afternoon hours of 20 August 2019, lasting into the early evening. This area of convection produced overshooting tops with IR brightness temperatures less than -90C (Fig 1)!

Analyzing a nearby sounding, these storms developed in an extremely moist environment (TPW=2.49″), very strong instability (MLCAPE = 4660 j/kg) and a very high equilibrium level (around 16.5 km). Based on the sounding, the -90C observed IR brightness temperatures likely put the OTs over 17.5 km above the surface!

Corresponding visible imagery of one of the storms is equally impressive. The final frame shows an OT penetrating deep across the equilibrium level casting an obvious shadow on the anvil (Fig 3).

Finally, a VIS/IR comparison of a -90C OT (FIg 4).

Bill Line, NWS

A severe thunderstorm developed quickly in the Pueblo, CO area during the early evening of 10 August 2019. GOES-16 imagery and lightning data provided clues to a rapidly intensifying thunderstorm with the potential to be strong to severe. While the environment was not all that supportive of large hail, steep low-level lapse rates and anomalously high Total Precipitable Water (TPW) values (~130% of normal) indicated the potential for strong downdraft wind gusts and heavy rainfall. Indeed, there were reports of heavy rain with this storm, as well as a 76 mph wind gust recorded at KPUB Pueblo ASOS (Fig 1). One-minute satellite imagery was available to forecasters during this event.

Analyzing 1-min visible and IR imagery from GOES-East over a 99-min period, we see several areas of cu initiate in the moist environment just west of Pueblo before organizing into into a single strong updraft (Fig 2 and 3). The updraft quickly reaches the tropopause and spreads out radially. An overshooting top is first apparent around 0000Z, while an above anvil cirrus plume starts to become noticeable after 0015Z.

The sandwich RGB combines the VIS and IR into a single image (Fig 4) into a single high-resolution qualitative product.

Looking at semi-transparent GLM Flash Extent Density (5-min accumulation updating every 1-min) gridded product overlaid on the VIS, we diagnose a rapid increase in total lightning between 2335Z and 2344Z (7 fl/5-min to 31 fl/5-min), or a lightning jump (Fig 5). This jump is an indicator of a strengthening updraft and often a precursor to hazardous weather at the surface. Cloud tops cooled from -45C to -64C during the same 9-min period. Lightning activity remained relatively stable through around 0000Z, while cloud tops continued to cool. Between 2345Z and 0000Z (after the lightning jump and during the continued cloud top cooling), very heavy rainfall (at least ~0.5 inches in 15 min) and gusty winds (estimated around 50 mph) were reported with this storm, in addition to improved presentation in radar imagery.

A second lightning jump occurred between 0002Z and 00009Z when FED increased from 23 fl/5-min to 56 fl/5-min). Cloud top temperatures bottomed out at around -76C at this time as well. During this lightning jump, very strong winds associated with the storm were measured at the surface, including the first severe wind gust of 59 mph at 0008 UTC, the first significant severe gust of 75 mph at 00010 UTC, and maxing out at 76 mph at 0016 UTC. The storm advanced into primarily rural areas thereafter, with lightning activity slowly dropping off, and cloud tops warming. See graph of IR and GLM trends for this storm in Figure 6, made using the AWIPS Tracking Meteogram Tool.

Bill Line, NWS

A line of severe storms progressed southeast across the upper midwest during the early evening hours on 5 August 2019. One-minute visible imagery from GOES-East provided an excellent view of storm top features and trends, including numerous overshooting tops and above anvil cirrus plumes, indicating active and exceptionally strong updrafts. Overlaying a semi-transparent 1-min GLM Flash Extent Density, we have even more information about individual updraft trends, augmenting the visible imagery whose details are still noticeable. Trends in the GLM lightning field tell us which updrafts are increasing in intensity, and which are decreasing, sometimes slightly prior to those trends becoming apparent in satellite and radar imagery. The GLM gridded lightning fields are on the same grid as the ABI data, so share the same degree of parallax. Also noted are periodic long flashes extending well away from the main updraft regions, indicating the potential for distant cloud-to-ground lightning strikes.

Bill Line, NWS