Karen, the eleventh tropical storm of the season, dissipated along the central Gulf coast Sunday morning. This was a relief to that area of the country, but was not expected from the forecast three days before. The Federal Emergency Management Agency (FEMA) had already begun to recall personnel on furlough from the government shutdown. Keep in mind that meteorologists at the National Weather Service (NWS) and National Hurricane Center (NHC) were already on the job, just not getting paid.
So what happened?
The tropical storm formed Thursday morning north of the Yucatan Peninsula and in my last post you can see what the track models were suggesting. However, the storm was already being affected by a southwesterly shear aloft and dry air covered much of the western Gulf of Mexico.
There were some models suggesting that Karen would intensify and possibly become a hurricane. It did intensify to sustained winds of 65 mph late Thursday, but this was the strongest that it would become.
|Intensity forecast from 12z Thursday, October 3, 2013. Click on the image for a larger view. Image Credit: UCAR.|
Southwesterly shear aloft and dry air increased west of the storm and it was clear that Karen was struggling on Friday. Dry air cover everything to the west of the center by Saturday and there wasn’t enough moisture feeding the storm to overcome the large-scale factors. Karen continued to weaken on Saturday becoming a minimal tropical storm and then a depression. It finally became a remnant low Sunday morning.
|The water vapor image for 21z Saturday, October 5, 2013. Note the orange and red areas west of the center denoting the dry air in the atmosphere. Click on the image for a larger view. Image Credit: UCAR.|
How did the models do?
It will take some time to sort through all of the information. The GFS (American) model did a decent job of forecasting rainfall for the upstate of South Carolina, but was too far north. The ECMWF (European) model did a better job on placement and on rainfall for the coastal plain of South Carolina. These models did better on intensity than some of the hurricane models.
|The GFS 96 hr forecast (valid 12z Monday) from 12z Thursday, October 3, 2013. This forecast depicts the surface pressure pattern along with 6 hr rainfall (colored areas). Click on the image for a larger view. Image Credit: WSI.|
|The ECMWF 96 hr forecast (valid 12z Monday) from 12z Thursday, October 3, 2013. This forecast depicts the surface pressure pattern along with 6 hr rainfall (colored areas). Click on the image for a larger view. Image Credit: WSI.|
All of the models struggled a bit. The best consensus seemed to be on Thursday. Thereafter, the track models began to diverge. In all fairness to the NHC their forecast seemed reasonable and they had included discussion about the possible effects for the shear and dry air. Thus, what happened to Karen was not a big surprise.
|The surface map for 12z Monday, October 7, 2013. The remnant low of Karen is attached to the cold front meaning it has been absorbed into the frontal system. Click on the image for a larger view. Image Credit: NOAA.|
The remnants of Karen became absorbed by an approaching cold front Monday morning. Having lost its tropical characteristics does not mean the end of the remnant low. The low pressure system is expected to slowly move across north Florida and may stall for a time off the Southeast coast. This could prolong the period of clouds and rain for parts of the Carolinas into southern Georgia. The cool air coming in the backside of the system may result in a cool air wedge that could last until Thursday; a dramatic change of temperature from what occurred over the weekend.