Fredericksburg geography and winter weather
First, a quick weather update. Today’s (Wednesday’s) clouds are courtesy of a surface low pressure that formed off the coast along the cold front that passed through here yesterday. We won’t see any rain from this system but the clouds – especially the high thin variety – will remain over the area today and keep the high temperature suppressed to the upper 40s (F). This afternoon there may be a peek of sun but it will still be chilly.
Now for the geography part. As most longtime Fredericksburgians know wintertime precipitation events are usually an iffy proposition for the city and its surrounding vicinity. While DC gets snow and Richmond sees rain, our moisture can be very much a mixed bag of snow/sleet/freezing rain…or just plain rain. This is due to several geographic factors that mess with the critical freezing point (32 degrees) of water:
(1) Latitude: Intuitively the weather is colder the farther north one travels and that is exactly why Richmond can get rain while DC sees snow. The 32 degree isotherm (yes, you can look it up) typically parks itself in and around Fredericksburg, and the exact location of that line greatly affects what type of winter precipitation we see here.
(2) Elevation: Also somewhat intuitive the higher the elevation the colder it is, and the typical rate at which this cooling occurs (the dry adiabatic lapse rate) is roughly one degree (F) per 200 feet in elevation. Given Fredericksburg’s location on the fall line of the Rapphannock River the land elevation westward tends to slope upward toward the Blue Ridge Mountains. Thus Shannon Airport is at 85 feet, Culpeper Airport is 312 feet, Orange Airport is 466 feet, and Charlottesville Airport is at 640 feet. These elevation differences can make just enough temperature difference to create rain in Fredericksburg while frozen stuff falls not far to the west.
(3) Proximity to water: Large bodies of water are both heat reservoirs and moisture sources, so our proximity to the Potomac River, the Chesapeake Bay, and the Atlantic Ocean greatly affects our weather. Given that frozen winter precipitation requires both cold air and a moisture source the balancing act of how much of each is available is very complicated. The distance offshore that coastal storms – our main source of heavy precipitation – form and track is very difficult to pinpoint more than a few hours ahead of time, especially given that a track difference of 50 miles can make a difference whether the ‘Burg sees rain, snow, or some other frozen concoction.
So all of these factors (plus a few more that I won’t detail here) play into wintertime precipitation conditions that may be very localized for the ‘Burg. It’s almost like trying to predict a day ahead of time exactly when one single summer thunderstorm will pop up and exactly what part of the city it will dump rain on. Is that an excuse for weather folks not making better forecasts? No, but it is a reason that further research is required to develop better understanding of how to provide more accurate localized forecasts for population centers like Fredericksburg.