Friday, April 19, 2013

Tornado Watch until 10 P.M.

A Tornado Watch is in effect for all of central Virginia until 10 P.M.  A strong cold front moving in from the west will produce thunderstorms that are capable of producing damaging winds, hail and isolated tornadoes.  A Tornado Watch means severe weather is possible in and near the watch area. 


Tuesday, March 5, 2013

Quick update on the approaching storm...

My thoughts haven't changed at all regarding the impending storm.  Rain will increase in coverage and intensity across the region tonight.  Tomorrow, the rain will eventually mix with and change to wet snow, although rain could mix in through the afternoon from Metro RIC and points east. 

Accumulation in the Metro will be minimal at best.  With luck, the Metro might get an inch or two, but I wouldn't be surprised to see many places from Metro RIC south without any snow on the ground.  The farther north and west you go, the greater the risk of significant snowfall, particularly north and west of a line from Ashland to central Goochland.  The winners in all this (for snow lovers) will be the favored areas from Charlottesville up through the Shenandoah Valley and north to DC.

Thoughts on the upcoming storm...

The much anticipated storm will be begin exerting its influence on our area later today.  It looks like some rain will move into the area by later in the afternoon and will increase in both coverage and intensity at night.  As much colder air aloft moves into the region, we'll see the rain changing to wet snow during Wednesday morning and midday from west to east.  The snow will have a tough time accumulating due to surface temperatures above freezing and the increasing March sun angle.  While it will snow in Richmond, I'm not confident we'll see much in the way of accumulation.  Besides a northward trend in the model storm tracks, several other things just aren't lining up to make this a significant event in the Metro.  The storm track is certainly critical and the northward trend (coming over south/central VA) means marine air will get drawn in, and areas east of Richmond will likely see more rain mixed in during the afternoon.  We've been focused on the potential for strong "lift" in the atmosphere, but the model data indicates much of this will be earlier Wednesday in the Metro, before it's cold enough to really get any snow going.  Wet bulb temperatures (a parameter often used in situations like this to determine precipitation type in March/April) are hovering above freezing on the model data.  When looking at atmospheric charts at the 850 millibar level (approximately 5000 feet), heavy snow typically falls on the northwest side of the 850 low.  This tracks over us, indicating the greatest chance for significant accumulation will likely be well north and west of Richmond.  Many folks look to the 850 mb temperature and whether it is at or below freezing to determine precipitation type.  This is usually a good indicator, and the 850 mb temperature dips below zero (C) in the Metro during the afternoon, so I definitely think snow will fall.  It's just that accumulation may be hard to come by, especially during the daylight hours, even though it could be a heavy wet snow for a period.

All said and done, there will be a huge difference in snowfall amounts from southern Virginia up through the Shenandoah Valley (where 12"+ are possible).  There may be a very sharp line between virtually no snowfall accumulation and a quite a bit.  While this line has a better chance of shifting north and west, there's always the possibility that things could shift in the other direction.  I just don't think it's likely at this time. 

Below is a map which summarizes my thoughts.  My best bet in Metro Richmond is 1 to as much as 3 inches, with much more north and west.  Southern Virginia could end up losing out entirely...that is, if you are hoping for snow!