HURRICANES

Objectives:

A hurricane is a tropical cyclonic (low-pressure centered rotating circulation) storm system that has maximum sustained surface wind speeds of 119 km per hour (74 mph, 64 kt) or higher. [A knot (kt) is one nautical mile per hour.] Hurricanes form over the warm tropical ocean and derive their energy from latent heat released when water evaporated from the sea surface condenses within the storm system. A typical hurricane is about one-third the size of a typical extratropical cyclone of the middle latitudes, forms in a uniform mass of warm and humid air, and has no fronts or frontal weather. When a hurricane strikes the coast, property damage is caused by a surge of ocean water above flood stage, strong winds, heavy rainfall often causing flooding, and sometimes tornadoes.

Hurricanes that threaten the East and Gulf Coasts of North America usually originate over the tropical North Atlantic off the West African coast, the Caribbean Sea, or the Gulf of Mexico. Most hurricanes initially are steered slowly westward by the trade winds, but eventually curve northwestward, then northward, and finally northeastward around the Bermuda-Azores semi-permanent subtropical High. Precisely where the curvature takes place determines whether the hurricane strikes the Gulf Coast, the East Coast, or turns out to sea. However, a hurricane may depart significantly from this “average” track. In some cases, a hurricane meanders about, even moving in circles or figure-eights. Such behavior greatly complicates the task of hurricane forecasting.

After completing this investigation, you should be able to:

• Describe the track taken by a hurricane that occurred in the western North Atlantic Ocean.
• Indicate the probable position of highest storm surge when a hurricane makes landfall.

Introduction:

The 2012 Hurricane Season in the Atlantic Basin (which includes the Caribbean Sea and Gulf of Mexico) was more active than usual. In terms of the number of tropical cyclones, there were 19 named storms (tropical storms or hurricanes) compared to the long-term average of about 12 and the record of 28 in 2005. Ten of the tropical cyclones reached hurricane intensity, which is four above average. And two of the hurricanes, Michael and Sandy, were major systems (Saffir-Simpson category 3 or higher) versus an average of three reaching major status. The major story of 2012, of course, was Sandy. Sandy was initially a large but mostly unremarkable hurricane (briefly Category 3 while traversing Cuba) that tracked generally northward off the Atlantic coast. This late season storm then interacted with a strong cold front in the Mid-Atlantic region and unusual upper-level wind patterns, losing its tropical characteristics while making a transformation into a post-tropical cyclone, while retaining hurricane force winds. Hence, it was commonly referred to as Superstorm Sandy. The storm turned northwestward slamming the Northeast coast near New York City with a high widespread storm surge and strong winds. The flooding, wind damage and power outages were catastrophic throughout the region. Preliminary estimates of damages were near $50 billion, the second costliest in the U.S. since 1900, and a direct death toll of 147 in the Atlantic Basin with 72 of them in the mid-Atlantic and northeastern U.S.

Responsibility for the forecasting and warning of tropical weather systems in the Atlantic and eastern portion of the Pacific Ocean basins resides with the National Hurricane Center (NHC) in Miami, FL. The NHC’s website, http://www.nhc.noaa.gov/ (), contains the latest information on tropical weather systems as well as a wealth of historical and other information regarding hurricanes. The forecasting and warning of tropical storms that evolve into post-tropical cyclones becomes the responsibility of NOAA’s Weather Prediction Center (WPC) with dissemination of information concerning the forecast done through local forecast offices. The WPC’s website: http://www.wpc.ncep.noaa.gov/ ().

This investigation involves evaluating the forecast track of an intensifying tropical cyclone that began as a tropical depression and quickly reached tropical storm strength. It formed over the Caribbean Sea at the beginning of the observation period.

Superstorm Sandy – Tropical Storm Phase:

The Figures 1, 2, and 3 are selected tropical storm advisories from the NHC Sandy Graphic Archives: http://www.nhc.noaa.gov/archive/2012/graphics/al18/loop_5W.shtml (). Each figure’s inset legend shows information, including the stage of development, date and time, along with the sequential number of the advisory issued. Color coding shows coastal and land areas where watches or warnings had been issued, as described in Table 1:

Table 1. Tropical Storm and Hurricane Watches and Warnings

Color	Tropical weather statement	Expected wind speeds
yellow	tropical storm watch	34 - 63 kts (39-73 mph) within 36 hours
blue	tropical storm warning	34 - 63 kts (39-73 mph) within 24 hours
pink	hurricane watch	64 kts or greater (>74 mph) within 36 hours
red	hurricane warning	64 kts or greater (>74 mph) within 24 hours

The center of the circulation at that time was plotted in each figure with a dot in an orange circle. Additionally, large black dots display the forecast position centers with times showing expected tropical storm (S) or hurricane (H) strength and the white cone of forecast uncertainty in future positions.

