In response to Reid’s Charley (2004) Analysis

Reid,

Thanks for the reply.  I think the image from the paper is a classic example of what we have to work on.  Two questions I have after looking at the image as I continue to develop an understanding of the current and past TC wind forecasting techniques.  First, it is my understanding the guidance from NHC best track data serves as a starting point for forecasters predicting TC winds during landfalling storms.  Forecasts must be consistent with best track data.  Has this always been the case?  If so, why are there two different forecasted locations for Charley (one near Jacksonville, one in the Gulf)?  This leads me to my second hypothesis:  did the two offices not use the same best track forecast as a starting point?  The images below show the changes in forecasted locations over a sequence of advisory periods.  Did the forecast office in Jacksonville not communicate with the office in Tampa?

Shown below is the NARR Reanalysis 1000mb winds for the forecasted time I plotted, FYI.

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3 Responses to In response to Reid’s Charley (2004) Analysis

  1. Frank says:

    Thanks for picking on us again, Reid 🙂 (I was a forecaster at the Tampa Forecast Office when Charlie hit).

    The NDFD forecast grids that showed 2 centers for Charley was due to a couple of factors working together. First, the NHC forecast track slowed down considerably between the 11 AM advisory and the 5 PM advisory on 8/11/04. Based on the 11 AM advisory, the center was supposed to be emerging from the northeast coast of Florida around 2 PM on 8/13. However, you can see on the 5 PM advisory the position at 2 PM on 8/13 was now forecast to be just off the southwest Florida coast.

    Tampa was using the latest forecast from NHC, and therefore had a position off the southwest Florida coast. The forecast grids were not updated as quickly at Jacksonville, so there “older” grids were still in the NDFD when this image was taken.

    There has been some progress with this issue. The offices are now supposed to update their wind grids within 2 hours of the issuance of the products from NHC, mitigating some of this problem. Also, there is a better smoothing method used in the script that creates the default wind grids based on the NHC forecast information. However, this really doesn’t solve the problem with the inland winds because those are usually modified significantly by the individual offices.

  2. Reid Hawkins says:

    Sorry about that Frank, I just remembered there was a good example in the assessment of some of the problems that had occurred. The example from the assessment was a case where consistency, and the mechanics of the National Digital Forecast Database raised its ugly head. These issues as Frank mentioned have sort of been worked out.

    The issue with our case was it appears that different reduction factors were used at all the offices. For track points we all get the points when the TCM or preTCM data is available. We have to follow these tracks there is no compromise on those. The reduction in wind speed is a variable for everyone. Our office was using a 20 percent reduction, even though in an empirical study by one of our UNCW students it showed that 10 percent was valid for sites that were within about 5-10 miles of the coast and 20 percent was valid inland. In this case the latter would have worked any better than a static values for all land areas.

    I also think that the wind reduction should be higher in these re-curving systems that only graze the coast. The starting point we use with the hurricane center in the wind quadrants is for the highest wind one could expect somewhere in that quadrant. We have rarely seen these maximum winds happen with these glancing blows with the center off the coast. Only in cases like Hugo, Fran, Floyd, etc, where the storm penetrates the coast would I expect the winds to be stronger.

    Also, in this case of MHX, it has a extremely difficult job with the large amount of real estate they have to deal with that are in the sounds. And reduction of winds versus land is a challenge especially with different wind trajectories encounter differing frictional components.

    Reid

  3. Gail Hartfield says:

    It’s great to see this dialogue on Earl’s wind and gust forecasts. Clearly, many issues and concerns remain with respect to TCs and WFO forecasts, and this is precisely why this CSTAR research is so needed. As Reid says, we have no wiggle room on the actual track center, but the inland wind reduction can be quite variable, even within the office. I recall that I used a 20 percent reduction for Earl, although I can’t say that everyone here did the same in preceding or subsequent forecasts. Gusts are even more variable from forecaster to forecaster and from office to office; some use a flat kts-over-sustained method, while others use a percentage of sustained (say, 110 percent). Coordination is always challenging, and the vast majority of WFO forecasters try their best to identify inconsistencies and resolve them before the forecast hits the NDFD. But, coordination for all weather elements takes an enormous amount of time, and as is the case when you have humans making their own forecasts, each with his/her own thoughts on how it will all play out, there will always be some differences. Reid, I was unaware of the UNCW study you mention – was this written up anywhere? We would love to see it. I agree with you that a graduated reduction factor for winds and gusts as you go inland would, in theory, make the best forecast. I would hope that sometime soon, we can have a tool in GFE (the Graphical Forecast Editor, which is the application in which we construct our gridded forecast graphics, for those who are unaware) that works off the topography and friction to calculate the inland wind reduction and gusts, based on the data analysis resulting from this CSTAR research. I’m particularly interested in seeing the historical datasets over the mountains, since it seems that the inland reduction would be quite complicated and challenging with valley channeling and less reduction at the higher elevations.

    -Gail

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