Moisture across the northern Plains, upper Great Lakes into northern New England will likely bring a period of snow, sleet and freezing rain through this weekend. Meanwhile, heavy rainfall continues along the Gulf Coast with areas of flooding. Fire weather conditions continue for the areas of the Plains, southern Appalachians into portions of Florida. Severe thunderstorm potential increasing. Read More >
Overview
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Mid-afternoon on August 16th a strong supercell thunderstorm, which had already caused extensive crop damage in southern Saguache County, spawned a tornado northeast of Alamosa. The tornado was witnessed by many, and occurred around 8 miles east-southeast of Mosca (2 1/2 miles south of San Luis Lake), in a rural area. The tornado only lasted a few minutes, but, remarkably targeted and destroyed a structure...an unoccupied mobile home. This was an unusually strong supercell for the San Luis Valley. Crop damage in southern Saguache County totalled at least $42 million. |
Zoomed-Out Location of Tornado
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Photos & Video:
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| Large Wall Cloud and Tornado | Image of the Tornado | Wall Cloud Forming | Base of the Supercell |
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| Damage of an Unoccupied Mobile Home; Image Courtesy of Willard Williams | Damage of an Unoccupied Mobile Home; Image Courtesy of Willard Williams | Damage of an Unoccupied Mobile Home; Image Courtesy of Willard Williams | Large Hail in Respect to the Dollar Bill |
Radar:
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| Base Reflectivity and Base Velocity Volume Scans from KPUX Radar |
Environment
The Alamosa tornadic event was a pretty rare event, since tornadoes generally do not form in the San Luis Valley. In this case, the environment was just right to produce a mesocyclone based tornado. The SPC's Convective Outlook maps did not hint at a threat for tornadic weather, but did include the state of Colorado under a "See Text" contour. The text described that there was a chance for thunderstorm development, but again, did not hint at a tornadic environment. There were a few hints, though. Looking at the 6:00 AM MDT upper-air sounding from KABQ indicated that just south of the San Luis Valley, the helicity values were at 289 m^2/s^2, which indicates that the environment was sheared enough to produce a rotating storms. Illustrated in the 6:00 AM MDT absolute vorticity plot from Plymouth State University, there is indeed a region of high vorticity located to the west of tornadogenesis. The differential absolute vorticity advection is one of the main lifting mechanisms, the other source of lift, which can be analyzed from the SPC upper-air maps, is the ageostrophic circulation of a jet streak just to the west of where tornadogenesis occurred. The location of tornadogenesis is located in the poleward exit region of the linear jet-streak, implying upper-level divergence. The only variable missing from this tornadic equation, is the instability. There were no good signs of surface warming during the day, or upper-level cooling, therefore, it is implied that there were small values of CAPE, since a supercell was produced. The combination wind shear, implied low CAPE, and the vorticity maximum/jet streak location was enough spawn a mesocyclone based tornado in the San Luis Valley.
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| 6AM MDT SPC Outlook |
6:00 AM MDT SPC Upper-Air Maps
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| Surface Analysis | 850-hPa Analysis | 700-hPa Analysis |
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| 500-hPa Analysis | 300-hPa Analysis |
6:00 PM MDT SPC Upper-Air Maps
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| Surface Analysis | 850-hPa Analysis | 700-hPa Analysis |
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| 500-hPa Analysis | 300-hPa Analysis |
6:00 AM MDT Skew-T Soundings (From Plymouth State University)
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| KABQ Sounding | KDDC Sounding | KDNR Sounding |
6:00 PM MDT Skew-T Soundings (From Plymouth State University)
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| KABQ Sounding | KDDC Sounding | KDNR Sounding |
Absolute Vorticity Analysis at 6:00 AM MDT (From Plymouth State University)
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| Absolute Vorticity Plot over the U.S. Southwest |
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