Wednesday, September 19, 2012

Rain, Wind, Flood But No Tornadoes

We didn't get a lot of rain yesterday in Philadelphia when compared to folks in the outer portions of the Delaware Valley and the Poconos. Our three-quarters of an inch at the Airport is a mere pittance compared to the five to eight inches of rain that fell across the Poconos.

Flooding was very common in the Poconos thanks to a prolific rains. This video was taken by Steve Shinko near Tamaqua yesterday afternoon. You can see how much water was covering the road as the semis are trying to ford the flooded roadway.

If one were to look solely at doppler radar estimates, one wouldn't know just how much rain fell across the Poconos as radar estimates were about half of what reality said fell across Carbon, Monroe, and Schuylkill County -- the highest totals by radar estimate were only in the two to two and a half inch range.  Yesterday's rain within the embedded squall line fell at an intense clip and outdid what doppler suggested fell.  It was also enhanced by strong southerly winds in the atmosphere pushing up into the Poconos, squeezing moisture out and enhancing the rainfall as it moved over these areas.  The result is the prolific totals outlined above.

Flooding was less an issue down into the outer north and west suburbs -- there were some minor flooding reports in the usual suspect areas in Pottstown and in the Brandywine Valley but these were short duration and the situation this morning is much calmer in many areas locally.  However, relatively strong winds accompanied the front as it moved through yesterday evening, knocking power out to 50,000 individuals in Montgomery and Chester Counties at the peak of the storm.  However, wind gusts yesterday with the squall line were localized in nature.

Philadelphia topped out at 44 mph winds with the front as it moved through before 6 PM.  Localized gusts may have been higher but winds with the squall yesterday were not much stronger than the pre-squall wind.  Philadelphia had gusted consistently in the 30's during the day, with winds as high as 50 mph at Dewey and 47 at Lewes yesterday afternoon before the line hit.

Regarding the tornado watch, while we did not have any tornadic development yesterday there was sufficient wind shear in the atmosphere that warranted the issuance of the watch.  The general "lack" of sun probably helped keep the tornado threat down as it prevented additional mixing of the atmosphere and prevented stronger storms from actually firing in advance of the line.  There were a few instances yesterday where radar indicated some rotating winds within the squall line or advance activity.  We dodged a bullet more than the storm system underperformed.  The criteria in yesterday's watch suggested a 50% chance of a tornado developing within the watch box and 90% chance of six wind damage reports.  We didn't achieve either but if we had a bit of sunshine around in the early afternoon things would have likely been different from a wind standpoint.   From a rain standpoint, the Poconos would definitely say that yesterday's storm overperformed for them.