Saturday, February 09, 2013

A Swing And A Near Miss

Snowfall gradients are awesome if you are on the right side of the line. If you're not, it's agonizing. In Philadelphia, a range of a few tenths of an inch of snow to nearly five inches of snow fell last night over the spread of the city (25 miles from SW to NE).  Yeah, agonizing for those of you in Delaware County, Southern Chester County, or south of the city in South Jersey that picked up mere tenths or traces while Northeast Philly cased in on a band of snow.  Such is life in tricky, complex storms.  We mentioned the gradient would be sharp and it was...however, there were factors that limited the snowfall and storm impact in our region and made it less of a pain to deal with than what was projected 24-36 hours ago.

1) The southern piece of this storm raced into the region ahead of modeling projections.  It was a fast mover, bringing rain into the city ahead of modeling and also pulling off the coast faster than projected by a few hours.  The track was pretty accurate, the strength relatively accurate (although not perfect) to modeling, but the timing was off a bit...the faster movement of this storm essentially allowed rain to shoot in and out of here before the northern piece could phase in completely.  Geographically, the storm moved by a bit too far for the northern energy to fully capture in and allow snow to take over. It still snowed at a really solid clip, especially north and east of Trenton last night, but had the southern piece of this storm been a bit slower in its approach and pass by our region, the storm would have behaved a bit closer to modeled expectations from Thursday.

2) One of the facets of the storm was the sharp cutoff on the western side of the storm. It was a bit sharper than modeled with the southern piece of this storm system.  Rainfall totals, especially west of the city and northwest, were much lighter than modeled but also it created a longer lull overhead in between rainy phase and the snowy phase last night.  That sharper edge of precipitation was partly enhanced by very heavy rains and snows that pushed north into Connecticut and Long Island, leading to the three inch to five inch per hour snowfall rates over Connecticut and Long Island last night and thundersnow for a healthy dose of New England.

The evening snowfall picked up after 6 PM from the west, in earnest after 8 PM as the northern piece of energy began to latch into the larger coastal storm.  While Connecticut and Long Island continued to get pummeled with heavy snow (and sleet in spots), energy from the west was getting enhanced by the coastal storm's energy and snowfall picked up and increased in intensity and coverage from the west.  The best snows fell generally along and north of I-195 in New Jersey and east of the Delaware River although this wasn't a complete clear cut guide -- that said, snowfall totals in parts of North Jersey did exceed a foot.  That didn't happen in Pennsylvania although a half foot and a bit more did fall in the Poconos.

Locally, snowfall totals ranged from trace amounts in Wilmington to a half inch to inch in most of Chester and lower Delaware Counties.  The city ranged from tenths of an inch to 4.7" from southwest to northeast, with an inch or two common across South Jersey.

We admittedly whiffed on the southwest flank of this storm across the suburbs west of the city -- the gradient was sharp and unfortunately set up a bit farther northeast than we thought (and for the snowstarved, would have liked).  That said, our forecasts in Central and North Jersey weren't terrible and pretty close to reality.  Across the burbs and in our area, it was akin to watching a guy mistime his swing by a few seconds...had the timing been a bit better, had the southern energy been a bit slower, that snow shield would have developed a bit more to the west and southwest and we would have all seen a decent snowfall.

More: Full Snowfall Report (NWS)