First, I need to note that I was not on the forecast desk on December 24th, but I did help with the forecast during a few of the days preceding the event. The blog post below is not intended to be an in-depth summary of the event but an opportunity to share a few images and comments about the event.
This event was characterized by a highly amplified upper trough across the Mississippi Valley during the afternoon of 12/24 with strong winds through a deep layer of the atmosphere (see image to the right). At the surface, deepening low pressure was moving across the Ohio Valley during the afternoon of 12/24 with a cold front extending south across the Tennessee Valley into the northern Gulf of Mexico while an inland penetrating warm front was moving across the Carolinas.
The 30 to 33 hour forecast of the SHERBS3 parameter valid at 18 UTC on 12/24 from the GFS, NAM, SREF, and Canadian models are shown to the right as depicted in AWIPS-2. The SHERB is a composite parameter composed of the surface to 3-km shear magnitude, the 0-3-km lapse rate, and the 700–500-hPa lapse rate intended to separating significant High-Shear Low CAPE Event (HSLC) severe reports from non-severe HSLC storms using a threshold of 1(with significant severe weather more likely for SHERB above 1).
The 36 to 39 hour forecast of the SHERBS3 parameter valid at 00 UTC on 12/25 from the GFS, NAM, SREF, and Canadian models are shown to the right as depicted in AWIPS-2.
Two areas of elevated SHERB values can be seen in the forecast images, the most prominent is located across the upper Tennessee and Ohio Valleys and another located across the eastern Carolinas. From the various sets of model guidance, the SHERB was most consistently forecast to be greater than 1 across eastern Kentucky, Ohio and West Virginia. The SHERB was forecast to be near but generally below 1 across the eastern Carolinas. The SPC Day 2 Outlook also included these areas in the marginal severe weather threat.
Regional radar imagery from 1630 UTC (top) and 2230 UTC (bottom) on 24 December, 2014 is shown to the right. The convection across the Carolinas occurred earlier in the day and was multi-celluar. The Ohio Valley convection occurred later in the day, primarily as a narrow convective line. HSLC severe weather was observed in the Ohio Valley along with a single severe weather report in southeastern NC. There were two tornadoes reported on 12/24 including an EF1 tornado in Castle Hayne NC at 1530Z and an EF0 tornado in Lancaster, OH that occurred at 2216Z. In addition, there were more than 20 thunderstorm wind damage reports in the Ohio Valley. A full summary of storm reports are available from the SPC web site.
The 0-6km shear values were very high during the times in which severe weather occurred with shear values ranging between 50 and 60 kts across southeastern NC and generally between 80 and 100 kts across the Ohio Valley. The SPC mesoanalysis of 0-6km shear values valid at 19 UTC on 24 December (approximately halfway between the two tornado events) indicate the strong deep layer shear across the eastern United States. Surface-based instability was very limited. The SPC mesoanalysis SBCAPE values were very low across both areas, analyzed at less than 250 J/Kg.
Plots of the SPC analyzed SHERBS3 parameter at 1600Z near the time of the Castle Hayne, North Carolina EF1 tornado at 1530Z (left side of first image), and the SHERBS3 parameter at 2200Z near the time of the Lancaster, Ohio EF0 tornado which occurred at 2216Z (left side of second image), along with all of the severe weather reports for that day (right side of both images).