Today, AccuWeather.com reports a weather pattern that developed over the central and eastern United States during early September will continue to act as a buffer against major tropical systems through the end of the month. There is also a possibility the same pattern holds through much of the balance of this Atlantic hurricane season.
According to Expert Senior Meteorologist Dan Kottlowski, head of theAccuWeather.com Hurricane Center, "Westerly winds have broken through over the middle and upper levels of the atmosphere over the central and eastern U.S. and nearby coastal waters."
These winds and the dry air they bring are generally too disruptive for tropical storms or hurricanes to form in nearby coastal waters and tend to cause tropical systems coming from afar to avoid the Gulf and Atlantic coasts of the U.S.
There are some areas that are still somewhat vulnerable to direct impact from a tropical storm or hurricane, if they form.
"The most likely candidates would be the Caribbean islands, Central America, Atlantic Canada and the islands of Bermuda and the Azores in the Central Atlantic," Kottlowski said.
It is not uncommon to see the westerlies break through later in October and November over the U.S. and to have brief episodes of the same during September and early October.
"We keep seeing an upper-level trough of low pressure reappear in the central and eastern U.S. this month with only very brief episodes when it is not present," Kottlowski added.
There is still a chance that a window of opportunity opens up for a tropical system to move close to or form near the U.S., but the persistence of the trough pattern tends to cut the odds.
According to Expert Senior Meteorologist Brett Anderson, "During this October, it is possible the southern extent of the trough become shallow enough for less wind shear and dry air in the Gulf of Mexico, but that in itself is not a guarantee for development."
All it takes is one storm sneaking in during one of those open windows, and as we have seen with Isaac, it does not have to be a Category 3 or higher hurricane to cause major damage and endanger lives. Isaac was officially a Category 1 hurricane at landfall.
As of the week ending Sept. 21, the only two systems of note in the Atlantic were Nadine, near the Azores, and a potential system between Bermuda and the Azores.
While the rest of September appears as though it will be quiet for the U.S. mainland, there have been some destructive and deadly storms during October.
"Tropical waves continue to roll off Africa, so by no means does this mean that there will be no additional named storms this season," Kottlowski added, "It is not uncommon to have a few pauses in development, since we are getting past the peak of the Atlantic hurricane season."
As a result, we cannot say that the recent trend is directly due to a developing El Niño.
Kottlowski pointed out that he has noticed a lower number of tropical waves moving off Africa this season, when compared to average, but the season is not over.
As of Sept. 20, there have been 34 tropical waves.
"During a typical season, there are between 50 and 60 tropical waves," Kottlowski stated, "The low number of waves thus far are mostly due to the very slow start prior to August and September."
We have produced a higher percentage of named storms (14) versus tropical waves so far this season, when compared to the average. However, several of these systems formed along fronts and were not associated with tropical waves.
"While wind shear and dry air are always issues during a season, but they seem to be significant players in limiting the intensity of most systems this season," Kottlowski concluded.
By Alex Sosnowski, Expert Senior Meteorologist for AccuWeather.com