GREMLIN – Page 2

Operational Feedback of Gremlin, Octane, and LightningCast during a Severe Weather Outbreak in Central Oklahoma

Posted in GREMLIN, LightningCast, OCTANE Speed Sandwich, SPG2025 on June 3, 2025 by Kevin Thiel.

I tested the OCTANE, GREMLIN, and LightningCast products during an actual severe weather event on 6/3/2025. My role during this testbed was that of the mesoanalyst.

Initial environmental analysis shows weak to moderate shear, which was determined via ARARS soundings and SPC Mesoanalysis, along with OCTANE imagery showing divergent / accelerating speeds within the storm anvils. VAD hodographs were used as convection developed to see rapid changes within the shear profile during the course of the event (as convection altered the broader environment). Shear increased as the event progressed. OCTANE and LightningCast were both useful showing the uptick in storm intensity as shear increased.

LightningCast was very useful picking out developing updrafts and embedded updrafts within broader areas of convection. We used this product to gauge which updrafts had the greatest potential to become severe in the near term. A strong uptick in lightning would indicate a rapidly strengthening updraft which would warrant further interrogation.

Similar to LightningCast, OCTANE was useful in determining which updrafts were trending towards severe. While in the mesoanalyst role, I would check to see which updrafts looked most intense (warmer colors paired with a very bubbly/convective appearance) and showed strong divergence. Radar analysis would then help us determine which individual cells to warn on, especially if the area of convection is multicellular and warning the entire thing isn’t ideal.

I didn’t use GREMLIN as much, since this area had good radar coverage. However, I did use it to keep tabs on its performance. The product seems to do well with picking out the strongest discrete/semi-discrete cells and potentially struggles with smaller/shallower storms and mergers.

Using these products, and working as a team with good communication, we were able to successfully warn a tornado in the Norman area along with various severe wind and hail.

– WxAnt

PUB LightningCast and GREMLIN Nowcasting

Posted in DSS, GREMLIN, LightningCast, SPG2025 on June 2, 2025 by Kevin Thiel.

LightningCast

For this first day, I started out looking at Lightning Cast to gain familiarity with version 2 and see how it compares to version 1. The first thing I noticed was in southwest Pueblo County, where there seemed to be fairly frequent lightning. Version 1 in the top left panel (Figure 1 below) actually decreased in probability from 70% to 50%, whereas Version 2 in the top right panel remained at 70%. With both GLM and ENTLN depicting ongoing lightning, I think both versions should be showing higher probabilities. I’m wondering if it’s because both versions are so focused on the convection moving into southeast Pueblo County that they’re less focused on the stratiform lightning/less mature convection?

Figure 1: Four panel comparing LightningCast v1 (left panels) and LightningCast v2 (right panels)

Additionally, I tested out using the LightningCast dashboard for Fowler, CO beginning at 3PM MDT. One interesting thing to note was that it seemed to match better with the version 2 LightningCast in AWIPS versus with version 1, however both versions weren’t too far off. In the Figure 2 below, the left panel (version 1) shows between 30-50% probability of lightning, whereas the right panel (version 2) shows Fowler (purple dot in the image) right on the border of the 70% probability. Comparing that to the dashboard (Figure 3) for the same time, the yellow line (version 1) depicts a 54% probability, with the green line (version 2) showing an 84% probability for 21:18Z. With MRMS reflectivity at the -10C level showing a cell up to 42 dBz just southeast of Fowler, I would tend to lean towards utilizing version 2.

Figure 2: LightningCast v1 (left panel) and LightningCast v2 (right panel)

Figure 3: LightningCast Dashboard

One final note on the LightningCast Dashboard – I thought it was interesting to see that version 1 in Figure 4 below, the yellow line (version 1) shows two separate upticks in lightning probability versus the green line (version 2) showing a steady decline in probability.

Figure 4: LightningCast Dashboard

GREMLIN

I was also able to look at GREMLIN, which was my first time assessing this product. Figure 5 below shows a four-panel, with GREMLIN (top left), MRMS Reflectivity (top right), Satellite IR sandwich (bottom left), and GLM Flash Extent Density (bottom right). Just looking at MRMS and IR, the first cell that draws my attention is the cell in southeast Pueblo County as it has higher reflectivities and cooler cloud tops. The cell in southern Otero county looks like the cloud tops are slightly warming with time. However once we start looking at GREMLIN, those two cells look to go back and forth in reflectivity, leading to less confidence in overall intensity. If I were located in an area with poor radar coverage, or if a radar was down and I had to rely on GREMLIN, it may not be straightforward as to which cell could eventually warrant a warning.

