Sunshine tomorrow, Rain Thursday

Surface Weather Map

I hope you all enjoyed our Indian Summer afternoon.  Indian Summer is defined as a day with temperatures above 70 degrees after the first freeze.  How about 70 degrees after our first snowstorm?  We have received zero precipitation since the Snow-tober Nor'Easter.  That run will continue into tomorrow before breaking on Thursday.  A warm front moved northward and helped steer high pressure offshore, bringing a southwesterly flow to Southern New England.  This is the easy part of the forecast because a combination of Tropical Storm Sean and an approaching cold front will spawn a low pressure system that will track through Southern New England.  Model Guidance is not in agreement in regards to Sean as the system is forecast to stall.  The official National Hurricane Center track curves the storm around Bermuda.  The short range North American Model (NAM) sends the storm into Nova Scotia, spawning a heavy Predecessor Rain Event (PRE) Thursday afternoon.   A PRE occurs because of the warm tropical air being pulled north over a colder air mass.  This rain is supported by mid level energy.  This will happen either way, but if the NAM is right the rain will be much heavier.  

NAM 18z 51 hr run 500 mb Vorticity, NCEP

The long range GFS and the GEFS (which has many members and averages the solutions together) develop a secondary low south of Long Island, turning Sean out to sea.  This would led to less rain here in Southern New England.  The SREF (another multi-member model) likes between 0.25-0.50 inches of rain.  This is a good compromise and its what I expect from this storm right now.  Tomorrow I will outline amounts as hopefully some agreement occurs between the NAM and the rest of the model suite.  The weekend will be dry as high pressure builds in.  Another cold front will approach next week but temperatures will be above average for November, with the exception of Friday/Saturday.  

15z SREF 6 hr precip Monday 5 PM 

So tomorrow is nice, temperatures will again climb into the mid 60's.  Some showers may break out overnight Wednesday.  The heaviest moisture will occur Thursday afternoon.  Highs will be in the low 60's on Thursday.  Windy but dry conditions move in behind the system and temperatures will be in the upper 40's/low 50's on both Friday and Saturday.  The air mass will warm for Sunday and Monday before we may see a cold front next Tuesday.

Tropics Discussion
Tropical Storm Sean is the 18th classified system of the 2011 Atlantic Hurricane Season.  The season will wrap up on November 30th, but things can form and persist in December.  2011 becomes the 7th season with at least 18 named storms.  The others are 1887, 1933, 1969, 1995, 2005,  and 2010.  This has been an above average season so far, no doubt.  I will have a complete analysis after the season of how 2011 stacks up.  However despite the gaudy classified storm total, 2011 actually doesn't belong in the same sentence as those first six seasons.  Take a look at these charts I created using data I've collected from the National Hurricane Center.

ACE is a measure of the energy released by a named tropical system.  As you can see, 2011 has a significant less amount of hurricanes and major hurricanes and a much lower ACE.  According to Dr. Ryan Maue of Policlimate, global tropical cyclone activity is down.  With the Pacific Ocean out of its warm phase, perhaps we are signalling a slowdown of this current active Atlantic cycle.  The larger question is why have so many weak tropical systems formed?  So far 12 of the 18 classified systems (likely a 19th when post season review is completed) have failed to reach Hurricane strength.  The answer, I believe, lies with better detection.  I believe several named systems in 2011 would have been unclassified before 2000.