Forecasting in the winter is one of the most challenging in the southeastern US. Even though a small amount of anticipation of snow is in the minds of people living in the south, the event creates big problems. If a forecaster misses the event, it is usually under the wrath of a critical public. In Nashville and middle Tennessee, we are usually looking at the possibility of a few winter weather events over the course of the season. Occasionally the characteristics are there but they just don’t come together to produce anything or it is a rain event. Even with that, reminds me of a weathercaster at our station, who one winter missed it and heard it immediately from a viewer. All things look to be passing on a track as to not influence Nashville with any precipitation. He said in his forecast “this system could have potential of a winter worry for us if the low wasn’t tracking so far south. With its track, we should see on partly cloudy skies here in Nashville, maybe a few more clouds in southern middle Tennessee.” You probably have guessed it, the low did turn north overnight and next day a viewer call him be see when he would be over to shovel the 6 inches of partly cloudy off her lawn. With the supply of moisture to the south with the Gulf of Mexico, a cold air mass advecting far enough south, lows developing over Texas and the southwest sliding across the area and lifting with the strong fronts that move through in the winter, it should seem easier than it is. In a discussion with Stu Ostro at the Weather Channel, the south has many things that have to come together. These things that have to hit just right include moisture to condense into precipitation with the lifting component when cold air is in place that is not being modified. There are times that the cold air is dammed up against the eastern side of the Appalachian Mountains which supplies a winter characteristic to the east and southeastern states. Nashville however being on the western side of the mountain range is not as affected by this pattern. The western side of the actual mountains, tends to allow cold air to settle in or be displaced by WAA flowing northeast from the gulf and no cold air damming. This too will have modifications on a winter weather event. The cold air mass must be in place over Nashville while a lifting mechanism is in place at the same time to produce the snow event. The temperature profile of the atmosphere can be modified and needs a close examination to see what the potential is for a winter event. That profile can be modified by the same dynamics needed to make the event evolve. A low tracking across the area enriched with cold air can modify the profile and take the winter potential completely out of the equation. If the low tracks slower and more north, it will draw in moist warm air from the gulf and this convergence and lift will modify the profile. The fore mentioned blocking of the Appalachian Mountains may have the dry cold air sitting behind it while the warmer gulf air is flowing in from the counterclockwise circulation of the approaching low. This will allow the temperature modification along with the dry air in place to be wrapped into backside or northern sector of the low which will change the profile. This change in the temperature profile will allow the examination of potential precipitation types. The soundings will show those areas of snow, freezing rain, sleet, rain or a mixture of all. This will allow you to see the complexity of just forecasting a snow event. Dan Baumgardt, a Science Operation Officer for the National Weather Service (NWS) uses an instructional element call Top-Down Precipitation Forecast Method which includes a flow chart that will assist in examining temperature profile along with the sounding information. This method is used in the NWS winter presentation in the Nashville area in teaching winter weather classes. Henry Steigerwaldt of the Nashville NWS office is the winter weather presenter at public instructional events. He points out the use of the Top-Down method is helpful to just understand the situation. Steigerwaldt pointed to the track of the low and its importance. A low tracking too far north will leave Nashville in the lower precipitation band or may alter the type all together as discussed earlier in the profile modification. An upper level low is more of a winter precipitation guarantee along with the cold air mass in place. The retreating high pressure also tends to lessen the ability to produce snow than when a prevailing high is in place. The prevailing high will continue to reinforce the cold air needed while the modifications of the temperature are less with this deep arctic high in place. The track of the low more to the south will allow the moisture from the gulf to wrap into the colder air and allow a now event. The heavier snow will fall to the north of the low. A track to far south will be without the moisture needed to support much snow at all. A small change in any of these elements will change, sometimes drastically, the amount of snow we receive here in Nashville. The rain/snow line is much easier to predict in areas of the north and plains states. The south has too many characteristics that all have to line up and then time out at the same time just to produce the winter event. It should be noted that it is possible to hit the snow forecast for Nashville, but more times than not, it is going to be altered somewhat by modification of the temperature profile as well as air mass, lifting component, energy and moisture supply. Information gathered for this assignment was: A Discussion with Stu Ostro of The Weather Channel. A discussion with Henry Steigerwaldt of the Nashville NSW office. A brief explanation from WTVF Chief Meteorologist Ron Howes Information from http://www.crh.noaa.gov/arx/micro/micrope.php the Wintertime Cloud Microphysics Review which includes the Top-Down Method explanation and flow chart. Information from http://www.theweatherprediction.com/habyhints/71/ a winter weather forecasting Haby Hint from meteorologist Jeff Haby. |