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Posted at 12:00 PM ET, 08/12/2011

Peak of hurricane season nears, tropical Atlantic firing up

Peak hurricane season spans mid-August through late October. As we enter this active period, the National Hurricane Center (NHC) is currently tracking four disturbances with potential to develop into tropical cyclones - that is, tropical storms or hurricanes. None represent any near-term landfall threat.

Over the last several days, large-scale conditions over the tropical Atlantic have become increasingly capable of sustaining organized thunderstorms - which can intensify under the right circumstances.

Four tropical systems that bear watching in the Atlantic (National Hurricane Center, adapted by CWG)
The last two easterly “waves” that have rolled off of Africa have been accompanied by concentrated areas of long-lasting convective (thunderstorm) complexes. These two systems designated as 92L (18N, 45W - midway between northern Lesser Antilles and the coast of Africa) and 93L (500 miles southwest of the Cape Verde Islands), are now showing signs of developing further.

Some of the meteorological conditions in the environment nearby 92L and 93L favor their intensification. In particular, the wind shear around them is light to moderate and the ocean waters are warm enough.

There is also some indication from a few of the global weather models that one or both of these disturbances could acquire a name in the next couple of days as they move generally westward at 15-20 mph. This is in stark contrast to the situation with Emily, which had virtually no support from these same models. With this in mind, however, it is important to remember that the models’ good “no-call” with Emily will not necessarily equate to an accurate forecast for development this time.

One of the factors that may destructively interfere with both of their futures is the nearby presence of a Saharan Air Layer (SAL) –the areas shaded in red in the picture below, just to the north of 92L and 93L.

Saharan dust layer lurks to the north and northeast of 92L and 93L. It’s also apparent to the west of 94L. ( University of Wisconsin Space Science and Engineering Center )
For many of the Atlantic storms in recent years, a very dry slug of air like this one has suffocated the cyclone genesis process and left many seemingly qualified precursors in the dust, so to speak.

Just last week, dust played a major role in braking Emily’s attempts to spin up, and it may very well play a similar role with 92L and 93L. The fact that the SAL has been a formidable opponent in recent times suggests large-scale mechanisms are in a stable place that will try to keep it this way until further notice.

This means that 92L and 93L may have to battle –and overcome- the formidable SAL if they ever want to make it out of the tropics alive.

The latest track guidance has 92L probably recurving out to sea, never reaching hurricane status.

As for 93L, the track guidance takes it on a more westward course as a potentially stronger system, partially because of its location further south and further away from the SAL.

Both systems, which the NHC gives 40% odds for development into tropical cylcones over the next two days, are still 4-6 days away from reaching the longitudes of the eastern-most Antilles Islands, and many days still thereafter from even thinking about approaching the U.S., if they ever do.

Since there are still lots of potential outcomes, many of which never threaten the U.S., I am hesitant to project a bullish outlook on either of these systems until they prove worthy of withstanding the harsh environment the Atlantic Basin uniquely provides.

The National Hurricane Center is also tracking two additional systems - 94L and 95L. 94L’s prospects for development are low (20% according to the NHC) due to its proximity to the SAL. Moreover, models track it out to sea. 95L, which the NHC gives 30% odds of developing into a tropical cyclone, is also on a track that takes it northeastward out into the open ocean.

As for the upcoming segment of the season, there are global-scale mechanisms in place –namely the Madden-Julian Oscillation (MJO)- that will likely be in a phase that supports an active tropical cyclone period in the Atlantic Basin through the end of August. With the peak of season still to come in the middle of September, we will be on top of the any developments for you for months to come.

By  |  12:00 PM ET, 08/12/2011

Categories:  Tropical Weather, Latest

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