Geostationary Lightning Mapper (GLM) Data Used to Aid in Warning Decision…

The NWS office in Huntsville, AL (HUN) has had a long history with the use of total lightning data in operations, which stretches back to the office’s inception (after NWS modernization) in 2003.  Back then, and until its removal to South America for GOES validation testing, the HUN office largely used data from the North Alabama Lightning Mapping Array (NALMA). Lightning data sources from NLDN and ENTLN have also been used to varying degrees, but the advent of the GLM aboard GOES-16 brought a new era of lightning observations.  Because of the office’s participation in early operational testing of the GLM, its use and familiarity have gradually increased over the past year.  This was probably made easier due to our familiarity with total lightning data from the NALMA network.  Generally, GLM data have been used in much the same way as those from the NALMA network, especially with regards to situational awareness purposes (i.e., airport weather warnings, real-time weather watches for EM partners, initial cell electrification, etc.).  The use of the data to aid in severe weather warning decisions has been a bit slower to evolve, as might have been expected.  After all, there are differences in the way the NALMA observes lightning as compared to the GLM.  Values from the GLM have typically been “muted” compared to those from NALMA, so forecasters have had to make internal adjustments and recalibrate, if you will, what is considered significant.  However, the physical mechanisms that generate increases in total lightning, that is, increases in mixed-phase updraft volume, are essentially observed either way.  Thus, GLM data can still be useful to relate important information about storm/cell evolution, and can help to “tip the scales” in the balance of evidence about whether or not a warning may be needed.

This particular application of the GLM data occurred this morning with operational meteorologists at the HUN office.  The short image loop below shows thunderstorms moving across northwestern portions of Alabama between 1226 and 1300 UTC.  The top panel of the image contains data from the KGWX radar (0.5 degree reflectivity), while the bottom panel contains GLM 1-minute Flash Extent Density (FED) data.  Notice that lightning activity is relatively limited initially as the storm moves across western Franklin County, AL (near center of image), with 1-min FED values ranging between 2 and 6 flashes per minute.  Then, at the 12:37 UTC time mark, flashes begin an increase that manifests in a statistical lightning “jump” (GLM sigma > 2).  The warning meteorologist at the time was watching this cell for potential severe weather, and observed the sudden increase in FED values.  This, together with other radar and satellite observations (now shown here), suggested that a severe weather warning was necessary as wind signatures aloft gradually increased.  A warning was subsequently issued at 1245 UTC.  Incidentally, this thunderstorm did end up producing some wind damage, with trees reported down in south-central portions of Franklin County.  Notice also that a number of strong cell signals were detected by radar as indicated by the higher dBZ values across the domain.  Another use of the GLM is allowing meteorologists to focus on the cells with the strongest updrafts, making the overall radar interrogation and warning process more efficient.  This case can help to demonstrate that the GLM can be used as an important indicator of storm evolution and as a useful operational tool for the evaluation of severe weather potential.

-Kris W.


[Top] KGWX 0.5 deg Refl with NWS Severe Thunderstorm Warning (yellow box), [Bottom] GLM 1-min Flash Extent Density, 1226-1300 UTC 17 July 2019.  

GLM Aids Convective Situational Awareness–at 37 degrees

There has been a great deal of focus on the potential for heavy rain and flooding across the southeastern United States.  However, today has been marked with an interesting mix of winter and spring.  The day began with reports of sleet, snow, and rain as the first wave of precipitation spread across the region.  Temperatures rose into the upper 30s to lower 40s later in the morning, and the focus shifted–to thunderstorms.

Multi-Radar/Multi-Sensor Reflectivity, valid 1846 UTC and GLM Flash Extent Density , valid 1847 UTC 19 February 2019

Flash Extent Density (FED) data from the Geostationary Lightning Mapper has been lighting up (literally and figuratively) over the Mid-South and Mississippi Valley as precipitation lifts northward.  While many of the flashes have been focused within the convective elements along the southern edge of the precipitation shield, there have been numerous “long” flashes (greater than ~30 km) advancing north well beyond the convective cells (such as the one indicated above at 1847 UTC).

Use of the GLM aided forecasters in issuing an airport weather warning for the Northwest Alabama Regional Airport (Muscle Shoals) at 11:52 AM.  While this was not much earlier than the first report of thunder at KMSL, it was much farther north of the convective cells than originally anticipated.  GLM aided the forecasters’ situational awareness of an unusual situation.

Multi-Radar/Multi-Sensor Reflectivity, valid 1750 UTC and GLM Flash Extent Density, valid 1751 UTC 19 February 2019. White circles denote the KMSL (Muscle Shoals) and KHSV (Huntsville) airports.

