A major winter storm is expected to impact the southeast US and the Triangle region of North Carolina this evening into Saturday morning and afternoon. As with most winter storm events of this magnitude and in this region, a serious concern is not the amount of precipitation, but precipitation type. The impact of this event will be strongly influenced by the timing of the changeover from rain to sleet to snow. Earlier forecasts suggested that warm-air aloft could be enhanced in advance of the low pressure system setting up along the coast. It appears that this warm layer is now eroding, keeping the air aloft below freezing.
We can utilize North American Model (NAM) guidance as an example (courtesy of Plymouth State University) model forecast sounding. First, a quick lesson… A sounding is a graphical representation of the temperature, dewpoint, and wind of the atmosphere above a point. The red line represents temperature aloft, the orange line is 0ºC/32ºF. Any point where red crosses to the right of the orange line is where melting can occur. With a deep enough layer of warmer-than-0ºC air, snow can melt into sleet and further melt into rain. Snow, once turned to sleet, cannot turn back into snow. It will either continue to melt and fall as rain or refreeze and fall as sleet. If it hits the ground as rain and refreezes, it is freezing rain.
As we’re running models that predict not just what’s going on at the surface but also aloft, we can generate future soundings of the atmosphere. This animation shows the evolution of the forecast sounding at KRDU (Raleigh-Durham International Airport) from the NAM. The most-recent (6 January 18Z, 1PM EST) run suggests that air aloft at this location will just barely stay below freezing this evening into tomorrow morning. This should allow precipitation at the airport fall through the entire atmosphere as snow. However, you may notice how precariously close to the freezing line the temperature is forecast to be. If the temperature-aloft forecast has a one or two degree cold-bias, the entire forecast changes; the atmosphere ends up being warmer, more precipitation falls as sleet/rain, overall snowfall totals are reduced, and travel is made more dangerous. As a result, it is extremely difficult to say with confidence the amount of snowfall any location along the rain/snow corridor will receive. This is the difficulty in winter forecasting in this region, especially when winter storms impact major metropolitan areas.
Hopefully that conveys the difficulty involved in answering the question, “So, how much snow are we going to get?” Having said all of that as an extensively long disclaimer, our CNAPS model is predicting 5″ of snow to fall at the airport which puts it along with the averages predicted by the Short-Range Ensemble Forecast (SREF). Time will tell how accurate that prediction is.