Tuesday, August 13, 2013

Historical Case Study of February 5, 2001 Winter Storm

What would happen if everyone with a car suddenly got on the road at the same time?
And then promptly got stuck in the snow?
On Page 3 of the February 6, 2001 edition, the Philadelphia Daily News posed this tongue-in-cheek question. And then promptly answered it:
Well, we found out the answer yesterday afternoon, whether we wanted to or not.
Philadelphia's roadways locked up tighter than a meter maid's icy smile.
Trips home from work took three, four, five and even six hours.
Kids were stuck at schools late into the evening.
February 5, 2001 saw significant amounts of rain and snow cause major traffic problems for the evening rush., for which it is memorable.

500mb maps:
As there often is with East Coast winter storms, there was a deep trough in the East.
7:00 a.m.
A surface Low is deepening on the North Carolina coast.

Heavy precipitation was already falling at 7 a.m....

...but for most areas, that precipitation was falling as rain:
Light snow was already being reported in Allentown and Reading. But Philadelphia was seeing rain as the morning commute began, because...
...from about 875 mb down to the surface, we were above freezing.

10:00 a.m.
The surface Low continues to deepen and move northeastward.

Although still at 33 F, Doylestown has now turned over to snow. Trenton, Northeast Philly, Philadelphia, and Wilmington were all still seeing rain even after the morning commute ended.

1:00 p.m.
The surface Low has deepened significantly, falling at least 8 mb over the past three hours.

Allentown, Reading, and Doylestown are still seeing snow. Everywhere else is still seeing rain.

Although at the surface they are above freezing, Doylestown's 18 UTC sounding shows that only a small region of the atmosphere is above freezing.

Meanwhile, at Philadelphia, the atmosphere below roughly 875 mb is still above freezing:

2:00 p.m.
How things change in just an hour! Trenton, Northeast Philly, Philadelphia, and Wilmington have all seen temperatures fall a critical 3 degrees between 1 p.m. and 2 p.m. - from 36 to 33 - and now rain has changed over to snow all the way down to about I-295. Also of note, Atlantic City fell 5 degrees - from 43 to 38 - in that hour.

4:00 p.m.
The surface Low has deepened a little more by 4 p.m.

The heaviest precipitation is now pulling off to the northeast...

...but the precipitation is now all snow, and is still falling quite heavily, just in time for the evening rush hour. Notice that Atlantic City's temperature has fallen all the way to 33, and heavy snow is even falling all the way down there by now.
You can see why the changeover has occurred at PHL. Compare the 21 UTC sounding, below, to the 18 UTC sounding:

Unlike 3 hours ago, now almost the entire column is below freezing, except for a very small region near the surface.

7:00 p.m.

Precipitation has mostly come to an end, but the damage has been done.

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The storm's changeover coincided with the period of heaviest precipitation (which is not a coincidence), and also happened during the hours before and during the evening rush hour. This unfortunate coincidence is what caused such a mess.  

Our friend Ray Martin has many more maps and images from the storm on his Winter Storm Archive page here. NWS Mount Holly also has a listing of snow totals and a map of the amounts here.