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Weakening Flossie Closes in on Hawaii; ex-Dorian More Organized

By: Dr. Jeff Masters, 3:40 PM GMT on July 29, 2013

A weakening Tropical Storm Flossie is closing in on Hawaii, where tropical storm warnings are flying for the entire island chain. Satellite images show that Flossie has lost most of its heavy thunderstorms, and continued weakening is expected as the storm traverses the islands, due to moderate wind shear and dry air aloft. Flossie's winds of 45 mph will likely drop to the 35 - 40 mph range as it passes through the islands on Monday afternoon and evening, and it is possible that no location in Hawaii will record sustained winds of tropical storm force (39 mph or greater.) Monday's 11 am EDT wind probability forecast from the Central Pacific Hurricane Center gave Honolulu a 16% chance of tropical storm force winds, 20% for Kahului on Maui, and no odds for the Big Island. The storm's main threat will be heavy rains. Rainfall amounts of 6 - 10" are expected over The Big Island and Maui, and 4 - 8" in Oahu. Rains of this magnitude are capable of causing dangerous flash flooding and mudslides. Radar loops show a respectable area of heavy rain associated with Flossie, approaching the Big Island.

Figure 1. MODIS satellite image of Tropical Storm Flossie taken at approximately 5 pm EDT Sunday July 28, 2013. At the time, Flossie had top winds near 65 mph. Image credit: NASA.

Tropical storms are uncommon in Hawaii
On average, between four and five tropical cyclones are observed in the Central Pacific every year. This number has ranged from zero, most recently as 1979, to as many as eleven in 1992 and 1994. August is the peak month, followed by July, then September. Tropical storms and hurricanes are uncommon in the Hawaiian Islands. Only eight named storms have impacted Hawaii in the 34 year period 1979–2012, an average of one storm every four years. Since 1949, the Hawaiian Islands received a direct hit from just two hurricanes--Dot in 1959, and Iniki in 1992. Both hit the island of Kauai. Only one tropical storm has hit the islands since 1949--an unnamed 1958 storm that hit the Big Island. A brief summary of the three most significant hurricanes to affect Hawaii in modern times:

September 1992: Hurricane Iniki was the strongest, deadliest, and most damaging hurricane to affect Hawaii since records began. It hit the island of Kauai as a Category 4 on September 11, killing six and causing $2 billion in damage.

November 1982: Hurricane Iwa was one of Hawaii's most damaging hurricanes. Although it was only a Category 1 storm, it passed just miles west of Kauai, moving at a speed of nearly 50 miles per hour (80 km/h). Iwa killed one person and did $250 million in damage, making it the second most damaging hurricane to ever hit Hawaii. All the islands reported some surf damage along their southwest facing shores, and wind damage was widespread on Kauai.

August 1959: Hurricane Dot entered the Central Pacific as a Category 4 hurricane just south of Hawaii, but weakened to a Category 1 storm before making landfall on Kauai. Dot brought sustained winds of 81 mph with gusts to 103 mph to Kilauea Light. Damage was in excess of $6 million. No Dot-related deaths were recorded.

Figure 2. Tracks of all tropical storms and hurricanes to pass within 100 miles of the Hawaiian Islands, 1949 - 2012. During that time span, the Hawaiian Islands received a direct hit from just two hurricanes--Dot in 1959, and Iniki in 1992. Both hit the island of Kauai. One tropical storm also hit, and unnamed 1958 storm that hit the Big Island of Hawaii. Image credit: NOAA/CSC.

Remains of Dorian growing more organized
The remains of Tropical Storm Dorian, located just north of Puerto Rico, are headed west to west-northwest at 15 - 20 mph. Satellite images show that ex-Dorian has a moderate area of heavy thunderstorms that have grown more organized this morning, with an intense clump of thunderstorms that have created a circular area of high cirrus clouds called a Central Dense Overcast (CDO) over the disturbance. There are no signs of a surface circulation on visible satellite images, but an AIr Force hurricane hunter aircraft is on call to investigate ex-Dorian Monday afternoon, if necessary. In their 8 am EDT Tropical Weather Outlook, NHC gave Dorian's remains a 40% chance of regenerating by Wednesday. I put these odds higher, at 50%. The primary impediment to development is the presence of an upper-level trough of low pressure to its west that ex-Dorian is running into. Strong upper-level southwesterly winds associated with this trough are creating a moderate 15 - 20 knots of wind shear and driving dry air into the west side of ex-Dorian. This shear is not expected to relent at all during the next few days. None of the reliable computer models for tropical cyclone genesis predict that ex-Dorian will regenerate, though the HWRF model, which is not reliable for predicting genesis, does show Dorian regenerating. Dorian's remains should continue moving west-northwest during the week, spreading over the Bahamas on Tuesday and Wednesday, and over Florida and Central Cuba on Thursday and Friday.

Philadelphia sets its all-time single day rainfall record
An incredible deluge of 8.02" of rain hit Philadelphia, Pennsylvania on Sunday, breaking the all-time one-day rainfall record for the city. The previous record of 6.63" was set on September 16, 1999 during Tropical Storm Floyd. With a further round of rain after midnight in Philadelphia (bringing the 24-hour record storm total to 8.27”), July has brought 13.25” of precipitation to the City of Brotherly Love. This surpasses the previous July monthly record (since 1872) of 10.42”set in 1994. The wettest month on record for Philadelphia remains 19.31” in August 2011. Yesterday's deluge is an astonishing rainfall total for a location with such a long period of record, considering that it occurred without the benefit of a tropical storm being present. Remarkably, 6.46" of the rain fell in just 3 1/2 hours.

Jeff Masters


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