*Winter Storm Watch
* in Effect Area Wide Saturday morning through Sunday afternoon. Scroll down one post to see CapitalWeather.com's forecast through the weekend and various storm scenarios. PM update is off today.By Meteorologist Nabeel Keblawi
December 15-16: A significant winter storm is developing in the southwestern US that may potentially develop heavy snow across the Central Plains, Ohio Valley, the Finger Lake region, and the interior northeast. A wintry mix consisting of snow, sleet, and freezing rain, and rain may develop across the Mid-Atlantic, including Washington, DC and Baltimore on Saturday afternoon into Sunday evening. A strong upper level jet stream has been digging southward over the southwestern US, contributing to the development of a surface low pressure system in Texas today. A key player in this winter storm involves a strong high pressure system developing along the US-Canada border, which supplies the cold airmass needed to produce heavy snow and wintry mix in the storm's path. While it will provide a dose of cold air to our region, it may move away too quickly to allow for an extended period of frozen precipitation.
By tomorrow, this high pressure system is forecast to bring cold air southward as far as central Virginia, even east of the Appalachian Mountains. These mountains play will play an important role in this storm - funneling the cold air mass from the north southward into central Virginia without flowing over the elevated terrain. This phenomenon is known as cold air damming.
Tomorrow afternoon and evening, milder air arriving from the south ahead of the storm will rise up and over the low level cold air because warm air is lighter and less dense. This overrunning action will produce precipitation that may begin as snow just below cloud level. Depending on the existence of a warm layer at around 5,000 feet in the atmosphere as the snow falls, the snow may melt and become rain. If the low level cold air is below freezing and deep enough, the rain will freeze into ice pellets, or sleet. With a shallower layer of surface cold air, the rain will freeze on contact with the ground or trees or power lines, hence freezing rain. At least at the onset of precipitation, enough cold air will make sleet or snow most likely but the layer of cold air will be tenuous, with warm air attacking it at different layers and from different directions.
As the low pressure system heads towards the Kentucky-Tennessee border Saturday night, its position to our west will help steer warm southerly winds into region at five to ten thousand feet above sea level -- making it too warm for snow. Cold air damming at the surface may be strong enough to keep the far northern and western suburbs of DC and Baltimore at or below freezing for most of the event. But, as the high pressure area responsible for the cold air damming will be moving offshore (as opposed to staying anchored in place inland), the supply of cold air will erode a bit, probably causing a changeover to rain the further south and east you go. Pictured: Cold air damming will promote the formation of a wedge of cold air just along and just east of the mountains towards the north and west suburbs of DC. In the (NAM) model simulation shown, temperatures remain subfreezing north and west of a line (in pink) from Loudoun to the Montgomery/Frederick Co. border at 4am Sunday morning. Significant icing is possible in those areas.
The wild card here is the development of a secondary area of low pressure off the Mid-Atlantic coast. If it forms far enough south and east, and holds the wind in a more northerly direction, that could help tap some of the colder air and keep temperatures near freezing throughout the area (this is what happened during the Valentine's Day storm last year). However, most indications are that this low will be far enough inland to tap milder easterly flow.
Bottom line: This storm is not a snow-maker for Washington, DC due to the intrusion of above-freezing air aloft above the surface cold air. There is concern for significant icing due to a mix of sleet and freezing rain. Areas to the south and east of DC, including the district and immediate NW suburbs, will probably see a change-over to plain cold rain as surface temperatures likely rise above freezing. The further north and west you go from DC, the better chance for significant icing, higher snow accumulations, and more treacherous driving conditions. Near the end of this storm, there may be a change-over to snow -- especially north and west of DC -- as cold air moves in from the northwest. But significant snow accumulations are not likely for the Washington, DC area. All precipitation should end by Sunday afternoon.
This storm is still more than 24 hours away, and its track and cold air supply could change somewhat in that time frame, and that is what makes this forecast challenging. Any changes to track will result in changes in actual precipitation types and how much falls as frozen/freezing rather than just rain. A more southerly and easterly track would result in more snow and ice accumulations, while a more northerly and westerly track would result in less snow/ice and more rain. Nabeel Keblawi, who recently earned his Master's Degree in meteorology from the University of Maryland, is an air quality meteorologist for the Metropolitan Washington Council of Governments.