RAH:
SHORT TERM /THURSDAY AND THURSDAY NIGHT/... As of 500 AM Wednesday... ...Rain is expected to change to snow Thursday night before tapering off pre-dawn Friday... What has changed: Reduced snowfall amounts a bit across the southeastern zones and focused the peak accumulation amounts (1-2") a bit further north and west to better coincide with latest model trends. Our forecast remains much lower than most raw forecast guidance due to numerous limiting factors, including; (1) Warm surface temperatures, (2) Self-limiting CAA processes, and (3) Low rain to snow liquid ratios. Impacts should remain rather minimal, however, a few slick spots may form on area roadways during the overnight hours Thursday into early Friday morning, primarily along and east of US-1. If the forecast holds true, a Winter Storm Advisory issuance may be needed in future forecast packages (Our criteria is areas that are expecting to see 1" of snow or greater in a 12 hour period). Full Discussion: The surface front will settle in a nearly west to east orientation in vicinity of the SC/GA border, with ongoing surface CAA processes occurring north of the boundary. The front is progged to buckle back north as a series of weak disturbances begin to develop during the day Thursday, with strong coastal cyclogenesis beginning to initialize off the SC coast as early as Thursday evening. Around the same time, strong Canadian surface high pressure will continue to settle southeast through the central Corn Belt states, extending east through the Great Lakes and northeastern US states before beginning to south along the lee side of the Appalachians. Location and strength of the developing coastal low/trough will be key to the forecast going forward, as a stronger and closer to shore area of low pressure (NAM) will help to promote more rapid cold air entrainment from the north. Forecast models continue to trend increasingly wetter during the daytime hours Thursday, primarily as a result of strong convergence in the 850mb level where steepening temperature gradients and FGEN banding sets up in vicinity of the US- 64 corridor. Around this same time, a 925mb weakly defined cold front (marking the leading edge of the cold nose) will extend south into the area. This will aid, at least initially, in the ability for wet bulb/evaporative cooling processes to promote a light rain to snow changeover to occur in vicinity of the VA/NC border mid to late afternoon. From there, a gradual changeover to a rain/snow mix to eventually all snow will occur. At first, accumulations will remain little to none with more of a "white rain" type scenario unfolding. Eventually, especially areas east of US-1 where rain rates will remain higher for longer, the wet-bulb effect will help to cool surface temperatures below freezing, helping to promote accumulations primarily on grassy and elevated surfaces. Best timing for this will be between 02z/9pm Thursday through 10z/5am Friday morning. Snow amounts continue to remain highly uncertain in this event, however, thinking that raw model output remains way too high in this scenario. Several limiting factors remain at work, including: (1.) Persistent above freezing surface temperatures that will help temper/melt the majority of the snowfall. The exception to the rule may be areas closer to the VA/NC border, however, these are the areas that will likely see far less QPF throughout the event (2.) 10:1 snow to liquid ratios are not likely in an event like this, expect closer to 3:1 at precipitation changeover onset, 5:1 toward the middle of the event, and perhaps ending in vicinity of 10:1 if we are lucky. (3.) While the CAA process will be key in the original changeover, it will also become the ultimate limiting factor through the frozen ptype portion of the event. Gradual drying is expected to persistently take place in the lower-third of the boundary layer as the cold air slides south into the region. This will work to greatly reduce QPF from northwest to southeast in general. Higher amounts of QPF in the southeast will remain, however, dewpoints in the middle 30s will keep widespread snowfall accumulations from occurring. With these limiting factor in mind, have shifted the geographic location of accumulations a bit further north and west. Amounts are down a bit also with this run, topping out in the 1 - 2" range in northeastern Coastal Plain. From there, if you follow the US-64 further west, expect accumulations in vicinity of an inch with lesser amounts as you progress west of US-1 and south of US-64. Alterations and fine tuning to the accumulation forecast is highly likely to continue with sequential model runs and forecast packages, but still finding it highly doubtful that significant snowfall amounts will occur across central NC. For the snow lovers out there, the canary in the mine that could indicate the potential for higher amounts lies in surface dewpoints Thursday afternoon. If dewpoints become significantly cooler, a possibility advertised by the recently cold-initialized NAM, wet-bulb cooling processes may promote an earlier changeover and perhaps higher amounts somewhere across central NC assuming the moisture is available. Precipitation should clear out of central NC rapidly pre-dawn Friday leaving sub-freezing temperatures and a light northerly breeze in its wake until sunrise. With sustained northerly winds in the 6 to 10mph range, thinking that drying processes in the coldest areas should limit the black ice/flash freeze potential to only patchy areas during the overnight hours. Sunshine and a breeze will help to clear and dry any leftover frozen surface moisture rather quickly Friday morning.