We will examine three advisories issued by NHC as it tracked the storm that became Hurricane Sandy and then went on to devastate so much of the Northeast.

Figure 1 is the first advisory issued by the NHC when this tropical system reached enough strength and potential to merit attention. The advisory was communicated to emergency managers and the public at 11 AM EDT on 22 October 2012. As shown in the lower legend, the center of the system’s circulation was located over the central Caribbean Sea. The maximum sustained wind speed was 30 mph and the system was moving toward the southwest at 5 mph.

Figure 1. Advisory 1; 11 AM EDT, Monday, 22 October 2012.

1. At 11 AM EDT on Monday, 22 October, the status of this tropical weather system, as described in the figure legend, was a [(tropical depression)(tropical storm)(hurricane)].

2. The forecast path showed that the system was expected to travel generally northward, threatening Jamaica on Wednesday and then eastern Cuba on Thursday. At the time of landfall in Cuba, the system was expected to be a [(tropical depression)(tropical storm)(hurricane)].

3. Note that there are no colored shadings indicating watches or warnings shown along the Cuba coastline. This time of predicted landfall on the Cuba coast was about 69 hours beyond the advisory time. The definitions of watches and warnings given in Table 1 indicate that there [(could)(could not)] have been a watch or warning posted for this area at 11 AM on Monday.

4. If you lived in the threatened region of eastern Cuba and knew of the counterclockwise circulation of the advancing storm as seen from above, you should expect the highest storm surge to occur to the [(east)(west)] of the point of the center’s landfall (intersection of the heavy track line with the coast).

5. While the heavy black line is the forecasters’ most probable track of the center of the storm’s circulation, NHC forecast models indicated a distinct probability that the center may pass elsewhere within the cone displayed, in decreasing probability to either side away from the center track. This white cone of potential track location in Figure 1 shows that in three days (at 8 AM Thursday) the center might possibly be located somewhere between [(western Cuba and the Mexican Yucatan Peninsula)(western Haiti and central Cuba)].

Figure 2. Advisory 7A; 2 AM EDT, Wednesday, 24 October 2012.

6. Figure 2 is the forecast map issued at 2 AM EDT, Wednesday, 24 October 2012, 39 hours after the Figure 1 advisory. The Figure 1 forecast position for the Figure 2 time was approximately 17 N 77.5 W. Compare this forecast position with the Figure 2 storm center location stated in the legend portion of the map. The Wednesday forecast longitude position was quite near the Figure 2 value and the forecast latitude position [(was)(was not)] within a degree of the Figure 2 value.

7. As shown by Figure 2 at 2 AM EDT on Wednesday, the weather system’s strength was that of a [(tropical depression)(tropical storm)(hurricane)].

8. Note the maximum sustained winds reported in the Figure 1 and 2 legends. Over the 39-hour period, from 11 AM EDT on Monday to 2 AM Wednesday, the system (in terms of wind speed) [(weakened)(remained the same)(strengthened)].

9. At the time of the Figure 2 advisory, it was reported in the figure’s legend that the center of circulation was moving towards the north at 10 mph. According to the forecast map, over the next four and a half days, the system was expected to [(head north and then curve northeast)(head northeast and then curve northwest)(head northwest and then curve west)].

10. The Figure 2 forecast track projects the system to reach the southern coast of Cuba late on Wednesday evening or early Thursday. In the hours before reaching the Cuba shore, the storm was expected to be a [(tropical depression)(tropical storm)(hurricane)].

11. Therefore, in its passage from Jamaica to Cuba, the system was expected to [(weaken)(remain the same)(strengthen)].

12. Following the system’s landfall and its passage over the island of Cuba, the strength of the system was projected to be that of a tropical storm (S) by the time it moved offshore. This was less than the strength at landfall. Weakening of the system’s circulation was likely due to [(lessening of the latent heat source over land)(increased friction over land)(both of these factors)].

13. After passing over Cuba, Sandy was expected to be a tropical storm and located in the Bahamas at about [(8 AM Thursday)(8 PM Thursday)(8 PM Friday)].

