INTRODUCTION Forecasting snow in Hampton Roads has long been a challenge for meteorologists. The region often sits on the line between rain and snow, sometimes getting a messy mix of rain, sleet and snow. Often, even when snow is falling 20 miles inland, slightly warmer onshore winds deliver rain to the region. In this paper I will be discussing the geography of the region, predominate synoptic and mesoscale features need for snow, forecast challenges that a forecaster may be confronted with, and rules of thumb that help the forecaster when predicting snow in the Hampton Roads region. THE GEOGRAPHY OF SOUTHERN VIRGINIA Hampton Roads is located in the extreme Southeastern part of Virginia. The region is also known as "The Tidewater Region" and runs from north to south along the Atlantic Ocean. This area of lowlands stretch about 100 miles inland and is covered with salt marshes and swamps. It's often called the Tidewater region, because of the flow of water up and down the coastal inlets and bays as the tide moves in and out. The region is divided into a mainland in the west and a peninsula called the East Shore, which is separated by the Chesapeake Bay. The regions lowest points are from sea level to 22 ft above sea level in the extreme eastern portion along the Chesapeake Bay and the Atlantic Ocean. The region gradually slopes to the west where the "Tidewater" transitions into the Piedmont of Virginia, where elevations are 200-300 ft above sea level. THE CRUCIAL ATMOSPHERIC SET-UP FOR SNOW Synoptically, there must be an area of low pressure over the St. Lawrence / Nova Scotia region, with the trailing cold front extending southwest off the Atlantic seaboard extending into the Gulf of Mexico. The second, and most important synoptic feature, is an area of high pressure centered over the Great Lakes Region ridging over the eastern 1/3 of the United States. Lastly, there must be a weak area of low pressure or trough located over the Southeast coast. The combination of the high pressure to the Northeast and low pressure to south of the Hampton Roads area plays a significant role on the type of precipitation that the area will observe. The strength of the North- to- Northeast winds and the length in time that the wind blows determines the amount of cold moist air that is advected into the area. At the 500mb level, a long wave trough must be located over the Great Lakes Region and extends into south Texas. There also must be a major short wave trough propagating through the long wave. This pattern allows the development of the weak surface low pressure in the Gulf of Mexico and helps to steer the low into the mid-Atlantic region. FORECASTING DIFFICULTIES The vertical temperature structure, along with the horizontal temperature gradient, often determines the types of precipitation (rain vs. snow vs. sleet vs. freezing rain) which will occur in the Hampton Roads region. The chief difficulty associated with forecasting winter precipitation in the region is; determining just how much warming will occur in the boundary layer, due to proximity of the Atlantic Ocean. Sea surface temperatures, along the Southeastern Virginia coast, generally are in the lower 40's during the winter months. Consequently, surface temperature forecasting is very difficult and the slightest influx of warm air can change a forecasted snow event into a cold rain event. Therefore, the forecaster must determine the amount of warming that will occur in the boundary layer. If the forecaster can do that, then they may be able to correctly determine the precipitation type and amounts. However, this is not as easy as it may sound. Surface temperatures are a difficult field to forecast. The numerical models typically have a hard time with the lack of detail along the coastline. It is not unusual for the models to have a 2-4 degree error, on the cold side, during a winter weather event in the region. Therefore, a primary job of the forecaster is to determine the model weakness and to improve upon the model guidance when preparing their forecast. One way to do this is to look at the newer model run in greater detail and to scrutinize the initial conditions to see if any of the previous poorer forecasts are being carried into the new model run. FORECASTING RULES OF THUMB There are several forecasting strategies "Rules of Thumb" that forecaster's in the Hampton Roads region must follow to correctly forecast snow. Theses strategies have been documented in a number of studies. The first forecast building block that you must look at is the 850mb level to determine where the 0 degree Celsius isotherm is located. If the 0 degree isotherm is over the region, than there is an equal chance of rain or snow. If the -5 degree Celsius isotherm is over the region, there is a strong probability of snow. The next strategy is assessing the thickness of the different layers of the atmosphere. The 1000-850mb thickness of less than 1,300 meters, the 1000-700mb thickness of 2,800 meters or less and 5,400 meters or less for the 1000-500mb thickness are need for snow to occur. As mentioned previously, the wind direction plays a significant role in determining if the precipitation will be in liquid or solid state. Climatology indicates that snowfall occurs when the wind direction is from the northwest to northeast. If the winds are from the east, than the influence of the Atlantic Ocean significantly warms the boundary layer changing any snow quickly to an all rain event. SUMMARY Snow forecasting in Hampton roads is very difficult where even a slight onshore flow can make a major change to the low level temperature profile. There are several forecasting difficulties when faced with predicting snow in the region. The chief difficulty is the models temperature bias, due to the proximity of the Atlantic Ocean. The forecaster must study the models closely from run to run to provide an accurate forecast. As a forecaster in the area you must become familiar with the synoptic and mesoscale features that contribute to snowfall in the region. The type of precipitation that reaches the ground is dependent upon three key criteria. The first, being the thickness values. Second, the location of the 850mb 0 degree Celsius isotherm and -5 degree Celsius isotherm over the region. Third, being the wind direction. Although forecasting snow in the Hampton Roads area is very difficult, the forecaster can make an accurate forecast by applying the rules of thumb and by carefully studying the model forecast. References: Local area Forecaster's Handbook for NAS Norfolk, VA, NAVLANTMETOCCENINST 3140.2D The World Almanac of the U.S.A. by Allan Carpenter and Carl Provorse Operational Considerations of the December 26, 2004 , Snowstorm across Hampton Roads Virginia and Northeast North Carolina, Timothy Gingrich and John Billet, NOAA.NWS Wakefield, VA |