10:09 PM

I’ve been breaking down all the data this evening to nail down my “final” forecast on what I believe will transpire over the next three days with Tropical Storm Danny.  In short, I think we are at a point where Tropical Storm Danny is not the main feature in this pattern but an accessory to what will become a strong coastal storm for the forecast area.

Tropical Storm Danny this evening is significantly disorganized  as the majority of the convection with Danny is well to the east of the actual low level circulation.  As a result, Danny has weakened slightly with sustained winds of 50 mph.  Although Danny is entering the Gulf Stream and may have a brief period of intensification, the potential for Danny to become a hurricane is practically at a close.

The main feature in this pattern is rather clear on the water vapor this evening.  As we can see on the water vapor satellite image, a strong disturbance at 500 MB is diving into the Gulf of Mexico this evening.  It should be noted that this disturbance is stronger than forecasted by model guidance several days ago.  A jet streak of around 50 KTS has developed over the Gulf of Mexico and is positioned southwest to northeast through Florida and over the Gulf Stream.  This happens to be the same place where Danny is located.  The strong upper level winds from the southwest is shearing Danny apart and keeping convection away from the low level center.  The disturbance in the Gulf of Mexico though is currently separated from the Polar Jet Stream, seen in blue.  Meanwhile, a stronger trough is diving through the northern Plains and establishing itself over the Great Lakes.

The upper level disturbance is drawing a significant amount of moisture out of the Gulf of Mexico and the lifting is producing wide spread showers and thunderstorms over the Southeast.  This area of rainfall is completely separate from Danny, keep this in mind.

So here is how I think this whole situation will play out.

High pressure over Ontario will build over New England overnight and will remain to the north of the forecast area through early Saturday morning.  The stationary front, that remained over the forecast area for much of the day today, will return over the region, which will set up an area of weakness along the coastal plain and coastal waters of the forecast area.  Meanwhile, Tropical Storm Danny will continue to drift north towards the coastal waters of North Carolina.

By Friday night, Danny will start to interact with the upper disturbance over the Southeast and become absorbed in the dominant upper level feature.  The wind field of Danny will collapse on Friday night as the tropical characteristics of Danny are overwhelmed.  Meanwhile, the trough over the northern Plains will continue to intensify over the Great Lakes.  This is key to the forecast.  If not for this trough, the upper level trough over the Southeast and the remnants of Danny would exit into the Atlantic.  However, as the trough over the Great Lakes forms an upper low, the remnants of Danny and the upper disturbance are pulled towards the coast, prevented from exiting.

On Saturday, technically the low level circulation of Danny will be roughly 100 miles off the New Jersey coast.  However, if you consider this storm at this point as a hybrid/extratropical feature at this point, then this storm is going to track just east of the “bench mark” for Nor’ Easters.  If you look at the model guidance for this time period, the storm barely has any recognizable tropical characteristics by late Saturday morning.  As such, I am forecasting what will basically be a very wet Saturday with the heaviest rainfall along the coast along with windy conditions.

As for the rainfall expectations, the far northwestern suburbs of the forecast area will have the lightest and more scattered precipitation with up to a half inch of rainfall expected.  Locations from the immediate Philadelphia metro through northwestern New Jersey, into central Hudson Valley around Poughkeepsie, and northwestern Connecticut can expect up to an inch of rain.  Locations from southwestern New Jersey through New York City, the southern Hudson Valley including White Plains, New York, and through Central Connecticut can expect 1 to 2 inches of rain.  Finally, the immediate coastal locations can expect 2 inches or more as the remnants of Tropical Storm Danny passes to the south and east of Long Island.

As for the winds, the potential for tropical storm force winds is rather slim as the low level circulation slows down.  The immediate coast can expect gusts up to tropical storm strength, especially over eastern Long Island and the immediate New Jersey coast.  Further inland, winds can expected to drop off significantly to 10 to 20 mph over western New Jersey and less than 10 mph over the far northwestern suburbs.

A strong cold front will move towards the Mid Atlantic on Sunday.  This cold front is associated with the trough over the Great Lakes and will force the moisture associated with the Gulf of Mexico disturbance and Danny off the East coast.  The showers and thunderstorms associated with this cold front will be capable of very heavy downpours.

Through Sunday, the forecast area will have a high threat for flash flooding due to the heavy rainfall and minor coastal flooding due to the persistent onshore flow.  There is also a risk of strong wind gusts and rough seas through Saturday night.

A much drier pattern will unfold for next weekend with dominant Canadian high pressure in control and providing clear, dry, and cool weather conditions to start September.