14. Following its passage through the Bahamas, Sandy was expected to head northeastward into the Atlantic following a rather traditional track but threatening Bermuda. At this Figure 2 time, did there appear to be a threat to the U.S. East Coast? [(Yes)(No)].

15. Jumping forward a day and a half to 5 PM EDT on Thursday, 25 October 2012, Figure 3 shows the storm positioned in the northern Bahamas. Plot the actual Thursday position of the circulation center from Figure 3 on the Figure 2 map. Did the actual Thursday position fall within cone of uncertainty of the forecasted Friday position? [(Yes)(No)].

16. At 5 PM EDT on Thursday, the system was a [(tropical depression)(tropical storm)(hurricane)].

17. Compare the strength of Sandy over Atlantic waters north of the Bahamas as indicated on the Figure 2 and Figure 3 forecast maps,. Sandy was expected to be [(weaker)(the same)(stronger)] on the later Figure 3 forecast map than it had been in Figure 2.

Figure 3. Advisory 14; 5 PM EDT, Thursday, 25 October 2012.

18. Table 2 shows the Saffir-Simpson Hurricane Scale. The category of hurricane strength is related to the damage potential and the measured maximum sustained one-minute wind speeds in miles per hour (mph). At the Figure 3 advisory time, Sandy was a category [(1)(2)(3)(4)(5)] hurricane.

Table 2. Saffir-Simpson Hurricane Scale

Category	Damage Potential	Max. Sustained Winds (mph)
1	Minimal	74 - 95
2	Moderate	96 - 110
3	Extensive	111 - 129
4	Extreme	130 - 156
5	Catastrophic	157 or greater

19. The Figure 3 forecast track issued at 5 PM EDT, Thursday, 25 October, projected that Sandy [(would)(would not)] make landfall along the New Jersey coast.

20. Consider on Figure 3 the two locations: New York City (coastal indentation north of the dashed track) and Delaware Bay (southwest of dashed track). Based on the forecast track, one would expect the higher storm surge from the major landfalling hurricane to occur in the [(New York City)(Delaware Bay)] area.

Superstorm Sandy - Post-tropical Cyclone Phase:

NHC issued its last Hurricane Sandy Advisory at 5:00 pm EDT, Monday, 29 October 2012 and with its advisory issued 2 hours later announced that “Sandy becomes post-tropical”. The storm had not yet made landfall. Landfall occurred at about 8 pm EDT near Atlantic City, NJ with winds of 80 mph. The unusual behavior of this hurricane and its transformation to a post-tropical storm of such size and strength with storm surges, high winds, flooding rains, and even blizzard conditions took well over one hundred lives and made countless other tens of thousands homeless. The forecasting and response challenge from such phenomena is truly daunting.

Motivated by Hurricane Sandy and its landfall as a post-tropical cyclone, NHC made changes to tropical storm and hurricane watch and warning definitions effective 1 June 2013. While Table 1 definitions were valid in 2012, they have since been broadened. We recommend that you review these current definitions at http://www.nhc.noaa.gov/news/20130404_hsu_postTropicalChanges.php ().

As directed by your course instructor, complete this investigation by either:

1. Going to the Current Weather Studies link on the course website, or
2. Continuing the Applications section for this investigation that immediately follows.

Investigation 12A: Applications

The Hurricane Seasons in both the Atlantic and eastern North Pacific Basins ended for 2013 on November 30th. The Atlantic Basin Hurricane season officially begins on June 1st while the eastern North Pacific Basin season begins May 15th. The 2013 hurricane season in the Atlantic basin was a relatively inactive year in terms of tropical cyclone numbers and strengths. There were 13 named tropical storms, which included a mere two hurricanes with no major hurricanes (rated Saffir-Simpson Category 3 or higher). This compares to long-term averages for the Atlantic basin of 12 named tropical storms, six becoming hurricanes with three being major hurricanes. The 2010, 2011, and 2012 seasons were more active, with 19 named tropical storms each. For more on the 2013 Atlantic hurricane season, see http://www.noaanews.noaa.gov/stories2013/20131125_endofhurricaneseason.html ().

