8:00 AM

An area of low pressure off the South Carolina coast this morning is expected to intensify and slowly move north towards the New Jersey coast over the next three days.  This low pressure system is expected to produce heavy rainfall, strong winds, and the potential for some coastal flooding along the New Jersey coast and the north shore of Long Island.  As such, the National Weather Service has issued Flash Flood Watches, Wind Advisories, Gale Warnings, and Small Craft Advisories for the Philadelphia and New York City metropolitan area.

The water vapor satellite picture this morning shows a very complicated upper level pattern over the eastern half of North America.  However, the key to understanding this forecast can be found in the Canadian Maritimes.  That area of dry air seen as black over Maine and Quebec, Canada is due to sinking air associated with an upper low over the Canadian Maritimes.  This is a feature of the east based negative NAO and is currently supporting a strong area of high pressure over Quebec and northern New England.  If not for this block over the northern Atlantic, this entire upper level pattern would not be able to produce a heavy rainfall event much less a long duration rainfall event because the various upper level disturbances to the west would simple exit into the Atlantic with little delay.  However, due to the blocking in the Atlantic, several upper level disturbances will be able to interact and intensify along the East coast.

The main upper low is currently diving south-southeast towards the Mississippi Valley.  The upper low is currently interacting with an area of low pressure over the Arkansas/Mississippi boarder, which is producing very heavy rainfall over Tennessee and Kentucky this morning.  Meanwhile, the Sub Tropical jet stream over the Gulf of Mexico and off the Southeast coast is supporting the development of a coastal low off the South Carolina coast, which in turn is interacting with the high pressure over Quebec to produce an increasing easterly flow over the northern Mid Atlantic and enhancing the coastal front that is in place.  Thus the rainfall developing over the region this morning.  See, I told you this is complicated!

Now, because of the North Atlantic blocking, all of these disturbances will interact and phase into one storm.  How do we know this?  Well, keep an eye on the dry air on the water vapor over Virginia.  This area of dry air is moving east, not northeast, which means the entire lifting mechanism of the upper low over the Plains is going to move towards the coastal plain.  When that happens, the coastal low over the South Carolina coastal waters will become the primary low pressure system and then this storm really gets cranking!

As the strong lifting hits the coast, all winds from the surface to 500 MB will shift to the east and southeast as the upper low becomes strongly negatively tilted.  This means a strong moisture advection from the Atlantic will develop and drive right into the New York City and Philadelphia metropolitan areas.  This strong moisture advection will begin to intensify this evening and increase in intensity through Sunday morning.  Meanwhile, the surface low along the coast will slowly move towards the New Jersey coast.  As the low pressure system intensifies, the pressure gradient will intensify at the same rate leading to sustained winds of 20 to 35 mph along the coast with gusts exceeding 50 mph at times.  Rainfall amounts of 2 to 4 inches is expected by Monday morning with some locations exceeding 5 inches via heavy downpours.

The brunt of this storm will be from Saturday morning on through early Sunday morning where rainfall amounts of a quarter to a half inch per hour can be expected.  Winds will be very strong as well in this time period.  This storm will be capable of widespread moderate flash flooding if not major flash flooding due to the heavy rainfall on top of an already saturated ground from the snow melt of the past week.  This storm will also be capable of minor flash flooding due to the persistent easterly wind along the New Jersey coast.  Further, the high winds will produce the potential for power outages and significantly reduce visibility.  Travel conditions will be significantly impacted as well.

The storm slowly exits on Monday with lingering showers through potentially Tuesday morning.  High pressure will take hold over much of the eastern United States for next week into next weekend leading to dry conditions and temperatures moderating to near to above normal levels.  There is potential for some days to push into the 60′s for the end of next week!