Monday, August 19, 2013

Historical Case Study of July 12, 2004 Flooding

As wet and flood-prone this summer has been, nothing so far in 2013 has compared to the extreme flooding of July 2004 along the Rancocas Creek in Burlington County, N.J.

Heavy rain and areas of flooding affected the entire Philadelphia area that day, but the event is most remembered for the deluge that submerged towns like Medford Lakes, Lumberton, and Pemberton, when several dams broke upstream of these places on both branches of the Rancocas.

(Because I forgot about Daylight Saving Time when I ran my scripts, all the caption times on the maps are one hour behind.  They should be 2:00 and 8:00, instead of 1:00 and 7:00.)

500 mb:
Not much to note. There is a closed upper Low over Saskatchewan and relative speed max south of it. There is also a small vorticity maximum over Michigan.

There was a weak (1012 mb) surface Low over Lake Michigan associated with the aforementioned vort max.

By 18 UTC (2:00 p.m.) a warm front was more well-defined, stretching southeastward from the Low. By this point it was being enhanced by the cooler temperatures in eastern Pennsylvania left over from heavy rain earlier that morning.

The Low deepened slightly by 00 UTC (8:00 p.m.) to 1010 mb.

At 13 UTC (91 a.m.), heavy rain from overnight continues to fall over the northern and western suburbs.

Already 5.50" of rain had fallen over parts of Berks County, according to radar estimates. Despite the heavy rain in those areas, notice that Burlington County has not received any rain yet.

By 15 UTC (11 a.m.), rain has just started moving into Burlington County.

By 17 UTC (1 p.m.), Burlington County was mostly covered by moderate rain, along with a few pockets of heavy rain.

As rain began to break apart over the northern suburbs at 19 UTC (3 p.m.), rain was filling in across much of South Jersey.

By 21 UTC (5 p.m.), not much had changed.

Three hours later, at 00 UTC (8 p.m.), rain had dried up entirely in Pennsylvania, but the heaviest rain of the event was just beginning in Burlington County.

The dark gray just north and northeast of Tabernacle signify where the radar estimated that 3.75" of rain had fallen in just the last hour. All across the red-shaded area, 2.50" had fallen in the last hour.

That latest deluge increased storm total rainfall to over 5.50" in areas shaded in the darker gray. The radar estimated that 7.00" had fallen in Tabernacle by this point.

The following hour, an additional 2.50" of rain had fallen.

By 01 UTC (9 p.m.), the radar was estimating that 9.00" of rain had fallen in Tabernacle, with almost the entire Rancocas basin seeing between 5.50" and 7.00" on the day.

Heavy rain continued at 02 UTC (10 p.m.)...

...and at 03 UTC (11 p.m.).

By 05 UTC (1 a.m.), rain has entirely moved out of Burlington County.

But in its wake, excessive rainfall totals. Radar estimates indicated 11.00" had fallen over Leisuretown, and most of the surrounding area saw more than 9.00".

NWS Mount Holly's Event Archive has a higher-resolution image saved, that shows radar estimates of up to 13.20":

Observed totals corroborate these estimates (you may have to squint your eyes to see all of the totals):

10.71", 11.23", and 13.20", all in the Rancocas watershed. 

Model forecasts:

The first three images below are forecasts of 12-hour accumulated precipitation from three different runs of the ETA, for the same period, 2:00 a.m. on July 12 to 2:00 a.m. on July 13. 
The ETA was forecasting heavy rainfall from at least 54 hours out, but not of the magnitude that actually occurred, with a maximum of just 1.25" to 1.50" forecast. This would not change over the next three model cycles:
The run of the ETA the night before the was the most conservative yet.

Even the RUC never picked up on the incredibly heavy amounts. Here is its 18 UTC run, forecasting less than 1.25" of additional rain over the next 12 hours:

Even six hours later, the RUC had still not picked up on what was occurring.:

This illustrates how flash flooding events can catch everyone - even meteorologists - off guard.