9:10 AM

Showers and thunderstorms continue to develop over the southern Mid Atlantic and move north over much of eastern Pennsylvania, New Jersey, the five boroughs of New York, and the Hudson Valley.  Meanwhile much of Long Island remains high and dry this morning.

As expected, there are several factors that is driving this rainfall, and yes Hurricane Bill is having some positive and negative influences.  The GFS and NAM model guidance shows some interesting influence from Hurricane Bill for this afternoon.  Naturally, there are plenty of thunderstorm rain bands around Hurricane Bill, but note the lack of precipitation off the New Jersey coast through eastern Long Island and off towards Boston, Massachusetts.  This is an area of subsidence created by the hurricane!  Remember when I mentioned about the possible influences of the upper level high pressure circulation over Hurricane Bill.  Well, here is one of those influences.  The air at the upper levels must sink at some point, and it is in these locations listed above where that air will sink towards the surface and hinder precipitation development.  Why is this important?

Pardon the cruel drawing, but here is basically what is going on.  The sinking air over the coastal waters must go in one of two directions once reaching the surface.  One direction is back towards Hurricane Bill and back into the overall circulation of the storm.  The other direction is towards the New Jersey coast.  This is now how Hurricane Bill will influence the forecast area!  The strong push of moist air from the Atlantic is driven up against the cold front that has stalled over the forecast area.  The cold front alone produces strong enough lifting for showers and thunderstorms, however by this afternoon the lifting is significantly enhanced because of this interaction of air flow.  As a result, the lifting over New Jersey and eastern Pennsylvania will be enhanced through this evening until Hurricane Bill exits from New England.  The heaviest rain will likely fall between 1 PM and 6 PM and then begin to taper off as Hurricane Bill exits.  One other note is that the sinking motion around Hurricane Bill will likely keep much of Long Island on the dry side of this forecast and that it is likely that the rain shield ahead of the cold front will shift to the west as the sinking motion described above drifts closer to the coast later this afternoon.  So don’t be surprised to see rain ending along the immediate New Jersey coast like around Asbury Park, Sea Side Heights, and Atlantic City while Trenton, Freehold, and other interior locations continues to be impacted by the rain.  Much of eastern Pennsylvania is going to remain rather wet through tomorrow morning, no extended break is expected there.

On the radar look for waves of showers and thunderstorms to continue to develop.  A brief break is possible between each wave, but as we can see on the radar, when one wave exits another quickly begins to redevelop over the southern Mid Atlantic.