IMPORTANT NOTE ABOUT RADAR IMAGES: National Weather Service-Blacksburg radar continues to have technical issues and is operating in “degraded mode.” Therefore weather service radar images and the products based on them, such as the Radar / Future Cast on this blog, may not contain an accurate picture of the extent or intensity of precipitation that is falling.
We’re about to switch into a new phase with the storm, the one that will eventually lead to widespread and locally heavy snow in Southwest Virginia, according to all available forecast guidance. The radar grab at left shows the position of Roanoke wih the arrow. All of the rain that has gone through so far has been moving northeast, moisture streaming from the Gulf of Mexico, propelled by the circulation around the approaching upper low now in Alabama. We are now in a gap of a bit less rain, with a thick body of rain — and farther north, mixed precipitation and snow — to our west. If you loop our Radar /Future Cast at right, you will notice much of that precipitation is moving north to northwest, while the area itself slowly advances eastward. This is indicative of the upper-low going “negative tilt,” taking on a northwest to southeast orientation, and therefore the upper level winds ahead of it being bent more to the north and northwest. As moisture is drawn northwest, it will spread out against prevailing west/northwest winds behind the storm, creating a shield of precipitation known as a “deformation zone.” The surface low was located over Atlanta late this morning (map at left). As it moves east, we’ll be on the colder back side of the precipitation shield, with colder air being pulled southward. This is where the dynamic cooling and some cold air advection will be maximized, and we expect to see rain changing to snow northwest to southeast and from higher elevations to lower elevations through the afternoon. This may happen very rapidly, so don’t be surprised if you hear some distant report of snow and then, suddenly, big flakes are plopping outside your window. NWS-Blacksburg forecasts estimate at 3 p.m. changeover at Blacksburg and 4 p.m. at Roanoke, but those times may vary an hour or two. Most of our region west of the Blue Ridge Parkway probably changes over to snow by 5 p.m., and it will follow suit to the east not long after, if it hasn’t already.