Ice storms are not common in the Philadelphia area, since our proximity to the ocean usually keeps the surface warm enough to prevent such storms. The most recent occurred on February 2, 2011, but it was mild compared to the ice storms of the infamously icy winter of 1993-94.
One storm from that winter, and arguably the worst Philadelphia has ever seen, hit on January 7-8.
The winter of 1993-94 is known for being a "gradient" winter among weather enthusiasts, one in which fast, progressive mid-level flow provides a nearly endless stream of disturbances moving west to east across almost the same track, while cold air north of the track gets colder and warm air south of the track gets warmer, building up into a tight temperature gradient, hence the name.
A look at the mid-levels hours before the ice storm clearly shows this pattern:
500 mb maps:
Sure enough, instead of a wound-up storm, we have a weak 1004 mb surface Low in Texas and a warm front that stretches all the way northeast from there to New York state and New England:
If we zoom in to the East Coast, we see that by 7 a.m. on the morning of the 7th, there was at least a 1010 mb surface reflection in West Virginia:
Although it may seem too fast, that same system that was in Texas the evening before was in West Virginia the next morning. Remember, one of the characteristics of a gradient pattern is very fast mid-level flow.
Freezing Level Maps:
The maps below plot the freezing line at three different levels: the surface, 925 mb, and 850 mb. In a typical winter storm with just snow falling, the surface freezing line would be the farthest south, and the 850 mb freezing line the farthest north, since temperature cools with height in the atmosphere. But as you'll see, this was not a regular snow storm for Philadelphia.
You can also watch the surface obs. Any time the symbol below shows up, freezing rain is falling. (Two dots would indicated moderate or heavy freezing rain.)
No precip falling yet, but notice how much larger that goldilocks region between the pink and red lines has grown.
With still no precipitation falling, and the area where the red line is south of the pink line shrinking again, it might at first look like Philly dodged a bullet. But notice how the 850 mb freezing line surged northward over the past six hours. Even where temperatures are below freezing at the surface and at 925 mb, they are still above freezing at 850 mb in many places, which guarantees that some mixed precipitation will still fall.
Precip is now falling, and we're right back to where we were at 1 a.m. But also notice that the gridded data I'm using for the freezing lines appears to be placing the surface freezing line too far north, based on surface observations. In reality, the entire Delaware Valley from roughly a LBI-to-Vineland line on north is in that unfortunate sweet spot, above freezing just off the ground but below freezing at the ground.
Now I don't know what happened to my lines. Obviously Trenton, Northeast Philly, and Philadelphia are all still below freezing at the surface. The surface obs confirm sleet and freezing rain continue to fall everywhere north of Millville and Atlantic City.
By early afternoon, Philadelphia International has hit 32 F, but the surface ob indicates that freezing rain continues to be the precipitation type.
Also notice the tight temperature gradient between Philly (at 32 F) and New York (at 25 F in the city, but 19 F just across the river and away from the moderating influence of the ocean.)
With precip still falling, the sun goes down and surface temperatures begin to cool again, despite never having actually risen above 32 F anywhere in the city or north of it.
Freezing rain fell through the night in most areas, before finally changing over to light snow by sunrise the next day.
Since this is starting to get repetitive, we'll skip ahead to 7 a.m. the next morning.
7 a.m., Jan 8th
By this point, the original wave had transferred its energy to the coast and redeveloped a new surface Low there.
By now, precipitation has changed over to snow in almost all areas. As the cyclone wraps up offshore, we return to a more normal-looking temperature pattern, with the surface freezing line north of the 925 mb and 850 mb freezing line. Unfortunately for snow-lovers, only about an inch of snow accumulated on top of the ice.
According to the National Weather Service, the ice storm of January 7-8, 1994 caused 590,000 PECO customers, or roughly 40% of all customers, to lose power, making it the worst power outage in PECO's history to this point, although not the costliest storm on PECO's end.
As always, our friend Ray Martin has more cool stuff about this winter storm on his archive page.
Well, this is my last entry as I'll be heading back to school next week. I would like to thank Tom for letting me kill some time this summer by writing for the blog. I had a lot of fun sharing what I know and learning some new things as well. Even though I'll be in Indiana, I'm still wishing for a break in the streak of lousy winters for you guys.