Hurricane Ava (1973)

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Hurricane Ava
Category 5 major hurricane (SSHWS/NWS)
Hurricane Ava (1973).PNG
Satellite image of Hurricane Ava
Formed June 2, 1973
Dissipated June 12, 1973
Highest winds 1-minute sustained: 160 mph (260 km/h)
Lowest pressure 915 mbar (hPa); 27.02 inHg
(fourth lowest in the eastern Pacific)
Fatalities None
Damage None
Areas affected Southern California
Part of the 1973 Pacific hurricane season

Hurricane Ava was one of the strongest tropical cyclones ever recorded in the northeastern Pacific Ocean. It was the first named storm of the 1973 Pacific hurricane season. Forming in early June, Hurricane Ava eventually reached Category 5 intensity on the Saffir-Simpson Hurricane Scale, the first Pacific hurricane to do so in June and the earliest ever in a season. Its central pressure made it the most intense known Pacific hurricane at the time. Despite its intensity, Hurricane Ava stayed at sea without significant impact.

Ava was given the most advanced measurement and reconnaissance available at the time. Recon flights were conducted and meteorological equipment was tested. The hurricane was also photographed from space by satellites and Skylab astronauts.

Meteorological history[edit]

Map plotting the track and intensity of the storm according to the Saffir–Simpson hurricane wind scale

On June 2, 1973, a tropical depression formed about 250 miles (400 km) south of Salina Cruz, Oaxaca. It started out nearly stationary, and became a tropical storm late on the same day it formed, the first named storm of the 1973 Pacific hurricane season. Ava then slowly moved westwards away from Mexico and became a hurricane on June 3. Ava became a major hurricane on the afternoon of June 5. The next day, a United States Air Force recon flight measured a wind speed of 150 mph (250 km/h) and a central pressure of 915 mbar (27.0 inHg). These measurements made Hurricane Ava by far the most intense storm of the season.[1]

At its peak, Hurricane Ava had winds of 160 mph (260 km/h).[1] These winds made it a Category 5 on the Saffir-Simpson Hurricane Scale, the highest possible category[2] and the first Category 5 since the 1959 season.[3] Ava was also a hurricane with windspeeds rapidly increasing the closer to the eye they were measured. Over a distance of 4 miles (6.5 km), wind speeds increased from 70 mph (110 km/h) to 158 mph (255 km/h), and they increased from 105 (165 m/h) to 158 mph (255 km/h) over half that distance. The reading of 915 mbar (27.0 inHg) was roughly 100 mbar (3.0 inHg) lower than the ambient environment far from the storm.[1]

After its peak, Hurricane Ava started weakening on June 7 as it continued its westward path. Its winds were 140 mph (220 km/h) on June 7 and 115 mph (185 km/h) on the next day. It was no longer a major hurricane after its winds fell to 105 mph (165 km/h) on June 9. Later that day it weakened to a tropical storm. Tropical Storm Ava became Tropical Depression Ava on June 11. The system then turned north and dissipated on June 12. Its remnants then became embedded in the trade winds as a tropical wave.[1]

Forecasting and observation[edit]

Radar during a recon flight

In terms of how well it was forecast, Ava had the largest error of any cyclone during the season. This 14° error five days out was mainly due to its northward turn when it was a weakening depression.[1]

For a few days, Ava was directly underneath Skylab during its first manned mission.[1] Astronauts acquired photographs of the hurricane,[4] which was big enough for Science Pilot Joseph Kerwin to describe it as "an enormous spiral" that was big enough to dominate the view outside the space station's window and prevent anything else from being seen.[5] Astronauts also provided microwave data through Earth Resources Experiment Package sensors.[4] Skylab also used a scatterometer on the system.[6] Unfortunately, Skylab's scatterometer data was harder to use than normal as it was degraded.[7]

Ava was also underneath the NOAA-2 and Nimbus 5 weather satellites. NOAA-2 provided photographs that were used to estimate Ava's maximum windspeeds. Satellite images were useful throughout the cyclone's existence, as did the wind reports of three ships when Ava was a young tropical storm.[1] Nimbus 5 carried an Electrically Scanning Microwave Radiometer and Temperature-Humidity Infrared Radiometer.[8] Both were used to study Ava. The main data provided by the THIR was data indicating cloud temperatures. The ESMR's main data was on rainfall rates, densities, and distributions. The observations also provided confirmation that clouds that are not vertically developed very much can produce tropical rainfall.[9]

