A Bret is born...
Posted July 18, 2011 7:05 a.m. EDT
Early season tropical cyclones in the Atlantic basin are often the result of circulations associated with decaying frontal boundaries that drift off the southeast coast of the U.S. over the Atlantic of Gulf of Mexico, stall, and then largely dissipate. Newly formed Bret is a good example, as the swirl of low pressure that has transitioned into a tropical storm was initially a baroclinic (meaning "driven by horizontal temperature gradients") low pressure wave along the front that brought us our pleasant break from heat and humidity.
As seen on the first image, taken from the interactive hurricane tracking map in our "Hurricanes" section, you can see the center of the storm early this morning was about 150 miles east of West Palm Beach, drifting very slowly east. The shaded fan shows the National Hurricane Center projected track that accounts for about 2/3 of the errors of historical forecasts from the past 5 years. The second image shows the same map with the "forecast models" option turned on. As you can see, there is pretty good agreement among most of the more sophisticated models that the storm will move to the northeast and gradually accelerate into the open Atlantic. Of course, there are always some outliers! The one that moves almost due west, in green, is a fairly simple advection model that assumes a very deep storm, when this is likely to be a rather shallow one for some time. Likewise, there is a dark gray track leading south-southeast that is a simple extrapolation based on the past twelve hour positions of the storm. Since it has drifted south on average during that period of time, it is projected to carry on that movement.
As another way to compare how some of the models are treating the storm, the remaining series of images all show model projections of sea-level pressure contours (black lines depicting isobars) and intensity of rotation around 2500 feet above the ground (shaded colors, with oranges being cyclonic and blue anticyclonic). Each image, from the tropical models section of Penn State University's web site, shows the pattern projected by that model (including the Canadian, American Global Forecast System, US Navy NOGAPS, and European models) for Tuesday evening at 8 pm. Most of them indicate a less organized system off the southeast coast, shown by a wave or nearly pinched-off contour in the pressure field, and more clearly by a maximum of the rotation indicated in orange, but it is notable that one model (the European) almost completely washes out the system. It will be interesting to see whether this model is out to lunch, or if it turns out that is the only one that correctly predicts dissipation of Bret - we'll know by late tomorrow!