Figure 5: Four Panel comparing GREMLIN (top left), MRMS Reflectivity (top right), Satellite IR Sandwich (bottom left), and GLM (bottom right).

That being said, Figure 6 below shows a screenshot of the same four-panel at 21:41Z, which shows GREMLIN having a pretty good grasp on the convection in Stanton and Morton counties (just outside of the PUB CWA). So in this instance, confidence in the GREMLIN product would at least be higher than the previous example shown.

Figure 6: Four Panel comparing GREMLIN (top left), MRMS Reflectivity (top right), Satellite IR Sandwich (bottom left), and GLM (bottom right).

Final Thoughts for Day 1

Overall I enjoyed testing out both of these products. I definitely want to get more hands-on experience with GREMLIN as well as the LightningCast dashboard in order to see these in different scenarios/environments.

– Fropa

GREMLIN and Lightning Cast – Observational Notes and Feedback

Posted in GREMLIN, LightningCast, SPG2025 on June 2, 2025 by Kevin Thiel.

SYNOPSIS – A broken line of thunderstorms lifted north through SE Colorado in a weakly sheared, high LCL environment with modest instability (1000-2000j/kg MUCAPE) and high DCAPE (1000+ j/kg). This environment appears to favor pulse severe potential, with primarily a gusty/damaging wind risk.

OPERATIONAL NOTES AND FEEDBACK – Using GREMLIN and Lightning Cast Together

I used a 4-panel to compare GREMLIN, satellite, radar, MRMS, and LTG Cast data. I’ve not typically used LTG Cast to nowcast the severity of convection, but when combined with GREMLIN, it kind of reminds me of looking for signals in model data. For the most sustained convection, for example, GREMLIN had a fairly consistent signal of 40-50dBZ echoes in tandem with consistently high LTG probabilities. In the past, I’ve typically just focused on GLM lightning data on its own separate from LTG probs. Overlaying LTG Cast probs with GLM data seems to provide a more uniform / smoothed view of the evolution of lightning within convection as opposed to using GLM on its own. GLM can be jumpy at times, which can give the impression that a thunderstorm is weakening. However, if LTG cast probabilities remain high, it may give the forecaster more confidence that a thunderstorm is not weakening. This seemed to be the case with multiple different thunderstorms in SE CO today.

OPERATIONAL NOTES AND FEEDBACK – GREMLIN

It was interesting to note how closely the increase and decrease in GREMLIN reflectivity was tied to the increase and decrease in lightning. The developers noted that this is to be expected. Since GLM data can sometimes be jumpy, and isn’t always reflective of the severity of a storm at a given moment in time, it might be interesting to see if there is a way to offset this. Perhaps there is some way to mesh GLM data with Lightning Cast data (reference the notes in the observation section about nowcasting convective strengths) or through some other means (longer averaging time, etc.). When GLM data isn’t jumpy, GREMLIN seemed to compare very nicely with MRMS. But, when GLM data was jumpy, GREMLIN seemed to struggle some, showing more rapid increases and decreases in reflectivity that what MRMS showed. As an alternative, I could see where simply overlaying LightningCast data on top of GREMLIN data could provide a more “smoothed” and uniform trend in convection over time, in a way that could still provide useful information for warning decisions.

From an operations standpoint, GREMLIN seemed to provide a great overview of convective evolution, especially when overlaid with LightningCast data. It’s possible this could translate to warning decisions, but this initial runthrough with the product suggests its biggest advantage may be nowcasting the general evolution of convection as opposed to making specific warning decisions. Admittedly this is my first use of the product, and I’m looking forward to trying it in future days of the HWT to see if anything different stands out.

– NW Flow

Day 1 – Analyzing GREMLIN

Posted in GREMLIN, SPG2025 on June 2, 2025 by Kevin Thiel.

GREMLIN

An area of showers and thunderstorms formed across southern Colorado this afternoon. A dryline was just to the east and coincided with a gradient of mixed-layer instability. Thunderstorms developed along an arc from Pueblo county to Baca county and slowly moved north-northeastward before weakening in the late afternoon hours. The Clean IR-CH13 band depicted a strengthening storm to the west while warming cloud tops were occurring over Otero county. GREMLIN picked up the deepening convection while quickly weakening convection to the east.

-Eagle

Storm Initiation for FWD and Gremlin gets Going

Posted in GREMLIN, OCTANE Speed Sandwich, SPG2025 on May 22, 2025 by Kevin Thiel.

