Sep 2, 2011

DAY 93

Hurricane Katia has been downgraded today, back down to a tropical storm. She has been struggling with a little dry air and some moderate wind shear. Katia's maximum sustained winds are at 70 mph and her pressure has risen to 990 mb. 

(Tropical Storm Katia)

The Hurricane Center is forecasting Katia to regain hurricane status tomorrow, despite the fact that she is entering into a more sheared environment from an upper level low to her northwest. Infrared satellite imagery tonight would seem to support that forecast, as Katia is displaying some impressive convection in her core. The track from NHC has not changed much. They are still forecasting a WNW movement over the next 4 days, and then a bit more of a westward movement on day 5. Still far to the north of the Caribbean islands.

The computer models however, are displaying  a drastic change in the extended forecast for Katia's track. They are starting to suggest that Katia will have much more of a close encounter with the US coastline than they were originally predicting. With as dynamic as the atmosphere has been this season, the computer models are constantly changing their tunes as we go through time. We should only focus on a 72 hour forecast with these things anyway in my opinion. In other words, just focus on the official NHC forecast track in the part that is shaded in solid white. That track has been untarnished this season.

Extended forecasts can be useful, as long as they're not taken as gospel. They can point out things that we need to keep an eye on as tropical systems progress. For example, the latest run of the GFS, which has been one of the more reliable models this season, is showing us 2 things. One is a quick rebuild of the Subtropical Ridge, which would keep Katia on a more westward track. And it is also showing that the remnants of TD#13 or possibly Tropical Storm Lee, could supply some magnetism, pulling Katia into the Atlantic coast.

This would be a nightmare scenario for New England. Especially with the recent criticism that has surrounded the fear factor that was used for Hurricane Irene. There might be a little distrust for the forecast there if those people are asked to evacuate again. I think what the public has learned up there is that when they are told that they should evacuate low lying areas, that goes for inland river and lake areas as much as it applies to the coastline. The highest danger from tropical systems has always been, and will always be rising water levels.

(TS Katia)

Another feature that we've been watching is what is now Tropical Depression #13. It is listed as a 35 mph storm with a central pressure of 1007 mb. It is moving northwest at a whopping 2 mph. It is forecast to loop back toward the northeast and eventually run aground in Louisiana this weekend. With the lack of steering currents in the area this movement could take even longer. 

The Hurricane Center is not forecasting this storm to become a hurricane at the moment, but if it does start to get organized, this still remains a good possibility. TD#13 is already producing tropical storm force sustained winds, but it is not showing any signs of organizing much of a closed surface circulation and the lowest pressure is 1007 mb. You can see that it is being heavily sheared from the west. Once this shear moves out, this system should be able to become Tropical Storm Lee.

(Tropical Depression #13)

The greatest threat from this system will be heavy rainfall. The circulation over the state of Louisiana is being caused by an upper level low, and not a surface circulation. The surface low is likely developing just to the west of the largest cluster of convection. Most of the rain will likely remain to the east of the system. Not much precipitation is being picked up on radar yet, but this will be an extended rain event over the southeastern states. 


Another feature being mentioned by the Hurricane Center tonight is an area in the north Atlantic. It is being given a 40% chance of becoming a tropical cyclone in the next 48 hours, but if it does develop, it will not effect any land areas.

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