Expected Timeline

Right now, based on model data, here's what I expect in terms of timeline and accumulations:

Late Morning to Noontime Friday:  Snow begins from southwest to northeast expanding in coverage and intensity through the afternoon.  3-6 inches on the ground by evening.

Friday Evening - Saturday Morning:  The heaviest snow develops and we could be looking at a foot to a foot and a half by morning on Saturday.

Saturday:  Snow will continue, lessening in intensity through the afternoon and ending overnight.

Total accumulations?  I think, so long as we avoid any issues with sleet mixing in - and I think, at this point, that we will - you can expect a general range of 18-26 inches.

Predicting totals in any one spot is tough because very small-scale events take place within storms like these - almost like storms-within-a-storm.  Mets refer to this as "banding" because their presentation on radar is of darker bands within the existing precipitation area.  If enough bands set up over one place, I can see localized areas 28-30 inches as possible.

This storm is a big deal.  We haven't seen one like this in a number of years.  It won't quite be the Superstorm of 1993 in intensity at its center, but the impact could be nearly as big for areas around DC.

"Variables", or, "What Could End Up Causing This Forecast to Suck"


At this point, confidence is as high in a forecast as I can remember it being.  There are no outlier models, no strange European model screaming for the storm to come to our west and douse us in rain rather than snow, no bizarre forecast model created by some dude in his garage that forecasts the storm to go out to sea and miss us entirely.  All the models, to some degree, agree.  That's unusual.

In an uncertain science, this is as close to certain as it gets.  But stuff can still go awry. 

1. Storm Track - the big one.  Forecast models have been pointing to a track that puts the bullseye over Washington DC and Baltimore, MD.  You've seen the output from these models below in my previous posts.  But, especially as you go further north, the gradient of total precipitation gets VERY tight.  In other words, a few miles difference makes a huge difference in storm totals. Right now, Philly is forecast to have 12-18 inches but most models put New York City in the 3-6 inch range and some miss it entirely.  So, too much of a shift south and we end up in lighter snow.  This doesn't appear likely.  Too much of a shift to the north and we're back with that sleet mixing in problem again.  This doesn't appear likely either.
2. "Dry Slot" - If you look a few posts down, you can see some satellite images I posted of when the storm was still southwest of Texas.  See the comma shape?  Notice that between the head of the comma and the tail there's a southwest to northeast aligned area of no clouds.  That's the dry slot.  If you get under that, there's no precipitation.  Now the atmospheric setup here should preclude that from being an issue.  But it's the only other thing I can think of to go wrong with this forecast.  We'll put this in the "not likely" column as well.

 More model runs still coming later this afternoon.  I'll try to stay on top of them.