On Friday, January 9th 2015, areas of freezing rain fell across the southern and central Coastal Plain of North Carolina during the pre-dawn hours. The precipitation was driven by convergent low-level flow in a region of isentropic up-glide that resulted in a small region of ascent and saturation across portions of southeastern North Carolina. With surface air temperatures in the mid to upper 20s, following a period of 24 to 36 consecutive hours of sub-freezing temperatures, the ground across the area was largely frozen. This resulted in icy conditions on many roadways with numerous car accidents across Sampson, Bladen, Duplin and nearby counties during the morning.
The precipitation developed a little after 08 UTC between Fayetteville and Wilmington NC, with the areas of freezing rain expanding and moving northeast. The coverage of precipitation peaked at around 10 UTC (left panel of the image above) with the precipitation generally weakening and exiting the area to the northeast at 12 UTC (left panel of the image above). Of some concern was the area of reflectivity that was expanding near and to the southeast of Raleigh at around 12 UTC, highlighted in the yellow circle.
Given the air temperatures near 30 degrees in the Raleigh area and the history of car accidents produced by the freezing rain in the Coastal Plain, forecasters were concerned that this area of expanding reflectivity may result in accumulations of freezing rain near Raleigh and the potential for more car accidents. The morning RAOB from 12 UTC at Newport NC (MHX) shown above, highlights the environment across southeastern North Carolina with an above freezing layer centered near 925 hPa as well as a shallow, subfreezing layer near the surface with a surface temperature of 26 degrees.
Forecasters during the event noted that the character and structure of the reflectivity signature near and southeast of Raleigh, seemed peculiar as it appeared to expand in all directions. The forecasters used dual-pol radar data (see image to the right) to examine the radar returns near Raleigh, highlighted in the yellow circle, more closely. In the dual-pol radar imagery below from KRAX at 1200 UTC on 09 January 2015, the reflectivity in the upper left included returns of up to 19 dBZ. The ZDR values in the upper right varied considerably but were sampled at -1.75 dB. The CC values in the lower right were very low and ranged and were sampled at 0.43 while the KDP values were removed because of the low CC values. This information, especially the very low CC and low ZDR indicated, indicated that the these returns were from birds taking off at daybreak and not a developing or expanding area of freezing rain. The dual-pol products were a considerable resource for forecasters and they allowed forecasters to correctly avoid unnecessary forecast updates or special weather statement issuances.