Wednesday, January 19, 2011

Update on Friday's Snow Potential

An upper-level trough currently (at the time of this entry) located over the Rockies will move eastward and deepen over the next 48 hours. Strong lift associated with the approaching trough will cause a broad area of snow and rain to develop and move through the Mid-Atlantic region. Temperature profiles in central Virginia will be supportive of snow as the primary precipitation type, but this snow will be falling into surface temperatures that are just slightly above freezing. It's common to see snow in central VA with above freezing surface temps when you have sub-freezing conditions just off the surface, it just melts on contact without accumulating. Friday will be a situation where I think we'll see snow (if you are up early), but any accumulation will be limited to grassy surfaces. Depending upon the relationship between the temp and dewpoint at the time the precip begins, there will be the potential for areas north and west of Richmond to evaporatively cool to the freezing mark at the surface. These areas will have the best potential to see slick roads. Whatever we see will move out of the area early in the morning, and we'll have a sunny, windy, and cold afternoon.

It continues to look like our best shot at seeing a significant winter weather event (one that would result in school and business closings) will be on Tuesday. This next system will likely bring a mix of precipitation to central VA, and in an order that will keep us pretty busy. The combination of a strong retreating surface high over New England, and several upper-level disturbances embedded in southwesterly flow will make it very interesting. Right now it looks like it will start as snow late Monday, then go to a mix of sleet and freezing rain, then over to all rain, and then back to snow before ending Wednesday morning. I'll have more updates on this system in the next few days.

Fog Wednesday Morning

After overnight rain tracked through central Virginia, fog developed with decreasing clouds and light winds. A Dense Fog Advisory is in effect for these counties highlighted in gray, where visibilities are being reduced below a quarter of a mile in most areas.

The rest of central Virginia is experiencing at least light fog, with patchy dense fog in low-lying spots or near waterways (like rivers, creeks, ponds, etc). Please use caution this morning if you live or travel through the Dense Fog Advisory counties, and also be aware of the potential to drive into sudden, locally reduced visibilities elsewhere in the Commonwealth. Please note that I-95 and I-64 in northern and western parts of the state are especially impacted by the significantly reduced visibilities. The fog should dissipate mid-morning, with decreasing cloud-cover this afternoon.