Monday, February 7, 2011

Update on Rain Tonight & Snow Thursday

A storm system tracking through the Mississippi Valley region this morning is bringing us an increase in high cloud-cover in Virginia. As this system passes over us overnight, we'll have a chance for scattered light rain showers, which may have a few wet snowflakes mixed in there. However, precipitation will neither stick nor freeze because surface temperatures will only dip into the upper 30s to around 40 degrees Tuesday morning. Therefore, I do not expect this overnight precipitation to be disruptive. Most of the rain will be wrapping up between 5 and 7 a.m. in central Virginia, so I expect decreasing clouds after sunrise with plenty of sunshine the rest of Tuesday, but with cooler temperatures. Winds will also be from the Northwest at 15-25 mph, so it will feel a tad chilly despite afternoon highs in the mid to upper 40s.

The next couple of days will be relatively quiet until a storm system currently diving out of the Pacific Northwest tracks into the Southeast and Mid-Atlantic late Wednesday into early Thursday. Here is a view of the approaching snow (in blue) at 12 a.m. Thursday:

You can see that most of the snow is to our south, and it appears that the surface low will be generated off the southeast coast a tad too far to our south to bring us a significant snow event. However, here is one model (NAM) estimation of snowfall accumulation on Thursday:

This will occur in response to the upper vorticity max (spin in the atmosphere) passing overhead, aiding lift over central Virginia for snowfall to occur, even though the surface low will be to our southeast. Here's a view of that upper vort max from the same model Thursday morning (X marks the spot):

You can see the closed low pressure system offshore of the Outer Banks at the same time:

The dotted blue "540" line you see is a rough estimate for rain vs. snow. Anything south of the line could be rain, anything north could be snow. This is a rough guide, and forecast soundings must also be used to decipher the type of precipitation that will reach the ground based on the entire column of air it must pass through on its way to the surface. It does appear that North Carolina could see more snowfall accumulation than we do in Virginia because of them being closer to the coastal low. However, because we will have the upper vort max passing overhead, we could still squeeze out at least a little snowfall. Moisture will be sufficient and we'll certainly have the lift necessary. It just doesn't look like this will be "the big one" for Virginia. This storm system is still, as I mentioned, in the Northwest U.S., so we'll be tracking it as it makes its way across the country toward us, and provide updates on any shift in the track (which would significantly impact our snowfall potential).

No comments: