Tropical Storm Emily (8/4/2011)


Tropical Storm Emily officially formed on the evening of August 1, 2011 after a reconnaissance airplane had detected maximum sustained winds of at least 39 mph at the surface.  The National Hurricane Center had a 70%-near 100% area of tropical cyclone formation for about 3 or 4 days prior to the reconnaissance flight.

Models, as well as forecasters, have been having a little bit of trouble trying to figure out Emily’s path and intesity over the past few days.  According to earlier (4 or 5 days ago) forecast model runs, Emily should be a hurricane, by now, and moving in a northwestern direction, not a tropical storm moving directly west, as is the case right now.  There are a few reasons why this storm has been so hard to forecast.

The first reason is shear (winds with height).  Strong shear weakens tropical systems and makes it difficult for them to organize.  Some models can’t really pick up on shear intensity very well, especially on a mesoscale point of view.  There has been some shear feeding into Emily from the south and southwestern side.  This is why most of Emily’s storm convection is located on its northern and eastern side.

Another reason why Tropical Storm Emily is only a tropical storm, instead of a hurricane has to do with sea surface temperature (SST).  SST usually determines how much potential energy the storm can feed off of.  The warmer the water, the more energy the storms can potentially have, essentially.  Taking a look at SST, below, you can see that although Emily is in warm waters, the warmest waters are north, northwest, and southeast of Emily’s current location.

Figure 1. Current (00Z 8/4) sea surface temperatures in Atlantic Basin. Map provided by AccuWeather. Click image for a better view.

As far as the forecast track is concerned, there are a couple of issues with determining Emily’s track, but there is one main cause that determines its path.  Surface high pressure in the north-central Atlantic Ocean, commonly known as the Bermuda High, will affect where Emily goes.  Once again, models are having trouble determining its current position and its forecast position in the future.  Taking a look at past, present, and future position of this high pressure, it would be helpful in explaining what I’m trying to say.  Below, are 3 images.  The first two images show a split screen shot of 500mb data on the left and surface analysis on the right.  The Bermuda High will be located on the right side of the images.  I circled the Bermuda High in yellow and drew the flow around the high with yellow arrows as best I could.  The flow around a surface high is clockwise. Please make sure to click on each image for a better view.

Past (12Z 8/1/2011):

Figure 2. 12Z 8/1/2011 500mb data (left) and surface data (right) in the Northern and Western Hemisphere. This is to show the Bermuda High in the Atlantic Ocean. Courtesy of Penn State E-wall. Click image for a better view.

Current (00Z 8/4/2011):

Figure 3. 00Z 8/4/2011 500mb data (left) and surface data (right) in the Northern and Western Hemisphere. This is to show the Bermuda High in the Atlantic Ocean. Notice the high sifted a little more towards the east. Courtesy of Penn State E-wall. Click image for a better view.

Just looking at two and half days ago to the present, you can see that the Bermuda High has shifted east a little bit, and thus the flow around the high has moved.  It looks like this small shift will allow Emily to finally start moving northwestward fairly soon.  The shift comes about 2 or 3 days late to what forecast models predicted.

Future (ECMWF 12Z 8/6/2011)

Figure 4. Surface 12Z run of ECMWF on 8/6/2011. Shows the Bermuda high moving south and eastward. Eventually, Emily will get caught up in the jet stream. Courtesy of Penn State E-Wall. Click image for a better view.

The ECMWF shows Emily turning northwest around the flow of the Bermuda High.  Once Emily gets far enough north, it will get caught up in the jet stream coming out of the northeastern part of the U.S. and mostly likely will transform into an extratropical storm, where it will no long be a tropical cyclone threat to the United States.  We will have to wait and see what happens.

As of right now, Emily is forecast to side-swipe the eastern coast, including Florida.  However, I would not be surprised if Florida does get hit directly, because of the poor forecasting that the models have done over the past few days.  With Emily coming in contact with land (Haiti, Hispaniola, Cuba, etc) , I don’t believe it will become a hurricane anytime soon.  Once (or if) it gets back into open water, then there is a greater possibility that it may become a Category 1 hurricane.  Therefore, Florida would only have to deal with a strong tropical depression or a weak to moderate tropical storm, worst case scenario.

National Hurricane Center’s Forecast Track: 

Figure 5. NHC's forecast track of Tropical Storm Emily as of 2am EDT 8/4/2011. Click image for a better view.


About Brian

University of Oklahoma graduate with a degree in Meteorology. Follow me on Twitter: @WeatherInformer
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