Tropical Storm Hermine Follow Up with Lessons Learned, Wind Gusts and Gust Factors

tcm-wind-11am-0901

Four quadrant wind radii from the raw TCM guidance from the National Hurricane Center for Hermine issued at 11 AM EDT 01 September 2016.

To evaluate the performance of CSTAR related research to operations activities, members of the CSTAR Tropical Cyclone (TC) wind team recently discussed experiences with Tropical Storm Hermine (September 2016).  In addition, the sustained winds, wind gusts, and gust factors for Tropical Storm Hermine were examined across coastal and eastern Georgia, South Carolina, North Carolina and Virginia.

A conference call was conducted with team members from NWS offices in Charleston, Columbia, Newport, Raleigh, Wakefield, and Wilmington along with NWS Eastern Region SSD to examine the event in greater detail and to identify ways to improve the process over both the near and longer term. Hermine had some special circumstances that made the use of the TCM wind guidance more difficult than normal, including the TC’s weak intensity, the limited wind radii on the western semi-circle of the storm, and the fact that the storm was undergoing extratropical transition as it began to exit the region. The CSTAR TCM wind technique likely won’t be able to completely mitigate these issues. However, a couple of other issues were noted and some solutions are being developed to handle these items:
1)    Adjustments are being made to the CSTAR_Wind_from_WindReductionFactor smart tool so that the wind reduction is only applied to winds of 34 kts or greater (inside the TCM envelope).
2)    CSTAR TCM wind technique offices will explore the use of brief conference calls following the NHC coordination call, to discuss the PREAT guidance and ensure offices are using the technique appropriately and are well collaborated.

NWS Raleigh volunteer Victoria Oliva, examined the sustained winds, wind gusts, and gust factors for Hermine across coastal and eastern Georgia, South Carolina, North Carolina and Virginia. Hourly observations of winds and wind gusts from 48 regular ASOS or AWOS METAR locations impacted by the over land wind field associated with Tropical Storm Hermine were examined. The locations examined in this analysis extended from KSAV (Savanah, Georgia) northeast along and just inland of the coast to KWAL (Wallops Island, Virginia). The map below is a subjective analysis of the maximum wind gusts observed during Tropical Storm Hermine across North Carolina.

windgust-20160903

Subjective analysis of the maximum wind gusts (MPH) observed across North Carolina from 02 September to 03 September 2016 during Tropical Storm Hermine.

Only observations from routine hourly METARs were used (special observations and observations not at the top of the hour were excluded). In addition, gust factors were only calculated for sustained winds of 10 MPH or greater. For each observation, the hourly wind gust factor was computed. The gust factor is defined as the ratio between the wind gust of a specific duration to the mean (sustained) wind speed for a period of time.  A total of 860 gust factors were computed for Hermine.

In general, Hermine was characterized by modest sustained winds as it moved across the region although the strength of the wind gusts increased as the storm evolved during extratropical transition as it moved across North Carolina and off the coast. The maximum sustained wind contained in the Hermine data set was only 44 MPH and the maximum wind gust was only 62 MPH. The data set contained a large number of lower end sustained winds and wind gusts. Nearly 82% or 705 out of the 860 observations, contained in this data-set had sustained winds less than 25 MPH.  Only 64 observations, or less than 8% of all observations, had sustained winds of 30 MPH or more and only 3 observations had sustained winds of 40 MPH or more.

hermine-gust-factor-scatter

Scatter plot of sustained winds and gust factors for Tropical Storm Hermine for 48 METAR locations across coastal and eastern Georgia, South Carolina, North Carolina and Virginia

The chart to the right is a scatter plot of the sustained winds in MPH versus gust factors for the 860 observations included in the study along with a best fit regression curve (y = -0.375ln(x) + 2.6175). In general, the chart demonstrates an inverse relationship between the wind speed and gust factor. Not surprisingly, the gust factors with Hermine were rather variable at low sustained wind speeds and generally converged and decreased with increasing sustained wind speed. This chart is very similar to the database of 15 storms used to develop the CSTAR TCM wind technique.

hermine-gust-factor-bins

Histogram of gust factors from Tropical Storm Hermine for 48 METAR locations across coastal and eastern Georgia, South Carolina, North Carolina and Virginia.

A histogram of the frequency of gust factors for Hermine is shown to the right.  The average gust factor for Hermine was 1.58 which is somewhat higher than the average of 1.53 for the database of 15 tropical cyclones used to develop the CSTAR TC wind technique. The histogram data noted that the gust factors were most frequently noted in both of the bins between 1.4 and 1.5 as well as 1.5 and 1.6. Given Hermine’s relatively weak winds, it’s not surprising that the distribution of gust factors for Hermine is shifted to the right when compared to the 15 storm database, while the overall pattern and character of the distribution for Hermine is very similar to the 15 storm study.

hermine-regression-compare

Comparison of gust factor regression equations associated with Tropical Storm Hermine and the 15 storm database used to develop the CSTAR TCM wind technique.

We generated a regression equation using the gust factor data set associated with Tropical Storm Hermine and compared it to the regression equation used in the 15 storm database used to develop the CSTAR TCM wind technique. The figure above  compares the Hermine regression equation to the 15 storm equation. They show a similar trend and generally match each other well, but given the modest wind speeds with Hermine, it’s not surprising that the Hermine curve is steeper than the larger dataset.

To summarize, participants from the six WFOs using the CSTAR TCM Wind technique discussed the techniques performance during Tropical Storm Hermine, and while the feedback received noted that the process seemed to work well and forecasts were generally accurate and well collaborated, some issues were identified. Adjustments to the CSTAR_Wind_from_WindReductionFactor smart tool are being made so that the wind reduction is only applied to winds of 34 kts or greater.  In addition, CSTAR TCM wind technique offices will try to use brief conference calls to discuss the PREAT guidance and ensure offices are using the technique appropriately and are well collaborated. Finally, despite the modest strength of the tropical storm and its extratropical transition as it exited the region, the observed gust factors matched the 15 storm database used to develop the CSTAR TCM wind technique reasonably well and the tools provided good guidance.

 

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