The Calm Before the Storm

Irene is a huge hurricane.  Hurricane Watches are now in effect for the entire Massachusetts, RI, and Connecticut coast

What does that mean?  It means that Tropical Storm force winds extend 425 miles from the center.  It means that Hurricane Force winds extend over 100 miles.  It also means that you need to ignore what Category the winds fall in.  The central pressure is the true indicator of strength in a large Hurricane like Irene.  Right now the central pressure is 946 mb.  This can support winds of up to ~140 mph which is a category 4 hurricane.  So why are Irene's winds "only" 105 mph?  The answer lies in the large wind field.

Watches/Warnings and wind field

In small, compact Hurricanes, they can really spin up and when the pressure goes low, the winds respond.  However the area that experiences heavy wind is limited.  In the case of Irene, she is distributing her power across a much larger area.  This is very similar to what happened to Hurricane Ike in 2008.  Ike devastated Galveston and the North Texas coast as a category 2 hurricane.  It wasn't the strong winds that caused the devastation, it was the storm surge, heavy rain, and consistent winds of category 1 strength for hours.  Ike caused $30 billion in damage in the United States and killed 123 people.

Irene will only slowly weaken as she makes her way up the East Coast.  The shape of the Hurricane should morph into a Nor'Easter looking cyclone.  This is typical of Hurricanes as they transition from the tropics to the mid latitudes.  So here is my first "impact map" of what we can expect.

Green= flooding rains

Light Purple= Sustained Hurricane Force gusts over 100

Brown= Brief sustained Hurricane winds, gusts 80-100

Red= Tropical Storm force Sustained, frequent gusts over 75

Blue= Moderate Storm surge impact

Dark Purple= Major Storm Surge impact

Black= forecast track. Areas east Tornado threat

The slightest shift makes all the difference.  If it spends more time over water than over land while coming up the coast impacts will be even worse.  There is a flood watch for Worcester County because 4-6 inches of rain should create some problems.  Its just that area's west of the center will be receiving more rain. 

Anywhere east of the black line will suffer widespread power outages and numerous down trees.  A major coastal flooding event is going to occur on the South New England coast.  Here is the National Weather Service forecast discussion for Hurricane Irene.  

Rainfall forecast from HPC

Timing is still a work in progress.  Some models are speeding the cyclone up, which does make sense, but I want to see if this trend continues.