Historic late-season snowfall is expected to hit the East Coast tonight and into Tuesday. The National Weather Service expects 8 to 12 inches in Philadelphia, 12 to 20 inches in New York City and 12 to 18 inches in Boston. Even during the heart of winter, these totals would disrupt travel and lead to school closures. But such amounts this late in the season could make this storm something truly special.

But how special could it be?

Before we begin, let me note that we’re looking at snowstorms, not single-day snowfalls: The trick is that storms sometimes occur over multiple days, but climatological records are kept within a single calendar day. To determine how much snow fell in a multiple-day storm, I added up the 24-hour snow totals for each day of the storm. That doesn’t always yield perfect data because sometimes two storms happen on the same day with a gap between them. In a few cases, there are highly reputable sources that disagree on storm totals (though not by much). The lists that follow are the best estimates.

Let’s start in Philadelphia, which is expected to get the lowest total of these three cities.

YEAR DATES AMOUNT (INCHES) 1915 April 3-4 19.4 1993 March 13-14 12.0 1958 March 19-21 11.4 1888 March 12 10.5 1917 March 2-4 9.6 1941 March 7-8 9.0 2009 March 1-2 9.0 1981 March 5 8.8 1956 March 18-19 8.7 1941 March 1 8.6 1960 March 3-5 8.4 1915 March 6-7 8.2 Philadelphia snowfalls of 8 inches or greater in March or April Since 1885 Source: National Weather Service and national centers for environmental information

Storms that yielded at least 8 inches of snow have occurred in March or April 12 times since 1885. But look at the dates for most of those storms: They’re before March 14. In fact, just four 8-inch or greater snowstorms have hit Philadelphia this late in the season in the past 132 years. If this storm drops more than 9 inches, it will be more impressive than all but five March or April snowstorms in Philadelphia history. A snowfall of 12 inches would tie for the second-biggest March or April snowstorm ever in Philadelphia. Of course, the all-time late record snowfall of April 1915 looks to be safe unless the forecast is way off.

Snowfall from this storm is expected to break into New York City’s top five all-time March or April storms.

YEAR DATES AMOUNT (INCHES) 1888 March 12-14 21.0 1941 March 7-8 18.1 1960 March 3-4 14.5 1914 March 1-2 14.5 1896 March 15-16 12.0 New York City snowfalls of 12 inches or greater in March or April Since 1870 Source: National Weather Service and national centers for environmental information

There have been just five snowstorms of 12 inches or greater in March or April since record keeping started in 1870. More amazingly, none of them have happened in the past 57 years and just one of those happened at this point in March or later. That storm, back in 1896, dropped 12 inches. So as long as this storm has just slightly more snow than is predicted at the lower end of the forecast range, it will be the greatest snowstorm this late in the season in recorded New York City history. If the snowfall hits the upper end of the forecast range, this storm will challenge the infamous Blizzard of 1888 for the largest March or April storm ever.

Boston isn’t forecast to beat its all-time late-season snowfall (April Fools’ Day 1997), but this should still be a major late-season snowfall there as well.

YEAR DATES AMOUNT (INCHES) 1997 March 31-April 1 25.4 1960 March 3-5 19.8 1891 March 3-5 15.5 1956 March 19-20 13.3 2013 March 7-8 13.1 1993 March 12-13 12.8 1892 March 1-4 12.3 Boston snowfalls of 12 inches or greater in March or April Since 1891 Source: National Weather Service and national centers for environmental information

Boston has gotten a foot or more of snowfall in March or April only seven times since 1891. Only two of those storms occurred on March 14 or later. If more than 13.3 inches falls, this will be the fourth-largest snowfall in March or April in Boston and the second-largest ever on this date or later in the season.

But perhaps what’s most amazing about all these forecast snowfall amounts is that they’re expected as part of the same storm. If you look at the three tables, you’ll note that only one snowstorm appears on all three lists (March 1960). Usually just one or two of the large East Coast cities are affected by a major late-season snow in a historic fashion. In this case, all three could be. That means we’re not just expecting deep snow; we’re expecting it over a wide area as well.

Of course, we won’t know if we’re experiencing a storm of historic proportions until the snow is measured. The one thing I’ll be watching is how much precipitation falls as sleet or rain in these cities. More of that means a reduced chance of historic snowfall. If what’s coming down is mostly or all snow, this storm will probably be one for the record books.