A methane explosion occurred last Friday, January 20, in rural northwest Alabama (story from WAFF-TV). NWS Huntsville provided decision support services to the incident, which posed significant risks to emergency personnel. The active pattern last weekend created additional concerns, since several rounds of rain and thunderstorms were forecast to move across the area (though fortunately the significant severe weather from that weekend remained well to the south).
One such event arrived Saturday morning as stratiform rain pushed back into the area. Forecasters noted that there were indications of cloud-to-ground lightning from the National Lightning Detection Network along the leading edge of the rainfall, so we leveraged flash extent density data from the North Alabama Lightning Mapping Array to investigate further. Strangely, when loaded as an image in AWIPS-2, this showed little.
It took some time to discover why. The flash rates were so low (1 flash per ‘scan’) that the FED image interpolation was smoothing the data below what the color curve could visualize. After the interpolation was turned off or the color curve edited again, the flashes were much more apparent, as seen in the following GIF loop from AWIPS.
Adding the full flash extent density information from the NALMA helped the forecasters to visualize the lightning threat beyond what was otherwise available in AWIPS. This helped when it came time to brief emergency personnel on the approaching threat.
This event also helps to reinforce the potential utility of the Geostationary Lightning Mapper (GLM) aboard GOES-16 as it becomes available this spring. However, forecasters will have to visualize the GLM data wisely. It will likely more important to view low flash rates for an IDSS or safety mindset, versus higher flash rate changes for severe weather. Even with total lightning, context is everything.
NWS Huntsville is providing Impact-Based Decision Support Services (IDSS) to protect life and property at an outdoor sporting competition in the Decatur, Alabama area this week. A decaying Mesoscale Convective System (MCS) moved across north Alabama this afternoon, forcing a delay in the competition for several hours. While the North Alabama Lightning Mapping Array (NALMA) helped determine what to tell local emergency managers about the start of the lightning threat, the NALMA really shined in trying to figure out when the lightning threat would end.
The example images include NALMA Flash Extent Density data, which are represented by irregular pink and purple shapes displayed over the KGWX radar reflectivity. Both the 1949 and 2007 UTC indicate scattered very low flash rates extending over a broad area–including the Decatur area–suggesting occasional in-cloud flashes within the trailing stratiform region of the MCS. This is a known threat with MCSs, but it was not clear at the time how long the lightning threat would persist. Use of total lightning information from NALMA enabled NWS Huntsville staff to determine that the lightning threat would not subside until rain subsided.
With the launch of GOES-R and the Geostationary Lightning Mapper, these kinds of data will improve lightning-based IDSS across a much wider cross section of the CONUS.
The cold air outbreak over the eastern United States had impacts far and wide, including the development of snow showers all the way into northern Alabama. However, between unseasonably low 850 mb temperatures and northwesterly flow, the outbreak also caused a semi-persistent band of snow to develop along the Tennessee River (downwind of a reservoir known as “Lake Wheeler”).
While most of the river-effect monitoring occurred with radar, the late-morning MODIS overpass captured one of the narrow river-effect bands (and did so more effectively than the lower-resolution GOES-East Imagery).
Snowfall reports from underneath the band have indicated 2 to 3 inches of snow, compared to the 1-2 inches reported with heavy or persistent snow showers elsewhere. Unfortunately, orbit timing and cloud cover have not allowed us to view the snow swath using the Snow-Cloud RGB. However, the Snow-Cloud RGB from the edge of this morning’s MODIS pass still illustrated the river-effect band persistence.
The Huntsville office has a long history of using total lightning information from the North Alabama Lightning Mapping Array (NALMA) for warning decision-making. Since 2003, WFO Huntsville has been ingesting and receiving a source density product from NASA SPoRT. However, recently, we decided to begin migrating to Flash Extent Density (FED) data; this is more consistent with the Geostationary Lightning Mapper, more consistent with recent operational research, and easier to convey and understand. Typically we are trying to apply the “two-sigma” lightning jump algorithm suggested by Schultz et al. (2009, 2012).
On June 8, a weak front moved across the Huntsville forecast area, initiating development of strong to severe thunderstorms. An 1800 UTC sounding from Redstone Arsenal indicated a relatively high threat for wet microbursts.
One of the storms moved across extreme southern Jackson and northern DeKalb Counties in northeast Alabama. I was viewing the NALMA FED data and watched as the storm went from less than 10 flashes, to 40 flashes, in three “scans” (from 2214 to 2218 UTC). (Interestingly, despite using SAILS with the KHTX radar, AWIPS-2 matched all three lightning images to a single 0.5-degree radar scan.)
Since we cannot get the formal lightning jump algorithm into AWIPS-2 at this time, forecasters need to do some quick mental math to decide if jumps such as these constitute a real jump. I was certain this did (and later Excel work verified this) so I issued a severe thunderstorm warning, despite the storm being very close to the Georgia state border.
This storm produced structural damage in the Cartersville community near the state line shortly after the warning was issued, tearing the roof off of an apartment complex and downing trees and powerlines. There was not much lead time (there rarely is with these kinds of storms) but this reinforces our past experience with total lightning–and reinforces that lightning may be especially useful during a challenging warm season warning environment.
One night after a widespread dense fog event, we have been monitoring more fog formation very closely. Unlike the previous night, the visibility has not fallen quite so far, so fast at most of the airports across the Tennessee Valley; just a few sites in typically fog-prone valleys are reporting visibility of less than 1 mile. However, coverage is the question, and the default 11-3.9 micron satellite imagery was not particularly helpful in diagnosing that. There are hints of fog in the valleys of northeastern Alabama, but it’s tough to be sure how widespread the fog might be.
The Nighttime Microphysics RGB imagery was much, much more useful–and confirmed what the surface observations were telling us. The 0802 UTC pass indicated that much of the fog is confined to the river valleys in and around the Huntsville CWFA, especially in the northeast Alabama valleys and the Elk River around the Tennessee-Alabama border (near where the mouse pointer is located). Furthermore, the fainter gray-cyan colors surrounding the Tennessee River (bisecting the CWA) supported some of the less-dense fog reports coming from airports such as Muscle Shoals (KMSL) and Huntsville (KHSV).
This imagery helped confirm the surface observations, and helped with the decision to avoid a widespread dense fog advisory–at least temporarily.
Total lightning is often useful for situational awareness heading into the heart of the Southeast convective season. Typically, by late May, nearly all echoes on radar are producing lightning, and data from the North Alabama Lightning Mapping Array (NALMA) help to assess which of the cells require further attention.
This year was a little different.
On several days during the last week of May, a subtle warm layer and large dry layer aloft helped to cap the atmosphere, or at least limit vertical growth, across northern Alabama and southern middle Tennessee. We observed scattered “thunderstorm” development with 50 dBZ echoes at 0.5 degree elevation–but higher radar tilts yielded very limited vertical structure with these cells (very low reflectivity beyond about 2.4 degrees). While reviewing data from the NALMA, we realized that total lightning was giving us a clue–few, if any of these cells had any total lightning at all, as you’d expect with such shallow convection. The stronger cells with greater vertical depth (warranting further interrogation) were the only ones producing any total lightning whatsoever.
So, in this situation, total lightning data still provided situational awareness–but in a slightly different way. Instead of looking for the storms with the greatest flash rates or source densities (or changes thereof), we were looking for storms with ANY flashes or sources. However, since then, we’ve been aboard the Mesoscale Convective System train, and we’ve returned to our traditional uses of NALMA data.)