(Weather Analyst)
Mon Nov 01 2010 02:20 PM
Why Tomas Weakened - Analysis

Using GFS....

At 01/00Z last night there was a broad upper low over all of Central America with relatively small shortwaves rotating from south to north around. There is a broad upper high axis generally extending east and west from multiple centers at 25.0N 82.5W and again at 24.0N and 79.0W. The trough in the SW Atlantic has dissipated.

At 01/12Z the inverted trough near Yucatan had become positive tilted...that is as a trough you see advancing east and south bulging east; not inverted like easterly waves with the bulge facing west. Easterly waves almost always have their weather behind the wave; not in front where conventional shortwaves over the US have their weather in front where positive vorticity creates rotation and lift.

The H=12 hours valid time 02/00Z has this wave cutting off and forming an upper low over the Yucatan Peninsula. We use the 200mb Upper Air Analysis(UAA) for outflow however we do use the 300mb UAA as we progress towards the environment becomes progressively colder; thickness decreases thus heights fall. I had a terrible time in USAF Weather Forecast School understanding this...the best way I can describe this to all is a stand up freezer; you open it and all that dense cold air comes tumbling out; where as a chest freezer only mixes at the top, not the bottom. When that cold air tumbles out imagine it were a balloon with a surface you can see; maybe blue-clear. The edge or surface of the balloon is a constant pressure that its edge either expands with heat or contracts with cold. As the westerlies sink south you will note the heights of these constant pressures also drift south. Ever ask yourself how forecasters know where the rain/snow line is; thickness, the magic number being 546 or 5460 decameters...back to hurricanes.

There is a gradience between these tropical features that is generating shear between the two and it is this wind that has Tomas decoupled...that is its mid-level circulation is tilted or separated to the NE of the surface.

Future track of Tomas....

Begins along the West Coast of the US as a strong jet maxima in excess of 160 knots will dive between the upper closed high off California and a deepening trough along the East Coast in the next few days; this piece of energy will carve a cutoff low over Eastern TX. A second piece of energy will move east and deepen the upper low and adjacent long wave trough. This second piece will kick the cutoff low over the GOM and as it does so surface cyclogenesis will occur along a surface front and deepen as it moves up the coast. This in turn will recurve and kick Tomas NE with possible extra-tropical transition and become absorbed into the frontal zone.

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