On January 19, I was in Silver Spring, Maryland that is just outside the District of Columbia almost due north from downtown DC. The morning began with an accumulating snow that quickly changed to a wintry mix before becoming a cold rain by late morning. The forecasters did state a changeover back to snow, and the snow would be significant. The rush hour began and the snow began to fall heavy. The forecasters knew this storm would be a quick hit, but the intensity was a surprise. Thunder and lightning became more frequent, and the snow became intense.
Because of the heavy snow, the rush hour was a disaster, wet snow accumulated on trees causing power outages, and visibility near zero. In approximately four hours, 5-7 inches of snow in addition to the one inch of morning snow fell across the Silver Spring area. The weather forecasters missed the intensity, and this relatively nominal snowfall event in comparison of the accumulation totals to the blizzards of 2010 became a high impact event in the chaos on the roads and loss of electricity to hundreds of thousands of residents.
In weather history, the high impact weather events are the events that the weather community cannot provide the accurate impact of the upcoming event. The date is now February 3, and I am currently in Raleigh, NC. My friend lives in Southeastern North Carolina, and he called me around 5:30pm this afternoon. He began to inform me that sleet was falling near Lumberton, NC. Lumberton is located in Southeastern NC. The precipitation shield is moving faster than the models or forecasters were predicting into North Carolina. Sleet being observed in far Southeastern Carolina spells potential problems for the Central part of North Carolina later tonight and early Friday morning.
As the week progressed, the weather forecast was advertising a cold rain for Friday into Saturday morning. However, precipitation is showing on the radar over Central North Carolina 12 hours earlier than predicted. The temperatures are above freezing in the 35-40 degree range from Greensboro in the NW to Goldsboro in the SE. The dewpoints will play the pivotal role in evolution of tonight’s event. The dewpoints are in the teens as of 8pm, and the low dewpoints are indicative of a dry air mass. The two impacts of the low dewpoint temperatures on tonight’s storm are as follows. First, the rain being detected on Radar is not reaching the ground. The precipitation is evaporating before reaching the ground, and the atmosphere must moisten before precipitation will be observed on the surface. Second, the dewpoint temperature in the teens allows for evaporational cooling throughout the atmosphere when the precipitation reaches the ground later tonight. The precipitation will fall from the cloud to the ground, and the evaporational cooling will force air temperatures lower as the dewpoint rises to meet each other at the maximum cooling temperature of the atmosphere.
In the early morning hours, the intensity of precipitation and the maximum cooling of the surface temperatures up through the cloud deck temperatures will dictate the type of precipitation later tonight. Will the dry air mass prevent measurable precipitation from reaching the ground to cause any winter related problems? Will the precipitation be intense enough to overcome the dry air mass and cool the atmosphere and surface temperatures to see sleet and snow? What will happen? We will soon know….