06/10/10  3:35 PM

In a surprise turn of events, a lone thunderstorm has reached severe status, the only one in all of the Northeast.  This thunderstorm is located in south Litchfield County of northwestern Connecticut and is moving southeast at 30 mph towards central and eastern New Haven County.

A TORNADO WARNING has been issued until 4 PM after Doppler Radar indicated the thunderstorm was capable of producing a tornado.  The potential tornado was located near Cornwall Bridge, Connecticut and will be near Thomaston at 3:35 PM and Oakville and Terryville by 3:40 PM.

The thunderstorm is capable of producing hail up to a half inch in diameter, heavy downpours, frequent lightning, and wind gusts over 50 mph in addition to the potential tornado.  The thunderstorms will move southeast towards the Long Island Sound by 4:15 PM and through Long Island between 4:30 PM and 5:00 PM.  The thunderstorms are expected to weaken over the Long Island Sound, but will still be capable of severe weather.

4:45 AM

Tropical Storm Grace

The Tropical Atlantic has produced a surprise tropical storm over the northeastern Atlantic, roughly 420 miles northeast of the Azores Islands.  Tropical Storm Grace is a compact tropical system that has developed along an old stationary frontal boundary.  The future of Tropical Storm Grace is pretty much nailed down as a deep trough over the central Atlantic will force Grace to move to the northeast towards Spain.  However, it is interesting to note how well developed Grace has become with a clear eye wall structure and a rather impressive outflow.  Tropical Storm Grace has sustained 65 mph winds and is close to becoming a category 1 hurricane.  Even still, the future of Grace suggest that this tropical cyclone will become extratropical by the middle of this week with no threat to anyone except the shipping lanes of western Europe.

The rest of the Tropical Atlantic remains quiet as a deep trough dominates from the western Atlantic to the central Caribbean.  High pressure is expected to build over the Gulf of Mexico, which will lead to more favorable conditions, however no development is expected over the next 48 hours.

11:20 AM

Tropical Storm Erika continues to weaken late this morning as strong southwesterly shear has sufficiently disrupted the low level circulation.  Sustained winds have fallen off from 60 mph to 40 mph and the pressure has now risen to 1008 MB.

There is a pretty wide range in the models as far as solutions for Erika.  Looking at the 12 Z model guidance, there is just as much chance as Erika being downgraded to a tropical depressions as there is that Erika becomes a major hurricane in the next three days. The difference in the model guidance stems from the uncertainty of the development of upper level shear.  The majority of “global models” like the GFS, ECMWF, and CMC suggest that shear will remain strong and thus limit the development of Erika to much more than a weak tropical storm at worse.  The usually reliable tropical model guidance suggest that shear will weaken as the upper low over the Caribbean shifts to the west, thus allowing Erika to rapidly intensify over the very warm waters around the Bahamas.  The NHC is reasonable in siding with the global model guidance considering the upper level wind shear this morning ended up being strong than forecasted by any guidance and the trends clearly side with the GFS, EMCF, CMC family.  However, I want to stress that there is a lot of uncertainty with this forecast and that the degree in which wind shear will have to decrease to allow Erika to intensify is not overwhelming significant.  My point is that Erika has the potential to pull a big surprise for everyone from the Bahamas to Maine, and as such, this storm needs to be monitored carefully.

6:47 AM

The main phrase to describe the pattern for the end of this week and into next weekend is progressive.  There are no sustained troughs that will drive the pattern, instead troughs will dive into the West and lift northeastward into the eastern Great Lakes.  This type of pattern produces fast moving low pressure system with strong upper level support, which can lead to some surprising severe weather potential for each cold front passage.  At the same time, if you don’t like the current weather conditions, you may only have to wait a few hours for a new air mass to come in.  

On Thursday, high pressure over New England will provide the forecast area with clear to partly cloudy skies and near normal temperatures once again.  However, the high pressure system will quickly exit New England as another low pressure system approaches from the Ohio Valley.  The low pressure system will track towards central New York, which will place much of the forecast area in the “warm sector” of the low pressure system.  There is potential, much like yesterday, for strong to severe thunderstorms to develop over the forecast area given the current forecasted track of the surface low pressure system and position of the upper level trough.  This situation will have to be monitored carefully.

Clear skies, light winds, and comfortable conditions will return for next weekend as strong high pressure over the Ohio Valley builds into the Mid Atlantic.  However, the pattern will not allow the high pressure system to sit over the forecast area for long and another low pressure system with showers can be expected by Monday.  

Temperatures through this period will average near normal as the predominant air mass will have origins from the Pacific with some introduction of a more tropical continental air mass on Friday morning.

5:07 PM

I have to admit I am pretty surprised that the SPC has not issued a severe thunderstorm watch at the very least over the Philadelphia metro.  I don’t think anyone can really question as to whether severe thunderstorms will form this afternoon, as they already have.  At this point, if I was at the SPC, I would at least issue a severe thunderstorm watch for the Philadelphia metro through at least 7 PM this evening.  

Of course, I don’t have that power.  What I can say is that all those in the Philadelphia metro and the central/southern New Jersey coast including Trenton and Wildwood, New Jersey should be prepared for some strong to severe thunderstorm through this early evening.