A mature upper level cyclone moved across the Deep South on Thursday, January 17th 2013. The upper-level low enhanced surface cyclonegensis across the Southeast with the surface low deepening and maturing during the afternoon and evening hours as it moved into the southern Carolinas. An area of light to moderate rain spread into central NC during the late afternoon with above freezing surface temperatures. As colder air aloft moved into the region and precipitation rates intensified, the precipitation changed to mostly wet snow before ending as mainly light rain or drizzle.
The change over from wet snow to rain/drizzle occurred as a dry slot associated with the upper level cyclone moved across the region and the precipitation intensity decreased and saturation in the ice nucleation region ended. The images below are the KRDU RAP forecast soundings valid at 05 UTC (1200 AM EST) and 06 UTC (100 AM EST) on January 18. The 05 UTC sounding shows a sub-freezing thermal profile (except at the ground) with saturation in the SFC-500 hPa layer (extending well into the ice nucleation zone). The observed and anticipated precipitation type was snow. The 06 UTC sounding again shows a sub-freezing thermal profile (except at the ground), however, saturation is largely confined to the SFC-800 hPa layer where temperatures are +1C to -5C and ice is not likely contained in the clouds. The observed and anticipated precipitation type at 06 UTC was rain or drizzle.
The short movie below provided by NWS forecaster Brandon Vincent shows the end of the precipitation in Apex NC between 12:14 AM EST and 12:30 AM EST on January 18 as the wet snow ended as light rain and drizzle. The top part of the movie shows a video of the precipitation in Apex with the video synched with KRAX radar imagery and the time stamp. The location of the video is shown in the radar imagery by the range rings near the center of the radar products. The radar product in the lower left is the KRAX reflectivity (Z) at 0.5 degrees and the other radar product is the dual-pol correlation coefficient (CC) at 0.5 degrees. The changeover occurs as the intensity of reflectivity decreases and more telling, the CC values decrease from 0.97-0.99 (plum color) to 0.88-0.95 (orange and yellow) colors. This change in the CC indicates a change from a more uniform precipitation type snow to more mixed precipitation and eventually just rain. As the short term forecast soundings suggest, this likely occurs as the cloud depth and precipitation rate decrease and the clouds lose their glaciation.
High-resolution movie (29MB) – http://www4.ncsu.edu/~nwsfo/storage/cases/20130118/2013.01.18.comparison.hq.wmv
Low-resolution movie (12 MB) – http://www4.ncsu.edu/~nwsfo/storage/cases/20130118/2013.01.18.comparison.lq.mp4