Tuesday, September 27, 2011

[californiadisasters] Would You Be Prepared For a Tropical Cyclone in SoCal?

Would You Be Prepared For a Tropical Cyclone in Southern California?

By Eric Boldt
NWS - Los Angeles/Ventura

The odds of a tropical storm or a minimal-strength hurricane hitting the southern California coastline are very low, but the reality is that it's definitely possible. Four tropical cyclones have managed to bring tropical storm–force winds to the southwestern United States during the twentieth century: a tropical storm on 25 September 1939 in California, Hurri-cane Joanne on 6 October 1972 in Arizona, Hurricane Kathleen on 10 September 1976 in Cali-fornia and Arizona, and Hurri-cane Nora in September 1997 in Arizona. Only the 1939 tropical storm made a direct landfall in coastal California near the San Pedro Peninsula. (Please see Page 4 for a com-plete list of tropical cyclones that affected Southern Cali-fornia in the 20th Century.)

The 1939 no-named tropical storm caught many boats by surprise as they headed back to the mainland from Santa Catali-na Island, following a prolonged heat wave across southern California. Un-fortunately, approximately 45 people lost their lives when their boats cap-sized before reaching shore and there were over $2 million (over $31 million today) in property damages to the shipping industry, coastal structures, and crops. Following this disaster, the Los Angeles Na-tional Weather Forecast Office was established (in 1940) to make daily weather forecasts and provide watches and warnings for the southern California region. National Hurricane Center scientists estab-lished that the 1939 tropical storm was the 38th most deadly tropical cyclone to hit the U.S. since 1851. Ironically, at least 90 people reportedly lost their lives due to a record-breaking heat wave that en-gulfed the southern California area with daily tem-peratures over 100 degrees during the week leading up to the land-falling tropical storm.

Many scientists have concerns that a Category 1 (sustained winds at least 74 mph) hurricane along the Los Angeles coast would be much more devastating than a tsuna-mi, given the historical record of both types of events in this part of the country. On October 2, 1858, a minimal strength hurricane skirted the San Diego coastline before moving north-west and weakening over the Channel Islands. This is the closest that a hurricane has come to making landfall along the California coast since record-keeping began in the nineteenth century.

Hurricanes need very warm ocean temperatures to survive a trip northward from their origination area off the southwest coast of Mexico. Sea surface temperatures of at least 80 degrees Fahr-enheit (27 degrees Celsius) are essential to power the heat engine of a hurricane. El Niño years can bring warmer ocean temperatures to the south coast of California, especially during the months of September and October. Wind shear, or the shift in wind direction and/or speed with height is a major contributor to the weakening of hurricanes as they move into more northern lati-tudes. Wind shear, cooler ocean temperatures or a combination of the two greatly influence the weak-ening of tropical cyclones into tropical depressions (sustained winds less than 39 mph) or dissipating tropical cyclones well before entering the coastal waters of southern California.
The main weather hazards associated with tropical cyclones are the following:

High Winds

Winds associated with tropical cyclones are measured on the Saffir/Simpson hurricane scale in categories from 1 to 5 as sustained wind speed values. Property damage is likely to occur at the low-est category and extensive damage at category 4 or higher.

Tropical storm force winds = 39-73 mph

Saffir/Simpson hurricane scale
Category 1 - 74-95 mph
Category 2 - 96-110 mph
Category 3 - 111-130 mph
Category 4 - 131-155 mph
Category 5 - > 155 mph

Storm surge

Storm surge is simply water that is pushed to-ward the shore by the force of the winds swirling around the storm. This advancing surge combines with the normal tides to create the hurricane storm tide, which can increase the mean water level 15 feet or more. In addition, wind driven waves are superimposed on the storm surge. This rise in water level can cause severe flooding in coastal areas, particularly when the storm surge coincides with the normal high tides.

Inland Flooding

Extreme rain events in the southwestern United States are the most frequent effect of tropical cy-clones in the region. High winds, coastal storm surge and wave action are much less frequent. Rainfall intensity will also increase at higher eleva-tions where recent burn areas are susceptible to flash flooding and debris flows. The 1939 tropical storm in southern California resulted in 5.66 inches of accumulated rainfall in downtown Los Angeles (5.24 inches in 24 hours) and 11.60 inches recorded at Mount Wilson, both continue to be records for the month of September.


Tornadoes are most likely to occur in the
right-front quadrant of a tropical storm or hurri-cane. Some tropical cyclones seem to produce no tornadoes, while others develop multiple ones. Studies have shown that more than half of the land-falling hurricanes produce at least one tornado.

Source: http://www.wrh.noaa.gov/lox/scripts/headline_download.php?get=20110927_1346.pdf

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