GLM data have also been used to assess convective potential for sub-severe thunderstorms as the more convective cells have moved into the Huntsville county warning area.  A persistent GLM centroid of 4-8 flashes per minute corresponded to a report of dime-size hail in Cullman, Alabama around 1945 UTC.

Multi-Radar/Multi-Sensor Reflectivity, valid 1924-1954 UTC and GLM Flash Extent Density, valid 1925-1954 UTC 19 February 2019

GLM “sees” apparent meteor flash in Western Cuba…

So, I was seeing some news reports on Twitter this afternoon about an apparent meteor that struck Western Cuba.  Pulling up data/imagery from the GLM in AWIPS, I was able to see some relatively high Flash Extent Density (FED) values from that area at the same time of the meteor report.  The first image below shows FED values (1818 UTC) overlaying GOES-16 Visible (0.64 µm) imagery at 1817 UTC.


Image 1. GLM data shows an apparent meteor flash over western portions of Cuba at ~1818 UTC 1 Feb 2019. The GLM Flash Extent Density overlay GOES-16 visible (0.64 um) imagery from ~1817 UTC.

Also, notice the large amount of lightning observed by the GLM in central portions of the Gulf of Mexico.  Here’s a short 30-min image loop around this time period (the suspected meteor flash shows up about midway through the loop).  Importantly, before the GLM sensor, the amount and extent of lightning activity over open ocean areas, away from ground networks, was generally not known, especially at such high spacial/temporal resolution.


Image 2. GLM (Flash Extent Density) and GOES-16 visible imagery (0.64 µm) loop from 1802-1830 UTC, 01 Feb 2019. An apparent meteor shows up in western Cuba at 1818 UTC in the loop. Also, notice the active deep convection and lightning over the Gulf of Mexico during the period.



GLM & Public Safety: An Important Caveat

As great as is to use data from the Geostationary Lightning Mapper, there is an important caveat forecasters have to consider when using the data.

During the afternoon of July 11, typical “air mass” showers and thunderstorms were developing across northern Alabama, including several south of the Huntsville International Airport.  At 12:18 PM CDT, the GLM Flash Extent Density data started to light up with these cells, including one larger flash at 12:21 PM.  (Huntsville airport is marked by the eastern concentric yellow circles.)

KHTX radar valid 1722 UTC 11 July 2018 and GLM FED valid 1721 UTC

As I’ve noted before, we issue airport weather warnings for Huntsville if lightning is within 5-10 miles, heading towards the terminal.  So forecasters were justifiably alarmed that GLM flashes were starting to show up within the 10-mile range ring, and just barely edging towards the 5-mile ring.

But here is where that caveat comes into play: the parallax effect.  Radar showed the actual echoes associated with these flashes to be well to the south of the GLM flashes.  Earth Networks Total Lightning data from the same time period showed lightning confined to these cells well outside the 10-mile range ring.  Furthermore, the cells were moving away from the field.

KHTX radar valid 1722 UTC 11 July 2018 and Earth Networks total lightning valid 1721 UTC

KHTX radar valid 1722 UTC 11 July 2018 and Earth Networks total lightning valid 1721 UTC

It’s worth noting that GLM showed a greater spatial extent during some of these flashes, but Earth Networks was much closer in location to the radar.

So while a cursory glance at the GLM data might lead to an airport weather warning, it was important to double-check GLM against the radar–and recognize that an AWW was not necessary in this case.  The same caution will need to apply as we begin applying GLM to other public safety situations.

GLM Detection of Anvil Lightning

During the evening hours of June 27th, we noticed that GLM average flash area data was indicating a substantial flash from the main core of storms over northeast AL/northwest GA towards middle TN.  This occurred around June 28th, 0130Z-0140Z, and I have included both a screen shot and animation for this flash.  Though no DSS events were scheduled over this particular area, it would be of great benefit to utilize this for potential lightning impacts at outdoor events–where lightning is occurring well away from the storm core/updraft. Also of note was the GLM flash extent density showing a noticeable jump throughout the evening with the stronger updrafts. This aided in the decision to keep a significant weather advisory in place for these storms.

For purposes of calibration and given my experience with utilizing GLM data at a recent HWT (May), I prefer having a four panel display in D2D. ENTLN data overlaid on Flash Extent Density while the other available parameters (Average Flash Area and Total Optical Energy) are on two other panels.

GLM_062818_0130Z_LightningFlashAnvil-AnnotatedFigure 1: Four panel of GLM data with MRMS Composite reflectivity in bottom left, annotated Average Flash area in bottom right, flash extent density with overlaid ENTLN data in top left, and Total Optical Energy in top right.


