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.
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| 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.
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| 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.
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| 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. |
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| 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.
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| 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.