Recon aircraft also penetrated Hurricane Ava. It was the first Pacific hurricane penetrated by National Oceanographic and Atmospheric Administration aircraft,[10] but not by aircraft from other agencies.[11] NOAA craft were laden with sensors and measured wave heights reaching 40 ft (12 m) with a microwave radar system and a laser altimeter.[1] That was the first time ever that sea conditions in a tropical cyclone had been measured that way.[1] United States Air Force planes measured central pressure, air temperature, and humidity in the eye pressures using dropsondes.[1] The collection of data from both space and the air was done in order to allow comparisons.[12] Collectively, all of this measuring made Hurricane Ava the best-measured northeastern Pacific tropical cyclone at the time.[1]

Impact and records[edit]

Hurricane Ava stayed at sea.[13] Consequently, no one was killed and there was no reported damage. However, when it was a recently-named tropical storm, Ava did cause sustained winds below gale-force to three ships called the Joseph Lykes, Hoegh Trotter, and Volnay.[1] In addition, large ocean waves churned up by Ava created hazardous surf and strong riptides at Southern California beaches on June 9 and June 10. Those waves reached heights of up to 9 ft (2.7 m) at Newport Beach, 6 ft (1.8 m) at Long Beach, and 8 ft (2.4 m) at Seal Beach. Those waves made beaches more hazardous, resulting in double-to-triple the usual contingent of lifeguards throughout Southern California beaches. At Seal Beach and Newport Beach, lifeguards made 35 and 75 rescues, respectively.[14]

When it was active, Hurricane Ava set many records. Several have since been broken, but Ava still holds a few. Ava ceased being a Category 5 hurricane on June 7, 1973.[3] 1994's Emilia reached Category 5 intensity on July 19, 1994.[15] This span of 7,712 days, which Ava began and Emilia ended, is the longest time between successive Category 5 hurricanes in the northeastern Pacific, and anywhere worldwide, in recorded history. When Hurricane Gilma reached Category 5 strength on July 24 also in 1994, it marked the shortest gap between Category 5 Pacific hurricanes recorded.[3] Ava was also a Category 5 hurricane for exactly 24 hours; a record at the time. Hurricane John broke that in the 1994 season, and hurricanes Linda and Ioke also lasted longer, tied with John.[16] In addition, Ava is the strongest June tropical cyclone in the western hemisphere north of the equator.[3][17]

A spokesperson from the American National Weather Service was quoted as saying that, "Ava had sustained winds of about 180 knots with some gusts at 200 knots when she [sic] was peaking".[14] However the official "Best track" data file and the seasonal summary in the Monthly Weather Review contradict that report and give maximum winds of 140 knots.[1][3] If Ava's winds were that high, they would one of the highest ever reported in a tropical cyclone anywhere. Like any report of winds that high it is suspect.[18]

At the time, Hurricane Ava's minimum known pressure of 915 mbar (27.0 inHg) was the lowest known in its basin, making Ava the most intense Pacific hurricane. Ava is now the fourth most intense, as Hurricanes Linda, Rick and Kenna recorded lower pressures.[3] However, Linda's and Rick's pressures were only estimated from satellite imagery,[19] so Ava held the record for lowest measured pressure until Kenna tied it.[20] However, the meteorological record for the eastern north Pacific is unreliable because geostationary satellite observation did not begin until 1966.[19] Ava's pressure record is itself incomplete; Ava was only a Category 4 when its 915 mbar (27.0 inHg) pressure was measured, and the only reading when it was a Category 5 is 928 mbar (27.4 inHg).[21] These two factors mean that Ava's lowest pressure may be below 915 mbar (27.0 inHg), and that there may be other cyclones stronger than Ava.[22]

See also[edit]

References[edit]

