Oct 4, 2013

Another Update on Karen

Karen continues as a rather poorly organized tropical storm, battling the nemesis of many a tropical cyclone: strong wind shear and dry air. As shown below on the lower left image strong westerly shear of 20-25 knots continues across the cyclone and this is displacing the convection off to the east of the lower level surface circulation. The dry air is evident on the the water vapor satellite imagery (shown to the lower right) and is highlighted in the brown color. This dry air is drawn into the storm's circulation.

Based on the latest recon data from hurricane hunter aircraft, the storm has weakened a bit; the central pressure has risen (up to 1003 millibars) and the winds have decreased to 50 mph. The system is also wobbling on NW-NNW heading of around 10 knots or so. The dilemma facing the forecast is two-fold: 1) the future track and where it eventually makes landfall and 2) its intensity changes - will it continue to weaken, hold its own or perhaps intensify slightly before land fall.

A weaker tropical cyclone, being not well developed tends to be steered by winds in the lower levels of the atmosphere. Two of the four primary global models that are used in tropical cyclone forecasting - the UK and EC models are more west with the track of Karen. The other two models the GGEM (from Canada) and the GFS (the U.S. model) are more east with the GFS the farthest east and also indicating both a stronger and better organized tropical system. Based on the past motion of the storm the EC and UK appear to be doing better on the track and also the strength ( indicating a weaker cyclone). Because of this the tropical storm warning has been extended westward along the coast of Louisiana. This warning is now in effect from Grand Isle to Morgan City. The hurricane Watch remains posted for the moment from Grand Isle Louisiana to west of Destin, Florida but MAY BE discontinued later today or changed to a tropical storm watch or warning later today. So ultimately where Karen makes landfall is still up in the air at this point in time.

The system will ultimately make landfall over the weekend, over southeast Louisiana late Saturday then turning toward the northeast during Sunday as a non-tropical trough of low pressure approaches from the west.  The more west track would now favors the remnants of Karen being absorbed by the trough's cold front on Monday over the western part of South Carolina with the remnants of Karen moving northeast along the cold front as the front itself slides east to the East Coast by Tuesday.

Right now it looks like the heaviest rains of the combined cold front plus Karen's remnants will extend from the Southeast States to the eastern half of Virginia  and the Chesapeake Bay and the Delmarva Peninsula during the Monday-Tuesday time frame. While only moderate rains are forecast for now across Pennsylvania to NY State and western New England (most of this falling late Monday night into Tuesday).

More updates to follow