From a top down perspective looking at Octane products down to Gremlin and actual radar data from KFWS, it provides a clear picture of what you might want to be looking at as far as what the Octane products can show you and to take a targeted approach to potential warning operations. Taking a look at some satellite data, we can see two areas of interest to the north and then a second one to the west where strong Cloud top Cooling and Cloud top Divergence is seen. In addition to all of this, the greatest gradients from the Octane Speed/Direction images show a rather robust cell to the north and west of the Dallas Fort Worth CWA and this is the cell we will be focusing on.Figure 1: Four panel with Octane Speed Sandwich in upper left corner, Octane CTC/CTD with no smoothing upper right, Octane CTC/CTD High smoothing lower right, and Octance CTC/CTD Mod smoothing lower left. Times were between 1902Z-1932Z

Taking a look at the Gremlin data we can clearly see the MesoSector began to pick up on the previously mentioned cells and further strengthening of these cells located to the north and west of the Forth Worth CWA a few minutes prior to MRMS showing some reflectivities, as this initial CI began to strengthen further. We can also see a bit of a lightning jump with these cells as well.

Figure 2: Four panel with GREMLIN Mesosector on the top right, MRMS reflectivity on the lot left, ECONUS Gremlin with GLM flash density on bottom right and finally GOES Meso sector 3.9 Micron Imagery on bottom left. Times were between 1921Z-1945Z

Finally with radar returns finally noting a strengthening trend with the cell of note to the northwest of the Dallas Fort Worth Metro, and with the previous signals from the stronger cloud top cooling and resultant Divergence, we could infer if these storms did develop in our CWA that they had an increasingly likely chance to become severe within the next 30 minutes or so.

GREMLIN Goes Over 60dBZ?!

Posted in GREMLIN, SPG2025 on May 22, 2025 by Kevin Thiel.

A very interesting thing happened with the GREMLIN tool with today’s supercell southwest of Wichita Falls, TX. After a very strong upward burst in the updraft and surge in lightning activity, the CONUS GREMLIN actually showed simulated returns over 60dBZ, despite the product supposedly having a hard cap at that level. Simulated returns as high as around 64dBZ were noted.

Fig. 1. Clockwise from top left:GREMLIN GOES-East Meso1; MRMS Composite Reflectivity; GREMLIN GOES-East CONUS; GOES-East Meso1 Channel 13 10.35µm IR imagery.

Looking at the loop before and leading up to this time (Fig. 2), we see persistent high overshooting tops in the IR imagery, and even evidence of gravity waves surrounding propagating away from the core of the overshooting top. The Mesosector GREMLIN was a bit less and jumpy, but did eventually show consistently stronger simulated returns. Meanwhile, the CONUS GREMLIN would continue to consistently show strong returns at least approaching 60dBZ even after this loop. The GLM Flash Extent Density (Fig. 2 overlay) does show higher values in the frames leading up to GREMLIN going over 60dBZ.

Fig 2. Same as Fig 1, for a loop from 2024z to 2035z.

Fig 3. Same as Fig 2, with GOES-EAST GLM flash extent density overlaid.

– Marko Ramius

CONUS GREMLIN Handling Convection Worse than MESO for a Change Over Western Montana

Posted in GREMLIN, LightningCast, OCTANE Speed Sandwich, SPG2025 on May 22, 2025 by Kevin Thiel.

Figure 1: CONUS GREMLIN. There is a lot of stratiform in TFX and MSO’s area, though it is not being reflected nearly as much as MRMS indicates in Figure 2.

Figure 2: WMESO GREMLIN. Although the Meso Sector doesn’t extend all the way north, it does a better job representing the arcing line of showers. Perhaps it has a tendency to do this on the edge of the domain?

Not a whole lot is happening across MT right now so I will shift my eyes to TX for a bit. OCTANE seems to be doing great with initiation across a puffy CU field across west TX in Figure 3.

Figure 3: Note the cooling tops across all the CU in the unstable environment. A few have already developed into robust storms.

Back to MT…

Cloud tops have cooled somewhat on the storm entering the southwest corner of the CWA with LightningCast increasing accordingly, Figure 4.

Figure 4: LightningCast V2 seems to be handling this better according to lightning obs.

Figure 5: Associated Radar

ECONUS GREMLIN also appears to have picked up on this well (Figure 6).

Figure 6: ECONUS GREMLIN

Figure 7: ECONUS GREMLIN continues to intensify accordingly with what radar and MRMS have.

Additionally, the parallaxing appears to be quite evident once again as shown in Figure 8 and 9. I’d imagine that it’s due to the ECONUS sector, but I am not totally sure. This would be a major issue in my CWA with flooding ops as the warnings tend to be very specific over slot canyon locations, and this could lead to a false alarm or a missed event entirely if we are solely relying on GREMLIN.

Figure 8: Notable parallax issues compared to where my SPS was issued.

Figure 9: Location of the storm according to the KTFX radar.