Figure 4 is the map of the tracks of Atlantic Basin tropical weather systems for the 2013 season from the National Hurricane Center. The track of each storm is shown numbered at the beginning and end of its track. The names of the storms corresponding to the numbers along with strength and dates of existence are in a table to the upper left. The storm strengths along their paths are color coded according to the scale to the lower left. Black circles represent the center of the storm at 00Z and with white circles the location at dated 12Z positions. Recall that hurricane wind threshold speed is 64 kts (74 mph). Latitude values are shown vertically along the left and right edges of the image and longitudes horizontally along the top and bottom edges. The original map is found at: http://www.nhc.noaa.gov/data/tracks/tracks-at-2013.png ().

Figure 4. 2013 North Atlantic Hurricane Storm Tracks [NHC/NOAA]

21. Typically, the majority of Atlantic tropical systems travel westward at lower latitudes and then curve to the north and northeast upon reaching the belt of Westerly winds. With this directional pattern, storms that do not curve until passing about 75° W, [(will)(will not)] likely make landfall before turning toward the north. Storms that form in or travel to the Caribbean Sea or Gulf of Mexico are virtually certain to impact land areas.

22. One function hurricanes and tropical cyclones perform in the global weather system is to transport heat and moisture from tropical oceans to higher latitudes. For example, note the 2013 path of Tropical Storm Andrea which originated over the Gulf of Mexico and later weakened while tracking toward the Canadian Maritimes. The final position implies that this disturbance [(did)(did not)] transport energy and moisture to higher latitudes.

23. While no major hurricanes formed in the Atlantic Basin this year, Super typhoon Haiyan (also called Yolanda in the Philippines) in the western Pacific Basin devastated the Philippine Islands in November 2013 causing more than 6000 deaths. Haiyan’s 1-minute maximum winds categorized the typhoon, the western Pacific’s local name for a hurricane, at Category 5. At Category 5, the wind speeds would have been [(74 - 95)(96 - 110)(111 - 129)(130 - 156)(157 or higher)] mph. [See Table 2 in the introductory portion of this Investigation 12A.]

24. Because of a Northern Hemisphere hurricane’s counterclockwise circulation around its surface low-pressure center as seen from above, such winds would have brought extensive wind damage, accompanying heavy rainfalls and a damaging storm surge mainly to the [(left)(right)] of the point where an advancing hurricane’s center would come ashore. In 2012, Hurricane and then “Superstorm” Sandy’s circulation center made landfall just south of Atlantic City, NJ, (see Figure 3) but catastrophic flooding along the shoreline primarily affected the area to the north, especially near New York City, resulting in at least an estimated $50 billion in damages and about 100 deaths in the landfall area.

25. Figure 5 is the map of the tracks of tropical weather systems worldwide from the NOAA Coastal Services Center’s website, http://www.csc.noaa.gov/hurricanes/ (). This interactive mapping tool can display all historic hurricane tracks in all ocean basins by color-coded storm strength as seen in Figure 4 or selected subsets by basin, year or storm name. Tropical storms forming in either the Northern or Southern Hemispheres [(do)(do not)] cross the equator from one hemisphere to the other.

Figure 5. Worldwide Tropical Cyclones.

26. This absence of tropical activity at the equator (0° latitude) [(is)(is not)] due to the absence of the Coriolis Effect that is essential in producing rotation during storm formation.

27. From Figure 5 one can note that many tropical cyclones also occurred in areas including the [(western North Pacific)(eastern South Pacific)(South Atlantic)] ocean basin(s). Super typhoon Haiyan was among those storms. The frequency of storms in that location is caused by the large expanses of warm ocean waters year-round in that part of the ocean. (You may recall the sea surface temperatures plotted in Investigation 9B.)

28. The energy for tropical storms is primarily delivered to the atmosphere by the water vapor evaporated from warm ocean surfaces. This energy is released to the atmosphere through condensation in thunderstorms and distributed by the persistent, organized wind circulation of the storm. From the relatively short paths indicated on the maps of the cyclones upon making landfall, the weakening of the storm’s energy is likely due to [(reduced evaporation when over land)(increased surface roughness slowing winds)(both of these factors)].

For information on Atlantic and Eastern Pacific storms, go to the National Hurricane Center’s webpage, http://www.nhc.noaa.gov/ (). On the menu under Tools & Data, Data Archive, track maps and individual storm reports for past years can be found. Another valuable item under Outreach & Education is a list of Frequent Questions. Tropical cyclones in the mid-Pacific can be followed from the Central Pacific Hurricane Center (http://www.prh.noaa.gov/hnl/cphc/ ()) and those of the western Pacific at the Joint Typhoon Warning Center (http://www.usno.navy.mil/JTWC/ ()).