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A lot of it has to do with the La Nina storm track.  During La Ninas, storms along the northern jet stream tend to dominate.  So when they get to the East Coast and redevelop, they do this far enough north that we just get skirted.  The happened on 12/26 and looks to happen tomorrow, but to a lesser extent.  I think we'll see some more snow this time around.

Depends on your work's closing policies.  We're expecting 1-3" from DC and to the west - mostly late afternoon Tuesday and Tuesday night, and 2-4" just east of D.C.  Is that enough to close your office Wed? 

The 12/26 storm - which crushed the coast but just grazed us - was a really tough one because we were right on the edge.  Just the slightest shift west would have meant more snow for us.  That looked to be the case 5 days out and never really changed.  When it's such a close call, you're right: when you get within 12-24 hours, your best bet may be to look out the window.  But that was about as close a call as it gets around here.

This storm is a close call, but we're not as  close to the edge.

By Wednesday morning, the snow will have long ended (it should wrap up by 1 a.m. Wednesday morning or even sooner).  Having said that, there may be backlash delays from snow between Philly and Boston - depending on where you're traveling.

Forecasting has made remarkable progress since the 1970s and, in general, forecasts are pretty accurate to 48-72 hours.  But despite these advances, snow forecasting is still challenging and, yes, in exact ... especially in D.C. where there are so many variables to consider.

Regarding statistical analyses, there have been a lot of studies which have looked at the accuracy of the different models.  These studies help us know which models are more skillful than others.

So far this winter has been drier than average and we don't see any strong signals this is going to change.  On the other hand, it has been colder than average and looks to remain that way for a while. That means when weather systems come through - we should have some more shots at snow.  February is usually our snowiest month.

I'd say 50/50 chance based on our latest forecast.  We think there's a good chance you'll at least have a delay.  Do your homework to be safe...

Yes.  We're likely to be impacted by a dry slot as low pressure to the west weakens as it approaches and skips over us to form a new area of low pressure offshore.  It will take a while for the low pressure offshore to get going and throw snow back at us. 

Yes.  I wouldn't freak out about this storm.  It may look impressive for a time tomorrow evening, but it will be pulling away just as it's starting to get its act together.

We've got Fredericksburg in our 1-3" area in our accumulation map.

Computer models are based on mathematical equations that describe processes in the atmosphere.  Here's a good basic overview: Model basics.

If you're in northern PG county, I think you're at the high end of snowfall amounts - so probably 2-4" rather than 1-3" forecast for west of DC.  If you're in southern PG, probably 1-3".  The further northeast you are with this storm, the more snow you're likely to get.

I think the factors you mention do make forecasting pretty challenging here.  The mountains effect weather systems by squeezing out the moisture but sometimes they help trap cold air at low levels over our region.  The ocean serves a source of a moisture, but ocean storm tracks can be tough to pin down and mild flow from the ocean can introduce precipitation-type forecasting problems during certain storms.  When you combine the ocean and mountain effects together, it can become a mess.

We're not expecting the accumulating snow in D.C. until late afternoon or during the evening.  So the earlier you leave tomorrow, the more likely you'll miss snow in DC.  You may experience some light snow as you head southwest.    

The slight shift would mostly impact the amount of snow we'd get tomorrow evening, which shouldn't impact your plans.  And I think such a shift might only mean we get 3-6" if storm develops more, or less than 1" if the storm develops less.  Not sure the change would be dramatic, even if significant.

You'll probably run into some precipitation as you head southeast...but it's unlikely to be crippling or severe.  You may first deal with some light snow through central Va. which may mix with sleet and chance to sleet and rain by the time you get to Norfolk.  Allow extra time and be careful, but you should be ok.

We think you have a good chance of getting 2 but there's about a 25% chance the snow is less than 1" .  It's all about managing expecations...

You make a lot of good points.  Every region has its own forecasting challenges.  I do think snow forecasting is particularly difficult in the mid-Atlantic because we're often so close to the rain/snow line and have to deal with so many types of precipitation.

The snow may not get going until the afternoon, which may make the call a little easier...  Wednesday is more likely going to be the challenge.