We haven’t had much snow this winter -- only 7” in Phila. That’s more than last year, but still WAAAY below average, and only a fraction of what we saw in the winters of 2009-10 and 2010-11. There have been a lot of small snowfalls, with the biggest a mere 1.5”.
With so many snow chances, we would expect at least a couple would be big ones. So far? None.
That could change next week.
Yes, it’s March, but we’ve seen some big winter storms in the past at this time of year. The three big March snows in Phila. were 1993, 1958, and 1888.
Usually, any storm this late in the season will have rain vs. snow issues. The sun is much higher in the sky, warming the ground even through the thickest clouds. The ocean is usually warmer, and any wind off the ocean can easily change snow to rain.
MARCH SNOWSTORM HISTORY
That’s just what happened in 1993, known as the “Storm of the Century”. Snows of 1-foot plus fell from Alabama to Canada. We got 12” in Philadelphia before the change to heavy sleet. More than 30 inches fell in the Poconos. Yet only 4 inches fell in Atlantic City and Southern Delaware.
The 1958 storm was perhaps the strangest in recorded history here. Only 3 inches fell at the Jersey Shore, 11 fell in Phila, and up to 50 inches fell in Chester Co. Yes, that’s no misprint! Fifty inches!
The “Blizzard of ‘88” in 1888 was the most famous snowstorm in the Northeast for more than a hundred years. Phila. was on the southern edge of the storm (similar to the blizzard last month). Still, we had 10 inches of snow. Of course, that was nothing compared to the 30”+ in the New York City area and more than 50” near Albany, NY.
THE CURRENT SET-UP
You can see from the above examples that March snows, when they do occur, have the potential to drop HUGE amounts of snow, especially in inland areas. We’re still 5 days from the storm, but this is why it has potential.
1. A big blocking pattern in the upper atmosphere, known as a negative NAO (-NAO). This puts high pressure between Maine and Greenland, blocking any storm from coming straight up the coast.
2. Very weak westerly flow in the upper atmosphere , which would not push the storm quickly out to sea.
With this type of pattern this late in the season, even small changes in the track can lead to BIG changes in potential snow amounts. With the storm blocked from going up the coast, the onshore winds north of the storm should bring enough warm air in to cause more snow to fall well inland than near the shore-perhaps a LOT more. The snowfall map after the storm will probably cover a relatively small area compared to most winter storms. That makes forecasts even trickier.
WHAT TO LOOK FOR
Of course, it’s way too early to talk about snow amounts. Since each computer model is run at least twice a day, we have another 8 versions of each model to look at before the first flake would fall.
The MOST likely things to expect are:
1. A rain/snow combination in much of the area, with more rain near the coast and more snow well inland.
2. A LOT of wind. The top wind speeds will be determined by the strength and track of the storm. But it looks like it will be strong enough and close enough to be pretty strong.
3. A threat of coastal flooding and beach erosion. Again, track and strength will determine how serious it becomes.
The things most up-in-the-air are:
1. How much snow you’ll get in your neighborhood. The computer models may vary a lot right up to the day of the storm
2. How much of the storm will be rain.
We’ve had several tough, tricky forecasts during the winter of 2012-13. It is only fitting that what could be the last winter storm threat could be the toughest of them all.