2. Basic Characteristics and Forecast The 500-hPa pattern for this event featured a deep low centered over Idaho. A composite analysis of past tornado events in south central and southeast Montana was done prior to the event, and it and revealed that a pattern with a deep 500-hPa low in the Pacific Northwest is very common with tornadoes in the area. Discrete supercells formed along a well-defined surface wind shift in an environment that had surface dewpoints over 60° F, which is another strong discriminator of area tornado events based on the composite analysis. Forecasters recognized the strong correlation with past events, which helped them create accurate local outlooks with confidence. The 00 UTC Storm-Scale Ensemble of Opportunity (SSEO) from the Storm Prediction Center depicted potentially extreme Updraft Helicity (UH) values across the region. The maximum UH values from any member in the ensemble were 500 m 2 s -2 and the neighborhood probability of UH >100 m 2 s -2 was 60%. Individual models in the SSEO guidance, like the NSSL WRF-ARW, depicted storm evolution well, albeit too slowly. Southeast Montana’s Rare EF3: Predicting and Warning for the Carter County Tornado Chauncy J. Schultz NOAA/National Weather Service Billings, Montana, Weather Forecast Office 1. Introduction The highest-rated tornado in the Billings, Montana, National Weather Service County Warning Area occurred on June 17 th, This tornado was well-forecast from the outlook through the warning phase. 4. Radar Evolution and Climatology Reliable storm chaser and spotter reports were used in tandem with radar data to correctly determine and convey the threat. A radar climatology of other EF2+ tornadoes in Montana and Wyoming revealed that most events have peak low- level rotational velocity values of at least 50 kt, even though the 0.5° elevation angle of many of the events was over 5,000 feet above radar level. 5. Storm-Scale Ensemble The Carter County EF3 was well-forecast and well-warned because of pre-event knowledge on favorable synoptic-scale signals for tornadoes in the area, use of the SPC SSEO, and recognition of the near-storm environment and radar signatures. Future events in the region can benefit from climatologies of peak low- level rotational velocity values of significant tornadoes and SPC SSEO output. Forecasters should be particularly concerned about significant tornado potential when peak low-level rotational velocity values exceed 50 kt in near-storm environments that favor tornadogenesis. Similarly, significant severe events are more likely when the SPC SSEO has a high probability of maximum UH values greater than or equal to 100 m 2 s Conclusions QUICK FACTS RatingEF3 Path Length10 miles Path Width0.5 miles Injuries0 Fatalities0 Can the SSEO help calibrate outlooks? The high-end UH values ahead of the Carter County EF3 tornado caught forecasters’ attention and raised their confidence in a significant event. However, forecasters weren’t sure how often values of that magnitude can be expected from the SSEO. In order to answer that question and to help calibrate forecasters to the SSEO output, a climatology of member-maximum UH and the probability of UH >100 m 2 s -2 for all warm-season 00 UTC SSEO simulations over the northwestern High Plains from April through September was developed using all available model runs from 2014 and There is a significant correlation between event-total severe reports and increasing values of max UH and the probability of UH >100 m 2 s -2. Forecasters may only see signatures like those simulated ahead of the Carter County EF3 about once a year, on average. There was only one event in the climatology with similar values. That event included an EF2 tornado and 6” diameter hailstones. The peak low-level rotational velocity determined using the 0.5° elevation angle data (which had a beam height of approximately 12,000 feet above radar level) underwent several cyclic undulations during the life cycle of the parent supercell. However, values generally increased from 2000 UTC to 2048 UTC, when the peak magnitude of 66 kt was observed. Rotational velocity remained above 50 kt for 39 full minutes, during which the tornado was at its largest size based on several spotter reports. Question: Can Regional Radar and SPC SSEO Climatologies Assist with Messaging? 3. Near-Storm Environment The tornado occurred in an environment with MLCAPE around 2500 J kg -1, a low MLLCL height, and moderate low-level wind shear. The degree of instability in this case was quite anomalous for the region. Moreover, the effective-layer Significant Tornado Parameter (STP) was 2 at the time that the EF3 tornado occurred. Forecasters used knowledge of the environment to quickly issue a tornado warning for the storm as soon as it attained supercell characteristics, rather than beginning with a severe thunderstorm warning and later upgrading, as is often done operationally. Parameter Carter County EF3 Environment Mean Montana Value based on RUC Mesoanalysis Data for 35 Tornadoes MLCAPE2500 J kg J kg -1 MLCIN-20 J kg J kg -1 ML LCL Height1000 m AGL1386 m AGL 0-1-km SRH125 m 2 s m 2 s km Bulk Shear 15 kt16 kt 0-6-km Bulk Shear 40 kt46 kt The tornado initially touched down at 2027 UTC in Carter County in far southeast Montana. The tornado was on the ground for just over 60 minutes (until 2130 UTC) and was well-documented by several storm chasers and spotters. The tornado destroyed a mobile home and a small, old A-frame school house, leaving behind only the foundation. Six vehicles were picked up and tossed up to 200 yards, and the tornado snapped about 20 power poles. Finally, a swath of trees was flattened in the Custer National Forest, on the north end of the damage path. Two horses were killed by the tornado. Damage Photos by NWS Billings Maximum 24-Hour Updraft Helicity 24-Hour Probability of Updraft Helicity >100 m 2 s -2 NSSL WRF-ARW 1-km AGL Reflectivity at 22 UTC 24-Hour Probability of Updraft Helicity >100 m 2 s -2 Maximum 24-Hour Updraft Helicity Storm Reports for 19 June 2015 KUDX 0.5 ° Reflectivity at 2056 UTC KUDX 0.5 ° SRM at 2056 UTC KUDX 0.5 ° Reflectivity at 2110 UTC KUDX 0.5 ° SRM at 2110 UTC Photo courtesy Ryan Lueck Carter County EF3 Tornado Tornado at 2112 UTC (Ryan Lueck)