Forecast 96 Hours until Landfall

For NHC Advisory 19, the center of Hurricane Gustav was forecast to make landfall and pass to the East of Franklin. At this time, the center of Gustav was 1105 miles from its forecast landfall point. Figure 4.8 shows the forecast track for Gustav at Advisory 19. Based on historical landfall error, the probability of Franklin experiencing minimum hurricane conditions was determined to be 26.2% per method of 4.3.

Figure 4.8 Landfall Forecast Advisory 19 (NHC 2008a)

4.4.1 Hazards at 96 Hours until Landfall

Gustav was forecast to pass over Franklin on a Northwestern track. The projected maximum sustained winds at landfall were 100 knots, or Category 3 strength, which was very similar to the 120 hour forecast conditions at landfall. A Category 3 storm on a Northwest track could potentially produce significant storm surge flooding for Franklin.

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Gustav was located 1105 miles from forecast landfall at the 96 hour forecast, providing an average translation of 8 miles per hour. Using Table 4.2 with 8 miles per hour translation speed, winds in Franklin could be expected to be 77 miles per hour sustained. Individual wind probabilities for Franklin from Gustav were also determined. Figure 4.9 shows wind speed probabilities for Franklin at the 96 hour forecast period. The probabilities from the 120 hour forecast are repeated here as well to illustrate changes as the storm and forecast progress. When compared with Advisory 15, the pdf for Advisory 19 has more mass around the mean, with less out toward the tail. This results in a higher probability of lower wind speeds and a slightly lower probability of higher wind speeds occurring.

Figure 4.9 Wind Speed Probabilities for Franklin Based on Advisory 19

Table 4.9 shows the probability of Franklin experiencing each storm category. Because of the change in the wind probability distribution, the probability of experiencing a tropical storm is higher, while the probability of experiencing any category hurricane ( 1 – 5) is lower. Table 4.10 shows wind speed probabilities from the NHC. For Advisory 19, landfall was forecast to be at 8 am on Tuesday, which corresponds to the 96 hour column on Table 4.10.

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Table 4.9 Probability (%) of Experiencing at Storm Classifications for Franklin Based on Hurricane Gustav Advisory 19

120 hrs 96 hrs Storm Category Adv 15 Adv 19

Tropical Storm 55.8 56.1 Category 1 28.1 26.0 Category 2 15.7 13.5 Category 3 9.3 7.1 Category 4 4.0 2.8 Category 5 1.2 0.7

Table 4.10 NHC Wind Speed Probability Table for Advisory 19 (NHC 2008a)

At the 96 hour forecasting period, no detailed rainfall estimation can be made. Using Section 2.2.2, 14 inches of rainfall over a 24 hour period may occur. Note that this estimation has no spatial variance. Because of the lack of rainfall flood potential in Franklin, rainfall flooding is a minimal hazard.

4.4.2 Vulnerabilities at 96 Hours until Landfall

As was stated in the 120 hour forecast analysis, vulnerability is difficult to determine at long forecast periods, which include the 96 hour forecast. For both rainfall and storm surge

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flooding, only approximations can be made. For storm surge flooding, there is still a potential high hazard based on forecast track and intensity, but the storm is too far from landfall to determine more detailed vulnerability.

For high wind hazards, winds could potentially be 77 miles per hour sustained, with a 26% probability that Franklin will experience minimal hurricane wind speeds. Based on these wind speeds, as well as individual wind speed probabilities, and using the model HAZUS curve, there is the potential of experiencing damage state 1 or 2, which relate to low and medium risk respectively. Again, because the storm is too far from landfall, no detailed conclusions can be drawn for vulnerability from high winds.

4.4.3 Risk at 96 Hours until Landfall

For this time period, risks for reaching particular damage states are slightly lower than in the 120 hour forecast period. For this forecast, the track of Gustav had shifted slightly away from Franklin, but essentially remains unchanged. While the forecast intensity at landfall remains unchanged from the previous forecast, slightly more forecast information is available with regards to the forecast wind field, which results in a more detailed wind field model. Table 4.11 shows the probability of experiencing at least each damage state, as well as the resultant risk and habitability corresponding to each damage state probability. When reviewing the probabilities and comparing them to the previous forecast, probabilities of obtaining each damage state are lower. When viewing the wind speed distribution, the mass of the distribution has shifted lower when compared to Advisory 15. As was previously stated, the damage accumulated from the damage functions occur at wind speeds above 80 mph. When comparing the wind speed pdfs for Advisories 15 and 19 for wind speeds above 80 mph, the probability of obtaining these wind speeds is lower in Advisory 19. This lower probability causes the risk to be lower at the 96 hour forecast period.

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Table 4.11 Risks from High Winds at 96 Hours P(%) Damage State Risk Habitable Adv

15

Adv 19

Resultant

Risk Habitable

1 - Minor Damage Low Yes 28.9 16.9 Low Yes

2 - Moderate Damage Low Yes 17.0 6.7 Low Yes

3 - Severe Damage Medium No 8.8 2.1 Low Yes

4 - Destruction High No 4.5 0.8 Low Yes

At this forecast period, the storm is beginning to approach the timelines for which decisions will be made. However, the hazards and vulnerabilities from flooding can still only be minimally quantified, and risks from the hazards and vulnerabilities cannot be quantified in a reasonable manner. The potential relative risks must be considered from each hazard. Table 4.12 shows the three hazards as well as their corresponding risks and associated actions based on the risks.

Table 4.12 Risks at 96 Hours from Landfall Hazard Type Risk Action

High Winds Low Monitor Rainfall Flooding - Monitor Surge Flooding High Monitor

4.4.4 Decisions at 96 Hours until Landfall

There is potentially a high risk from storm surge flooding and a low risk from high winds, as well as a low risk from rainfall flooding. Surge flooding risk is unchanged from the previous forecast. The potential risk is still high, but there is still uncertainty in the forecast. Probabilities of obtaining wind damage states and the resultant risks are lower than in the previous forecast. At this forecast period the storm is still far from landfall and the overall decision recommendation, as well as the individual decision for each hazard, is to monitor the storm as outlined by decision processes in Section 3.5. This is based primarily on the fact that

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errors and uncertainties in the forecast are too great at such a distance to make accurate decisions with the available information.