2012 Atlantic Hurricane Season Birdseye Discussion #9A (Special Update)
...APRIL 29 2012...
Caribbean Sea tropical disturbance emerges over the Florida Straits while upper-level winds quickly become more conducive for tropical development. Surface observations from south Florida show no signs of development at this time...but full birdseye discussions will resume immediately if this changes. This discussion supercedes full update #9 which had discounted potential for tropical development.
The special feature section of full update #9 mentioned the disturbance over the last several hours transitioning to a more subtropical state while supported by divergence E of the cut-off upper trough axis located over the E Gulf of Mexico/Yucatan. It was also mentioned that CIMSS products from University of Wisconsin showed a decrease in the southerly vertical shear associated with calm upper winds near the upper trough axis. However...tropical/subtropical development was discounted in full discussion #9 as the computer models showed an eventual de-amplification of the upper trough...which would have re-increased the vertical shear and reduced the supportive divergence east of the upper trough axis.
Figure 1 below instead shows what has happened to the upper trough between full discussion #9 (181Z Apr 28) and this special update #9A (1315Z Apr 29). The thunderstorms have gained great concentration to the NE of the upper trough axis...resulting in a new warm core upper ridge axis and associated cirrus outflow (which also suggests a trend from subtropical back to a more tropical state). In turn...this new upper ridge axis has caused the upper trough definition to become much more amplified by 1315Z today...to the degree that there is even a new cut-off upper low just N of W Cuba! The NW to SE tilting of the amplifying upper trough has changed the upper wind direction over the Florida Straits to a more easterly direction...which is reducing the vertical shear as the upper winds become more aligned with low-level easterly trade winds. The sharpening upper trough has also increased the divergence over the Florida Straits...and a surface trough has formed as a result as of 0600Z TAFB (kudos to the GFS model which had predicted this would happen as of full discussion #9).
The fate of this disturbance still will be associated with how this upper trough evolves. First off...radar imagery from S Florida (Figure 2) shows a spin developing SE of Key Largo. An isolated rain band moving SW suggests another possible spin due S of the Florida peninsula. This could mean the new 0600Z TAFB surface trough is developing a closed circulation supported by the divergence of the upper trough. Observations in S Florida at this time do not support this theory with stiff surface easterly winds across the board and surface pressures remaining steady at a high 1017 mb. When compared with Figure 1...the GFS computer model is initialized with an under-represented amplitude of the recently-sharpened upper trough. However...the GFS model has the upper trough getting even sharper thru 2100Z today. The GFS then gradually de-amplifies the upper trough while moving it east into Florida tomorrow April 30...and then dissipates the upper trough altogether by May 1. This means westerly vertical shear should re-increase and the favorable upper divergence reduce. This is why I am still iffy on tropical development as the window of favorable upper winds should last only today according to the models. However..one has to be a bit skeptical of the models as they did not predict today's dramatic amplification of the upper trough described in the third paragraph of this update. If surface observations from Florida or elsewhere indicate surface pressure drops and a developing closed circulation...then I will be resuming full birdseye discussions. Otherwise...this is my last discussion until hurricane season starts June 1 2012.
In lieu of these updates...I have to update the rainfall forecast from previous full discussion #9. The 200 mb GFS run has the upper trough getting even sharper thru 2100Z today...and then gradually dissipates the upper trough while moving it east thru May 1. The 850 mb GFS model run has the Florida Straits surface trough turn westward into the south-central Gulf of Mexico (while steered by the low-level ridge axis due N) and dissipates it May 2 (since the supporting upper trough is dissipated by May 1). So does the rainfall move east while supported by the divergence of the dissipating upper trough? Or does the rainfall move west while associated with surface convergence of the surface trough? I'll go with something in the middle...and say the rainfall will be largely stationary and gradually weakening with both the surface and upper troughs dissipating by May 1 and 2.
Steady heavy rainfall/flooding potential in the west Bahamas/S Florida will persist south of the red line in Figure 2 (line running from West Palm Beach to Belle Glade to Naples). Rainfall to the north of this line will be largely limited. Some of the rain shield may expand northward beyond the red line only if the thunderstorm mass expands in size under the supportive upper ridge axis outflow shown in Figure 1.
The Florida Keys may not see as much rain. Right now the Keys are in a dry slot to the SW of the radar rotations in Figure 2. They will be entering the rain mass NE of the rotations once the rotations travel westward. However...it is entirely possible the Keys never escape the dry slot...which is induced by upper-level convergence on the back side of the upper trough. With the models suggesting the upper trough moving eastward...the dry slot may follow suit and expand northeastward. If this dry air mass indeed expands northeastward...it will also begin to end the rainfall from south to north on the mainland of S Florida.
Figure 1: Visible image of Florida Straits disturbance...complete with surface analysis in red and upper-level analysis in light blue.
Figure 2: Radar image from south Florida taken around 1315Z today.