Warning Operations with Satellite Data Only

Posted in GREMLIN, OCTANE Speed Sandwich on May 21, 2025 by Kevin Thiel.

In the absence of radar data, satellite data can be utilized for warning decision-making by utilizing a suite of satellite-derived products.

GREMLIN (synthetic composite reflectivity) used in tandem with OCTANE can depict an area of strengthening convection that would be capable of producing hazardous weather.

In the example shown below, OCTANE indicated strong cloud top divergence and GREMLIN showed increasing reflectivity, which resulted in increased confidence that convection is intensifying (as shown in this case). As a result, a special weather statement (SPS) was issued for a strong thunderstorm producing small hail and gusty winds. This storm would initially produce half inch to nickel hail.

Later on, cloud top divergence intensified further on the same thunderstorm:

Eventually, this storm would produce severe hail. This demonstrates the utility of monitoring satellite trends. If you have storm reports, you can use those trends to calibrate yourself on warning-decision making.

-Vrot

Day 3 – The Train Is On The Tracks OCTANE

Posted in DSS, GREMLIN, LightningCast, OCTANE Speed Sandwich, SPG2025 on May 21, 2025 by Kevin Thiel.

I definitely felt more comfortable using OCTANE today and now that I understand it better, I was able to look at it compared to 10.3 imagery and I found that useful in differentiating which storms were still strengthening.

Jason showed us how to play around with the color tables for the OCTANE product and basically invert them so that the purples/pinks (cooler colors) represent cooler cloud tops, and the yellows/oranges/reds are the divergence. Conceptually I think this makes more sense to me putting cooler colors = cooling cloud tops.

GREMLIN

I feel like I saw very mixed results with GREMLIN with this event. There were times when the meso performed the best I’ve seen it all week (below)

…and then just two minutes later, not so much.

I’m not sure what attributed to the sudden drop off on what on radar appeared to be the strongest storm.

Later, GREMLIN seemed to be doing very well with the areas of more stratiform precip, which I don’t believe I’d gotten to see up until this point. Was curious if it typically does better in that type of environment.

LightningCast

I wish I had grabbed more of the LightningCast plots since it was probably the product I was looking at the most since I was doing the DSS messages, but the plot below was the only one I did grab.

I was curious about the sudden dip in the V2 product because I don’t think I’d seen it be lower than both V1 before.

– Lightning McQueen

Calibrating Satellite Signatures Prior to Small Hail Report

Posted in GREMLIN, OCTANE Speed Sandwich, SPG2025 on May 21, 2025 by Kevin Thiel.

A handful of severe thunderstorm warnings were issued for the Pittsburgh CWA by our team today given upticks in the satellite signatures. A pea-sized hail report came in at 1951z and this allowed us to calibrate the satellite signatures to the intensity of the storms. Thus, given the uptick in both Gremlin and Octane, we were able to realize that it would take a bit stronger of a signal to warrant large hail and potential a downstream warning.

Figure 1: Gremlin East CONUS reveals a rapid uptick in dBZ just before the pea size hail report at 1951z.

Figure 2: OCTANE already revealed CTD present in the part of the line that produced the hail report. Additionally, the cloud tops seemed to cool more just before the hail report.

– Aurora Borealis

In addition to my colleagues above, while in warning Ops today, it was noted as a group that after some initial calibration of what was going on weatherwise and looking at the Octane Products, the Octane products look to have at least done a slightly better job at noting which convection was just weak enough to not produce hail or wind reports while radar data might have struggled slightly more with pointing out wind and hail threats overall and what storms were the strongest.

Figure 3: Cells on the southern end of the map in the (orange circle) had more significant cloud top cooling but was much shorter lived and cloud top divergence was also slightly weaker. These storms failed to produce more than pea sized hail and no wind reports. While storms noted further north (pink circle) while they had slightly weaker cloud top cooling with a more sustained cloud top divergence was responsible for some wind reports and potentially a tornado.

Meanwhile focusing on one of the radar related four panel products I tend to use in warning ops when differentiating between what storms to look at, the farther south storms, tended to have higher Vertically integrated Ice, higher MESH values, and overall looked slightly more structured than storms at the far northwestern periphery of the area. However, the further north storms are the ones that ended up producing winds reports which correlated with the better divergence aloft signal found in figure 1.

Figure 4: (Panel Contents from Upper left to Lower left: Upper left panel is RALA, upper right panel is MESH, lower right panel is 30 min rotation tracks, and lower left panel is Vertically Integrated Ice.

So overall it appears at least in this instance, the trends in upper level divergence as well as cloud top cooling may have been a slightly better indicator for potential severe thresholds of storm activity across the PBZ CWA.

-Sting Jet

EWP Blog

Spring Experiments & Beyond

Category: GREMLIN