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98L well-organized; a 5-year drought in U.S. major hurricanes

By: Dr. Jeff Masters, 1:27 PM GMT on October 11, 2010

A strong tropical disturbance (98L) is centered near the northeastern coast of Honduras along the border with Nicaragua. This system is close to tropical depression status, but development is currently being hampered by the storm's proximity to land. Satellite imagery shows a well-organized system with plenty of spin, a modest amount of intense thunderstorm activity, and some respectable low-level spiral bands. Water vapor satellite loops reveal that 98L has been able to substantially moisten the atmosphere in the Western Caribbean over the past day, and dry air will be less of an impediment to development than it was yesterday. Wind shear is a moderate 10 - 15 knots.

Figure 1. NASA MODIS satellite image of 98L taken 12:35pm EDT Sunday October 10, 2010. Image credit: NASA.

Forecast for 98L
The west-northwest to northwest movement of 98L at 10 mph should take the center far enough away from the coast of Honduras this evening to substantially increase the storm's odds of development. The latest SHIPS model forecast calls for wind shear to stay mostly in the moderate range, 10 - 15 knots, through Tuesday afternoon, then increase to the high range, above 20 knots, for the remainder of the week. NHC is calling for a 60% chance of 98L becoming a tropical depression by Wednesday morning; I'd put these odds higher, at 70%. The computer models predict 98L will continue on a west-northwest to northwest motion through Tuesday, which would take the storm close to the coast of Belize/Mexico's Yucatan Peninsula by Tuesday night. At that point, it is possible that a trough of low pressure over the eastern U.S. will reach far enough south to pull 98L to the northeast across western Cuba and the Florida Keys by Thursday, as predicted by the latest 8pm EDT (0Z) run of the GFDL model. Two other models, the GFS and HWRF models, keep the storm confined to the Western Caribbean for the rest of the week, though. The ECMWF, NOGAPS, and UKMET models do not develop 98L into a tropical depression. In any case, Honduras, northeastern Nicaragua, and the Cayman Islands can expect heavy rain from 98L over the next three days. Heavy rains from 98L will begin to affect Belize and Mexico's Yucatan Peninsula on Tuesday, and perhaps northern Guatemala and the western half of Cuba as well. These rains may potentially last many days and cause significant flooding problems.

The U.S. drought in major hurricanes
On average, the U.S. gets hit by one major Category 3 or stronger hurricane every two years. This year, the team of hurricane forecasters at Colorado State University called for a 76% chance of a major hurricane hitting the U.S. in their June forecast. However, the odds of a major hurricane hitting the U.S. are rapidly dwindling. Over the past fifty years, the only Category 3 or stronger hurricanes to hit the U.S. after October 1 were Hilda (October 3, 1964), Opal (October 4, 1995), and Wilma (October 24, 2005). Hilda and Opal were already named tropical storms as of October 1, so Wilma was the major hurricane that formed after October 1 to hit the U.S. during this period. Although we still need to keep a wary eye on developments in the Western Caribbean over the next few weeks, the odds are that 2010 will join 1951 as the only year to have five or more major hurricanes in the Atlantic, but no landfalling major hurricane in the U.S. (1958 is also listed as such a year, but a re-analysis effort is showing that Hurricane Helene hit North Carolina as a major hurricane that year.) If 2010 finishes without a major hurricane hitting the U.S., this will mark the first such five-year stretch since 1910 - 1914.

Figure 2. Hurricane Wilma over South Florida as a Category 3 hurricane on October 24, 2005. Wilma was the last major hurricane to hit the U.S.

However, some caveats are required. Hurricanes Gustav and Ike, which both made landfall in the U.S. in 2008 as top-end Category 2 storms with 110 mph winds, would probably have been classified as Category 3 hurricanes had they occurred early in the 20th century. This is because in past, when there were not any reliable wind measurements in the vicinity of a landfalling hurricane (a common occurrence), the storm was classified based on its central pressure. Gustav and Ike had central pressures of 957 and 952 mb, respectively, which would have qualified them as Category 3 storms. Similarly, Hurricane Floyd of 1999 and Hurricane Isabel of 2002 (though not within the last five years) were strong Category 2 hurricanes with 105 mph winds at landfall, but had central pressures of 956 mb. These hurricanes would also have been classified as Category 3 hurricanes in the past. There are many storms from the 1930s, 1940s, and 1950s that will likely change their landfall classification once re-analysis efforts are completed over the next few years. One case is Hurricane Ten of 1949, which is listed as having winds of a low-end Category 4 hurricane (135 mph) just before landfall, which would make it the only October major hurricane to make landfall in Texas. However, the hurricane is only given a Category 2 strength at landfall, based on its central pressure.

Prior to 1960, there were five major hurricanes that hit Florida in October. Most notable of these is Hurricane King, which hit downtown Miami on October 18, 1950, as a Category 3 hurricane.

Record quiet hurricane and typhoon seasons in the Pacific
Over in the Western Pacific, it is currently the quietest typhoon season on record, according to statistics computed by forecaster Paul Stanko at the NWS office on Guam. On average, by this point in the season, there should have been 21 named storms, 13 typhoons, and 3 supertyphoons (storms with 150+ mph winds.) So far in 2010, there have been just 12 named storms, 6 typhoons, and no supertyphoons. The record lows for the Western Pacific (since 1951) are 18 named storms, 9 typhoons, and 0 supertyphoons. We have a good chance of beating or tying all of those records. Over the in the Eastern Pacific, it has also been a near record-quiet season. On average, the Eastern Pacific has 15 named storms, 8 hurricanes, and 3 intense hurricanes in a season. So far in 2010, there have been 8 named storms, 3 hurricanes, and 2 intense hurricanes. The record quietest season since 1966 was the year 1977, when the Eastern Pacific had 8 named storms, 4 hurricanes, and 0 intense hurricanes. Climatology suggests that on average, we can expect just one more named storm in the Eastern Pacific this late in the season, so there is a good chance that the 2010 season is over. La Niña is largely responsible to the quiet Eastern Pacific hurricane season, due in part to the cool sea surface temperatures it brought. La Niña also commonly causes less active Western Pacific typhoon seasons, since the warmest waters there shift closer to Asia, reducing the amount of time storms have over water.

I'll have an update Tuesday morning at the latest.

Jeff Masters


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