Snow Chances Look Slim

Our chance of seeing snow early Thursday isn't completely gone, but the odds of more than an inch are slim to none. The NAO has become positive, meaning that there is an absence of a North Atlantic blocking pattern. When we have a blocking pattern, the jet stream will dip south sharply across the eastern U.S. and then turn northward up the east coast. This pattern allows systems to intensify and ride along the coast (nor'easters). Without a block this Thursday, the flow across the Mid-Atlantic will be very fast and very west to east, not allowing the development of a coastal low. Any hope we have for snow is tied to the upper-level low, which is only moderate in strength. Based on the last few data suites that have been very consistent, we'll likely only see a brief period of light snow early Thursday morning. One thing that will be interesting to watch, is the potential development of a coastal low late Friday. This scenario is pretty unlikely, but the medium range models continue to hint at such development. A similar low developed a couple of weeks ago, dumping 4-6" of snow on the OBX. We'll keep you updated on both of these systems.

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).

Brief Mix Monday Night, Snow Possible Thursday

I want to first talk about Monday night. A small but potent storm system will race past the area and bring us rain generally from 7 PM Monday until 7 AM Tuesday. As colder air wraps into the backside of the storm, we could see a very brief mix of wet snow. As you can see from the maps below, temperatures will stay well above freezing, so all flakes (if any) will quickly melt on impact.





Now to the more complicated part of the forecast. Yesterday I talked about how the computer forecasts were in fairly good agreement on the track and timing of Thursday's storm system. The only wild card was the strength of the system. Well, it is looking like the computers are slowly trending to a solution: a weak storm. I've posted below snow outlooks from three different computer forecasts. None of them have a significant snowfall in our area.




We still have to watch this carefully. That patch of dark blue in North Carolina and southern Virginia represents about 1 to 3" of snow. If this storm shifts a little north, then we could have some light accumulations in central Virginia. This could cause problems for the morning commute on Friday. I encourage you to keep a close eye to the forecast through the week for the very latest information.

Now the discussion for all the weather geeks...

So you are probably asking yourself: why are storms falling apart in recent weeks? I think two factors are in play: La Nina and NAO. La Nina years are typically dry and mild. This wasn't the case earlier this winter because we had a persistent -NAO. NAO (North Atlantic Oscillation) is now positive, which I think is allowing La Nina to finally take over our weather pattern.

The map below is the overall upper-level weather pattern across the entire Northern Hemisphere (link: http://www.meteo.psu.edu/~gadomski/HEMI500/5dayloop.html). Notice the blue blob over Greenland and the Hudson Bay. This is an upper-level trough, or cold anomaly. In a -NAO pattern, it would be the opposite, a powerful ridge or blocking pattern. This allows storm systems to track up the East Coast, like what we saw earlier in the winter. We are not in this pattern any more.

Now, can we see snow in a +NAO, La Nina pattern? Yes, it's happened before. However, unfortunately for all you snow lovers out there, the odds are stacked up against you.