GLM_062818_0130Z_LightningFlashAnvil_AnimationFigure 2: Same as in Figure 1, just animated.

The nice thing is that there were generally no latency issues noted other than intermittent 2-3 minute delays to real time.

-Stephen Latimer (NWS Huntsville)


Using GLM for Airport Weather Warnings

As part of our impact weather responsibilities, NWS Huntsville issues Airport Weather Warnings (AWWs) for Huntsville International Airport (KHSV) and Northwest Alabama Regional Airport (Muscle Shoals; KMSL).  AWWs are issued for the threat to personnel working outside at the terminal and neighboring operations, not for the threat to aviation as a whole.  One of our criteria for AWW issuance is the threat of cloud-to-ground lightning within 5 miles.

On May 30, a relatively small line of thunderstorms developed over northeast Mississippi and tracked to the east, producing a few pockets of straight-line wind damage along the way.  The line approached the 10-nm range of KHSV at 2320 UTC (seen below).  At this point, GLM Flash Extent Density (FED) data wasn’t the most helpful data set for deciding to issue an AWW; the National Lightning Detection Network (NLDN) was already detecting a great deal of cloud-to-ground lightning with the storms.


Multi-Radar/Multi-Sensor (MRMS) Reflectivity At Lowest Altitude (RALA) valid 2320 UTC 30 May; 5-minute NLDN flashes valid 2321 UTC; and GLM Flash Extent Density valid 2321 UTC.  The 5 and 10-nautical mile range rings around KHSV are illustrated in coral near the center of the image.

The GLM FED data were far more useful later on in the event, after the line passed well east of KHSV and the Huntsville metro area.  With nightfall, the convective updrafts weakened and lightning along the line generally decreased–but flashes within the trailing stratiform region of the quasi-linear convective system (QLCS) did not.  One of the more spectacular examples occurred around 0059 UTC (seen below).  The FED product really illustrated the spatial threat simply and effectively, especially when combined with the NLDN data.  (This example uses a slightly different color curve from the GLM baseline to enhance lower flash rates.)


MRMS RALA valid 0058 UTC 31 May; 5-minute NLDN valid 0059 UTC; and GLM FED valid 0059 UTC.  5 and 10-nm range rings around KHSV are also noted.

GLM helped forecasters acquire and retain situational awareness of these trailing stratiform “long flashes”, which helped with AWW extension/reissuance.  As a result, the airport weather warning for Huntsville was re-issued until the trailing stratiform region cleared the airport and the threat subsided.

Geostationary Lightning Mapper detects lightning in the Volcanic Plume from the Fuego Volcano in Guatemala (3 June 2018)

During the eruption of the Fuego Volcano on the afternoon of Sunday June 3, 2018, the Geostationary Lightning Mapper observed a rare, but important, phenomenon: volcanic lightning.  A total of five lightning flashes were observed between 1814 and 1834 UTC with the rising plume.  The first three flashes between 1814 and 1818 UTC were 8 to 10 km north-northeast of the volcano. Then as the plume continued to advect to the northeast of the volcano, the position of the lightning followed. A flash at 1822 UTC was 13 km from the cone of the volcano, and the flash at 1833 UTC was farthest away from the cone of the volcano at 15 km.

Animation of GLM group densities from 1810 to 1835 UTC on 3 June 2018. The location of the volcano is circled.

One of the unique features of GLM is the ability to measure the size of the flash directly as each flash is observed.  In thunderstorms, flash size is a good indicator of vertical motion, which is often hard to directly measure within thunderstorms, or in this case, a volcanic plume.  Between 1814 and 1822 UTC, flashes were occurring approximately every 2 to 4 minutes, and their size was on the order of 500-1000 km2.  Then between 1822 and 1834 UTC there was a lull in GLM-detected lightning activity, followed by the largest flash detected by GLM during the event at 1500 km2 as the plume continued to expand over central Guatemala.

There was a 3 hour lull in GLM-detected activity, until 2141 UTC, when a second set of 21 flashes was detected by GLM between 2141 and 2203 UTC. These flashes were located between 1 and 8 km south-southeast from the highest point of the volcano and correspond in time and reported location of the deadly lahar and pyroclastic flow that came down the south side of the volcano. GLM flash sizes ranged from 64 km2 up to 1500 km2 and there was not as clear of an increase in size as observed with the volcanic plume.

Animation of GLM group densities from 2139 to 2205 UTC on 3 June 2018. The location of the volcano is circled.