Saturday, October 12, 2013

Coastal Storm Soaks Parts of Pennsylvania


72 Hour Accumulated Rainfall as of 1 P.M., 10/12/2013.

The coastal storm that has been impacting our region for an extended period of time is now weakening and moving to our south due to an area of high pressure building in from New England. The prolonged coastal storm has brought soaking rains to portions of the Mid-Atlantic, including parts of our area. But the “jackpot” for accumulated rainfall is in South-Central Pennsylvania. Despite some additional rain showers this evening closer to home, this “jackpot” location will probably not change. 

Modeling had initially suggested that the heaviest rainfall would fall south and east of Philadelphia with two to as much as four inches possible over the course of several days.  Locations well north and west of Philadelphia were modeled to receive much less rainfall. But that has not exactly been the outcome. Most of Southern New Jersey and Delaware did in fact see storm totals during the last 72 hours in the two to four inch range, but it wasn't limited to just here. Portions of our western suburbs also saw a decent soaking rain with 2 to 4 inches of rain during the past 72 hours. Then there is Central Pennsylvania. Portions of Pennsylvania's Lower Susquehanna River Valley have received more than a foot of rain over the past 72 hours! In Goldsboro, York County, a spotter has recorded 12.60 inches of rain for a storm total and that number was as of late this morning.

So what went wrong with the initial forecast and why were areas of heavy rainfall not just limited to the Southern New Jersey and Delaware? Deep easterly flow became established well inland due to an anomalously strong 850 Millibar Moisture Flux at the nose of a decent easterly Low-Level Jet. This supplied ample moisture well into Central Pennsylvania. The models didn’t key in on this occurring since it underestimated the strength of the easterly flow. Next, a Vorticity Maximum on Thursday Evening was able to lift the air and numerous heavy showers developed in response on Thursday Evening. The Vort-Max rang out the deep moisture in place across this area as if it was a human hand squeezing a soaked sponge. This lifting process was also aided by the oragraphic nature of this area and this factor could help to explain why the blossoming didn’t happen when the Vort-Max was closer to the coast. The showers had the appearance of slantwise convection at times and Infrared Satellite imagery showed very cold cloud tops were able to blossom. In addition to all of this, the radar and satellite imagery indicated that there were some tropical characteristics taking place which is likely attributed to the fact that this energy was part of what had once been classified as Tropical Storm Karen. These tropical characteristics allowed the heavy showers to organize into feeder-like bands of heavy rain which surrounded and moved towards the center of a small mesoscale convective vortex that developed. A mesoscale convective vortex is a term which describes a miniature area of low pressure that pulls winds into a circling pattern. The mesoscale convective vortex couldn’t move as the area of high pressure over New England wouldn’t allow the feature to move northward.  Therefore, the bands of heavy rain sat over and redeveloped over the same areas for several hours.