Understanding the Radar Situation Around Tullahoma
The rain radar network over middle Tennessee has gotten a lot better in the past decade, but if you're relying on it for anything precise, you need to understand how the pieces actually fit together. Tullahoma sits in a gap zone between two NEXRAD sites — Nashville (KMWX) to the north and Birmingham (KBHM) to the west. That matters more than most people realize. When I first started tracking storms around here in the early 2010s, the hourly forecast model runs were useless past six hours out. The 0z and 12z NAM nested models would agree on the general direction of a front moving through the Highland Rim, but precipitation timing and intensity were all over the place. What changed things was learning to layer multiple data sources instead of trusting any single feed. Here's what actually works. You start with the WRF-DA model output from the National Weather Service in Nashville. It runs four times daily and has a 2.5-kilometer grid spacing for the eastern portion of their forecast area. Tullahoma falls right inside that sweet spot. The hourly precipitation probabilities from this model are generally reliable up to about 36 hours out, which is far better than the official NWS hourly forecasts that tend to smooth everything into broad strokes.
I run a custom script that pulls the model data, overlays it on the KMWX radar mosaic, and flags areas where the model and radar disagree by more than two millimeters per hour. When they agree, you can usually trust the timing within a fifteen-minute window. When they disagree, you're looking at either orographic enhancement the model is missing or a dry microburst situation where precipitation evaporates before hitting the ground. Both happen frequently in the Tullahoma area because of the ridge lines running northeast to southwest through coffee county. The trick most people miss is understanding beam blockage. The KMWX site has elevation angles of 0.5, 1.4, 2.4, and 3.3 degrees. At the low angles, the radar beam is about two kilometers wide by the time it reaches the Tullahoma area, which means the lowest elevation scan is mostly useless for anything smaller than a full line of storms. I learned this the hard way during a June 2019 event when the radar showed zero reflectivity over Tullahoma while actual rainfall at my place was coming down at forty millimeters per hour. The storm was below the beam. It was a textbook case of a shallow cold-sector shower that the radar geometry couldn't catch. My workaround was to subscribe to the NOAA weather radio on frequency 162.525 MHz for the Tullahoma county warning area and cross-reference the spotter reports with the radar. When the NWS office in Nashville issued a small craft advisory for Percy Priest Lake, I knew the winds around Tullahoma were probably picking up even if the radar couldn't see it. That radio feed is about as real-time as it gets for this area.
For the hourly forecast piece, I also pull the HRRR model data directly from the Earth Systems Research Laboratory. It refreshes every hour with a 3-kilometer resolution and a four-hour spin-up time. The HRRR is noticeably better at capturing the development of afternoon convection along the ridge lines south of Tullahoma. But it has a known issue with over-scoring precipitation intensity during the first hour after each refresh cycle. The fix is to ignore the first twelve minutes of any given HRRR run and treat everything from minute thirteen onward as valid. That removes most of the artifact noise. If you want a complete setup, the key ingredients are: the NWS Nashville WRF-DA hourly precipitation field, the HRRR for short-term convection, the KMWX radar mosaic with elevation angle awareness, and the NOAA weather radio for ground truth. Put them together in a single dashboard and stop expecting any one source to be right all the time. That's just not how this geography works.
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