Tuesday, March 05, 2013

Winter Storm Watches, Warnings, Advisories

The fickleness of a March storm will be evident tomorrow.  It's going to disappoint some, thrill others, and be a soggy or slippery mess throughout the region...and a pain in the butt at the Shore on Wednesday and again Wednesday night with coastal flooding problems.  This is not an easy forecast...and the modeling spread between various guidance has huge implications for what ends up falling around here.

First, Winter Storm Watches are out for Philly and the immediate suburbs, plus points south.  Winter Storm Warnings are out for Chester and Cecil Counties where the forecast for four to seven inches of snow, to the National Weather Service, is a safe bet.  The Watches for our region are generally for 3-5" snow, which is a compromise between the aggressive NAM and conservative Euro and GFS.

Winter weather advisories are out farther to the north, generally for the upper halves of Bucks and Montco, plus Berks and Lehigh, also for three to five inches in general from the storm system.

There's a pretty hefty spread in the model guidance for tomorrow's storm, just for Philadelphia.  The GFS and NAM are in completely different camps as the NAM is suggesting an overall colder, stronger storm and more snow, with the GFS generally milder with less precipitation.   The NAM, just for Philadelphia, is suggesting 1.68" of liquid precipitation, nearly half of that falling in the form of snow, while the GFS suggests only 0.40", about half of which falls as snow.  Needless to say there's a helluva big difference between two models for the city of Philadelphia -- and those differences have HUGE implications on final outcome....even 24 hours out from the storm.

The reason the NAM is as aggressive as it is comes down to the fact that the model fires up enhanced precipitation across Delaware and South Jersey, fueled by the oceanic influence.  The storm then pivots and brings precipitation north and northeast through the region as heavy snow on Wednesday night after a soaking rain during the day.  It is more aggressive than the other models and farther north with precipitation than the other models.  It might be a bit overdone as the NAM has a tendency to overdo precipitation.

Remember the snowstorm last month that bombed out for Boston but bombed here, bringing us just mere pittances of snow.  The NAM suggested 10"+ in Bucks County 24 hours prior to the storm, with Bucks getting 3" of snow on average.   The NAM can be great at times but sometimes can be woefully wrong.

Prudence in forecasting is the best course of action...hence a conservative approach from us and others around town.

Here's what we think at this point...precipitation spreads northward later tonight, mainly as rain for the city and points east, with snow farther to the west.  During Wednesday, rain falls in the city and points east, with elevation dependent snow falling to the west.  Snow will probably have some trouble sticking outside of elevated areas during the day on Wednesday in those areas where it does snow.

We probably see that transition from rain to snow take place in the evening hours as the storm intensifies upon reaching the ocean, with rain changing to snow during the early evening (5-8 PM as a best guess at this point).  Snow falls steadily through the evening, with the bulk of the accumulations on Wednesday night through the region.

If you remember the March 1st, 2009 storm -- it snowed off and on during the day with very little accumulation through the region, transitioning to a heavier thump of snow after sunset.  There will probably be more rain with this storm but the thinking is similar for those areas that do get snow -- it may take until evening to get a significant dose of accumulations going.

We talked about the potential for a steady thumping of snow across parts of the region -- as of now, the thinking is that band sets up from Atlantic City and Long Beach Island west through Millville, Wilmington, Dover, and back into Chester County.  We're pegging two to five inches through this area as a general rule with the possibility for isolated higher amounts, with 1-3" for Philadelphia.  The Shore picks up less than 2".  The heaviest snows in general will still fall across Lancaster, York, and points on south and west of there with the potential for 5-10" out of this storm.  In the heavier bands south of the city tomorrow evening, one cannot rule out the possibility for thundersnow...especially wherever the heavier bands sets up.

Regarding the Shore, our impact discussion from last night still applies...significant coastal flooding, wind gusts to 60 mph, beach erosion, a miserable Wednesday afternoon high tide and Thursday morning (4 AM) high tide.

We'll have another update on this in the morning.