Sunday, February 6, 2011

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: 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.