  1. ^ a b c d e f g h i j k l m Robert A. Baum (April 1974). "Eastern North Pacific Hurricane Season of 1973" (PDF). Monthly Weather Review. American Meteorological Society. pp. 297–301. Retrieved 2008-01-04. 
  2. ^ "Saffir-Simpson Hurricane Scale". National Hurricane Center. Retrieved 2008-01-04. 
  3. ^ a b c d e f National Hurricane Center; Hurricane Research Division; Central Pacific Hurricane Center (July 7, 2014). "The Northeast and North Central Pacific hurricane database 1949-2013". United States National Oceanic and Atmospheric Administration's National Weather Service. Retrieved July 10, 2014. 
  4. ^ a b "Introduction". SP-399 Skylab EREP Investigations Summary. National Aeronautics and Space Administration. Retrieved 2008-01-04. 
  5. ^ "First Hurricane". The Daily Courier (Connellsville, Pennsylvania: The Hurricane Newspaper Archive). 1973-06-08. p. 13. 
  6. ^ Arthur K. Jordan, Charles G, Purves, & James F. Diggs (1975-05-02). "Analysis of Skylab II S193 Scatterometer Data". Defense Technical Information Center. Retrieved 2008-01-04. 
  7. ^ V. H. Kaupp & J. C. Holtzman (January 1979). "Skylab Scatterometer Measurements of Hurricane Ava: Anomalous Data Correction". IEEE Transactions on Geoscience Electronics. IEEE. Retrieved 2008-01-04. 
  8. ^ Lewis J. Allison, Edward B. Rodgers, Thomas B. Wilheit, & Robert W. Fett (September 1974). "Tropical Cyclone Rainfall as Measured by the Nimbus 5 Electrically Scanning Microwave Radiometer" (PDF). Bulletin of the American Meteorological Society. American Meteorological Society. p. 1074. Retrieved 2008-01-05. 
  9. ^ Lewis J. Allison, Edward B. Rodgers, Thomas B. Wilheit, & Robert W. Fett (September 1974). "Tropical Cyclone Rainfall as Measured by the Nimbus 5 Electrically Scanning Microwave Radiometer" (PDF). Bulletin of the American Meteorological Society. American Meteorological Society. p. 1077. Retrieved 2008-01-05. 
  10. ^ "Wea01151". NOAA Photo Library. National Oceanographic and Atmospheric Administration. Retrieved 2008-01-05. 
  11. ^ Gary Padgett. "Monthly Global Tropical Cyclone Summary August 2000". Retrieved 2008-01-05. 
  12. ^ David Shayler (2001). Skylab: America's Space Station. Springer, Google Books. p. 267. ISBN 1-85233-407-X. Retrieved 2008-01-04. 
  13. ^ "Hurricane AVA" (GIF). Unisys. Retrieved 2008-01-04. 
  14. ^ a b Tom Williams (1973-06-11). "'Hurricane surf' hits Southland; lifeguards busy". Long Beach Independent (Long Beach, California: The Hurricane Newspaper Archive). p. A3. 
  15. ^ "The 1994 Central Pacific Tropical Cyclone Season". Central Pacific Hurricane Center. Retrieved 2008-01-06. 
  16. ^ Neal Dorst (2004-08-13). "Subject: E8) What hurricanes have been at Category Five status the longest?". FAQ. National Hurricane Center. Retrieved 2008-01-06. 
  17. ^ National Hurricane Center; Hurricane Research Division (April 1, 2014). "Atlantic hurricane best track (HURDAT version 2)". United States National Oceanic and Atmospheric Administration. Retrieved November 19, 2014. 
  18. ^ Chris Landsea (2006-11-28). "Subject: E1) Which is the most intense tropical cyclone on record?". FAQ. National Hurricane Center. Retrieved 2008-01-06. 
  19. ^ a b Max Mayfield (1997-10-25). "Preliminary Report Hurricane Linda". National Hurricane Center. Retrieved 2008-01-06. 
  20. ^ Franklin, James L.; National Hurricane Center (December 26, 2002). Hurricane Kenna 2002 (Tropical Cyclone Report). United States National Oceanic and Atmospheric Administration's National Weather Service. http://www.nhc.noaa.gov/2002kenna.shtml. Retrieved May 26, 2012.
  21. ^ "Hurricane AVA". Unisys. Retrieved 2008-01-06. 
  22. ^ Max Mayfield (1997-10-02). "Preliminary Report Hurricane Guillermo". National Hurricane Center. Retrieved 2008-01-06. 

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