Not often does the National Weather Service post a "70 percent" chance of precipitation four days from when it's supposed to rain or snow, nor does it start issuing those heads-up advisories this far in advance.

But that is one indication that another powerful nor'easter next week is growing ever more likely, and this one could be a bigger deal at the Shore than its predecessor.

In its afternoon discussion, the weather service's Mount Holly office said a nor'easter that could affect the region Tuesday and Wednesday with potentially heavy precipitation, strong winds and coastal flooding was looking "likely."

If you have a reasonably reliable circulatory system, you can follow the hourly ups and downs of the computer modeling on See the "vernal equinox coastal storm" thread under "short-range forecast discussion." We've seen solutions ranging from 2 feet of snow near Philly to warm rain.

Yes, Tuesday is the first day of spring, and happens to be the 60th anniversary of the hall-of-fame "Equinox" storm of March 19-20-21, 1958.

Map showing the 1958 storm off Midatlantic coast.
NOAA Library.
Map showing the 1958 storm off Midatlantic coast.

The virtual versions of this one and the '58 storm bear some similarities. Computer models see the threatened storm, as with its predecessor, as slow-moving, with heavy precipitation and a long period of onshore winds from the northeast.

In the recent rash of nor'easters, the Shore has been spared the worst of onshore gales since winds were mostly out of the north.

As for snow, the biggest threat would be inland – and upland. This one is projected to form off the Carolina coast and possibly stall.

Since winds would be from the northeast – they blow counterclockwise around a storm's center – they would import some warmer air off the ocean where surface water temperatures are around 40.

Inland, particularly in places to the north and west where the land rises subtly, would stand the better chance of heavier snow.

That was the case in 1958. Just under 3 inches was measured in Atlantic City. And a Pennsylvania record of 50 inches was reported in Morgantown, Chester County, at an elevation of 591 feet.

Temperatures decrease with height, and also, elevation tends to give the rising currents that cause rain and snow an extra lift.

In Philadelphia during the '58 storm, the temperature never got to freezing, but heavy snow fell in darkness on March 19 and early on March 20, and 11.4 inches was measured at the airport.

The heavy, wet snow took down powerlines and branches, causing what at the time was the electric company's biggest outage event.

The weather service says a repeat is no sure thing, and the recent nor'easter run has been an eloquent reminder that storms like to make it up as they go along.

As we've mentioned, one reason that those marvelous computer models have trouble figuring out where the atmosphere is heading is the fact that they have a hard time figuring precisely where it is now.