Tropical weather analysis - November 8, 2011

By: KoritheMan , 2:36 AM GMT on November 09, 2011

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Sean

Tropical Storm Sean became the eighteenth named storm of the season early this morning. However, we are still sporting an abnormally low tropical storm to hurricane ratio, with only six of our storms having become hurricanes. As of the latest NHC advisory, the following was posted on Sean:

Wind: 50 mph, with higher gusts
Location: 27.8°N 69.8°W
Movement: Stationary
Pressure: 999 mb
Category: Tropical storm (Saffir-Simpson Hurricane Scale)

Although convection associated with Sean isn't particularly deep, it is attempting to wrap closer to the center, which shows that the cyclone is at least trying. There is little upper-level outflow, except to the north.



Figure 1. Latest infrared satellite image of Tropical Storm Sean, courtesy of NOAA's Satellite Services Division (SSD).

Shear above Sean is still high in the upper troposphere, but is relatively low in the mid-levels. Since Sean is a shallow system, its vertical circulation doesn't extend high enough into the troposphere to actually feel these winds. It is for this reason that Sean has not dissipated, as an ordinary tropical cyclone would. However, there are indications on satellite pictures that the upper tropospheric shear is decreasing, which may explain the transition of Sean to a tropical storm today. For about the next 30-36 hours, the GFS and SHIPS, which is based on the GFS forecast fields, anticipate a modest relaxation in the shear, enough perhaps, to allow for Sean to look more tropical and potentially strengthen. I think the NHC forecast of a strong tropical storm is good for now. I feel a bit better anyway, given that subtropical and/or loosely tropical entities like Sean tend to be easier to predict than your average run-of-the-mill tropical cyclone. After that time, a sharp increase in shear is forecast, which, when coupled with Sean moving over much cooler sea surface temperatures, should promote weakening -- possibly rapid.

At this time, it is pretty unlikely that Sean becomes a hurricane even if the shear relaxes, as SSTs aren't exceptionally warm. Although, as noted earlier by the NHC, the upper troposphere is anomalously cold due to the upper low in which Sean is embedded; this could mitigate the normal SST/tropical cyclone ratio.

The forecast track is pretty easy. A significant cold front moving into the Ohio Valley is forecast to continue deepening while moving eastward. The resultant synoptic evolution should act to deflect Sean out to sea, well away from the United States and Atlantic Canada. However, the storm may pass close enough to Bermuda to deliver tropical storm force winds there, especially in gusts; this would tend to hold true even if Sean manages to tighten its circulation and thus contract the wind field.

For this reason, a tropical storm watch has been posted for the island.

Global models suggest that Sean should become absorbed by the aforementioned frontal zone on day four.

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1. BaltimoreBrian
4:18 AM GMT on November 14, 2011
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About KoritheMan

I'm just a 23 year old with an ardent passion for weather. I first became aware of this interest after Tropical Storm Isidore struck my area in 2002.

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