Sunday Thunderstorms and Tropical Storm Irene

Hope everyone enjoyed the first part of the weekend.  Tomorrow will not be as pleasant as today was because a surface front will set off showers and thunderstorms.  The latest North American Model (NAM) is showing the potential for nearly two inches of rain in the next 48 hours. The majority of this will fall after sunset tomorrow.  Any outdoor activities will be in good shape until about 130 PM.  After that showers will develop and then thunderstorms will push through after 5 PM.  As usual, as the threat develops I will post blog updates.  The high tomorrow will again be in the mid 80's.  Monday may start gray due to the leftovers from Sunday night but these clouds will clear and the humidity will drop back into the 50's.  Combine that with highs in the mid 70's and it will be a comfortable day.  Some late afternoon clouds could spit out a sprinkle or two late Monday afternoon and the most likely spot for these is Northern MA/Southern NH.

Short Range NAM model 48-hr precip forecast (click to enlarge)- courtesy of NCEP

After tomorrow night's rains the pattern relaxes for a few days as all eyes will look to the south to track newly formed Tropical Storm Irene.  Irene skipped Tropical Depression status and went straight to a 50 mph Tropical Storm.  This is significant.  Irene is in a favorable environment to strengthen with warm ocean water, deep heat content in the upper layer of the ocean, and calm winds aloft.  The quicker Irene strengthens, the sooner it will turn north.  This makes the system more likely to turn north and therefore makes the system more of a Atlantic Coast threat and less of a Gulf of Mexico threat.

Tropical Storm Irene, 1045 EDT 8.20.2011- courtesy of NOAA

There is a big wild card in Irene's development.  The islands of Puerto Rico and Hispaniola may disrupt the circulation of Irene when she passes by.  That would be a big benefit for Florida as it would take time to regain its overall structure and therefore be a weaker storm.  At this time frame, its helpful to read the overall pattern and what the models are forecasting in terms of pattern over the next few days.  The most important factor is already in place, a strong area of High Pressure between Bermuda and the United States.  This is called the Bermuda high.  

Hurricanes follow upper level wind patterns.  Courtesy USA Today

The amount of hype (myself included!) for Irene is about to increase.  One thing I am not is a "model hugger" who changes a forecast with every model run.  Many people you will watch in the news will do this in the next week.  I can remember Hurricane Debby in 2000 basically being predicted to do the same thing as Irene is forecasted to do now; become a Hurricane and strike the East Coast.  The Weather Channel and the big time cable news networks were all over the development of the system.  Well Debby was torn apart by the mountains of the Caribbean and never made landfall in the United States.  Here is what I know about this system.

• Analog season's support East Coast landfall
• Bermuda High in place
• Trough is forecast to approach from the Midwest, another key sign for EC tropical threat
• The storm has formed*

*This seems obvious but many times models forecast a storm to develop and it does not.  

So my general prediction for Irene is that it will impact Puerto Rico and the northern Dominican Republic coast.  Then the storm will push through the Southern Bahama's turning North, brushing extreme southeastern Florida as a category 1 Hurricane and then striking the Outer Banks of North Carolina.  I expect New England to see some sort of impact from the system.  It could be heavy rain from a remnant system or it could be a direct hit.  For us, its still too early.  This is a blend of model runs and climatology.  More tomorrow.