Friday, October 26, 2012

Sandy: "Just the Facts, Ma'am"

So, here it is.  We have a storm coming.  Hurricane Sandy or Frankenstorm or whatever you want to call it, we could experience some potentially damaging effects.  So why tell you this, when it’s been pounded hard across all forms of media for the past week or so?  Well, ultimately, it’s to get you prepared.  Hopefully this blog post will summarize what to expect and when and how to prepare.

First, yes, this is a special situation.  With energy advecting from a negatively tilt trough in the Ohio Valley, the tropical system will turn into a hybrid nor’easter.  This isn’t energy on top of energy, but a transition of energy.  I would still handle this like a tropical storm, with the higher rainfall rates and winds, it will have those characteristics.  Early Friday, though, the NHC has kept the storm characterized as "tropical" through landfall.  What this means is that the National Hurricane Center will handle the tracking and official predictions, at least over the next five days. Don't get hung up on whether it stays tropical, nontropical, or whatever through landfall -- the point is the storm will be *some* sort of a storm.  It will be large geographically and impact a large swath of real estate.

As far as flooding goes, we could see three different modes of flooding overlapping each other.  As mentioned by, well, everyone – coastal flooding could be a big problem.  Combining the surge of water being pushed up from the storm to a full moon, we could see some real problems along the shore and other coastal areas.  More on that later.  For the folks living in the city and in Delaware County, the Delaware River from about the Ben Franklin Bridge south into the Delaware Bay are also considered tidal and coastal.  Places around the swamps near Philadelphia International are susceptible to flooding.  Street flooding is also a possibility.  If your area saw flooding in Hurricane Irene, you should plan for similar flooding now ahead of this storm.  Also, make sure you go out and clear out your storm drains from leaves and debris.  Sandy will most likely clog them back up anyway, but starting off clean will make a big difference.  Finally, river flooding – with the heavy rains over a large area river flood could occur, but not immediately.  River will start to rise with the possible of flooding to occur well into the storm or after.  Stream flooding in the "usual suspect" streams is pretty likely as well.

From rain itself, the models vary on totals but a general three to five inch total looks more likely with this storm.  Highest totals appear to be leaning towards south through west of the city as transitioning tropical systems generally produce more rain west of their track...and with a trough in place helping enhance rainfall, some rather prolific rains are possible in Virginia and Maryland...down there, eight inches plus can't be ruled out. However, if Sandy's track is up towards Long Island those higher rainfall totals will need to be nudged towards us.  "Lower" rainfall totals are possible up towards New York City assuming the Euro track into the Delmarva is correct.

Winds are another factor.  Not since 2010 has Philadelphia and the immediate surrounding counties seen a severe windstorm.  Yes, we’ve had storms roll through with gusty winds and trees down and whatnot, but I mean strong to severe damage.  Now, in my conversation with the Warning Coordination Meteorologist from the National Weather Service in Mount Holly, he described the possible wind effects as this: "Take what happened in South Jersey from the derecho in June.  That damage was caused by strong, sustained winds over about 10 minutes.  Take that wind and keep it sustained for over a few hours or more."

Again, not a scare tactic, but another way to describe the possible effects from wind.  I’d also be concerned with the way the strong winds interact in urban areas and even in Center City.  A sustained round of 25 to 40 mph winds over twelve plus hours inland, 30 to 50 mph sustained at the Shore...and higher gusts.  Those higher gusts could reach 75 at the Shore.  For point of comparison, Irene's strongest winds in Philadelphia were sustained at 34 mph with a gust to 51.  A bad storm but increase the duration of the storm out a good six, eight hours and that's the direction things could go with Sandy from a wind standpoint.  Even if the winds don't verify to that level, it is looking like a rather miserable time of things ahead from a rain and yuck standpoint.

While it doesn’t seem likely that Sandy will completely miss the East Coast altogether, there is a wide array of possible effects and varying degrees of severity.  Worst case in terms of coastal impacts is if Sandy tracks onshore over the Delmarva.  A prolonged period of east winds will cause significant coastal flooding to a greater magnitude than if the storm were to track in over Long Island (the outlier scenario at this point).  Worst of worst impact areas on a Delmarva 'landfall' would include Delaware Beaches, Delaware Bay, and the Jersey Shore -- even up towards Sandy Hook and towards Metro New York...although the magnitude of impact gradually lessens as you get north of LBI in a Delmarva landfall scenario.  If the center crosses near Cape May, those impacts will be felt into Long Island in terms of worst coastal impacts.   In terms of planning and prep, assume the Delmarva hit takes place.

Tom talked about the Ash Wednesday storm and Ida as benchmarks at the Shore...think of those and other historical nor'easters that the locals talk about and that's the impact Sandy *may* bring to the coast on Sunday night, Monday, and early Tuesday.

So what can you do to prepare?  Well, the American Red Cross of both South Jersey and Southeastern PA, as well as FEMA have several resources to help you plan and prepare.  Also, the Ready Region website also has links to some local county webpages where you can sign up for emergency text alerts and emails.  Finally, although some think they hype the situation more, local television stations will help you get through the storm with up-to-date information.  And finally, you can keep it here to!