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Posted at 10:30 AM ET, 07/14/2010

Tropics: Caribbean may not be quiet for long

By Greg Postel

* More storms today?: Full Forecast | Hurricane Tracking Center *

The tropics are eerily quiet. But for how long? Image courtesy NOAA

After a quick burst of tropical activity just a couple of weeks ago, with Hurricane Alex and Tropical Depression (TD) #2 rolling across the western Caribbean Sea and Gulf of Mexico, much drier air has settled in across that region. In fact, recent satellite images across this part of the tropics show hardly any thunderstorm activity at all.

Keep reading for a preview of what may be brewing in the tropics...

Just to the west, however, over the Eastern Pacific, the atmosphere appears a bit more agitated. Showers and thunderstorms are widely distributed across this region. The National Hurricane Center (NHC) is closely monitoring an area of disturbed weather off the southern coast of Mexico for possible development. Environmental conditions are indeed favorable for tropical cyclone genesis there (defined as a tropical depression or stronger), with wind shear values insufficiently strong to provide much resistance to the coherent organization of thunderstorms, and high relative humidities through a deep atmospheric column.

As we look at the state of the tropics as a whole, one might expect to see the global-scale circulations that are capable of promoting hurricane development in the eastern Pacific favorably configured, given the degree of thunderstorm activity we're seeing in that region. This is indeed the case with the MJO (Madden-Julian Oscillation). The MJO is a tropical weather system the size of an entire ocean basin. It is intricately involved with varying the wind, sea surface temperatures, cloudiness, and rainfall in the hurricane development regions. Right now, the MJO is in a part of the tropics that favors tropical cyclone growth right where NHC is looking (the Eastern Pacific).

But here's the key. The forecasts for the MJO push it along toward that same area in the next 1-2 weeks where it fostered the eruption of thunderstorms over the western Caribbean Sea and Gulf of Mexico that ultimately yielded Alex and TD#2.

While it may seem unlikely that activity will pick up in those same regions that are largely experiencing tranquil conditions with scattered tropical cumulus clouds right now, some of the signs that point toward change are sufficiently clear to raise awareness of this potential outcome.

By Greg Postel  | July 14, 2010; 10:30 AM ET
Categories:  Tropical Weather  
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So the dust blowing off the Sahara and across the Atlantic isn't going to impact this system? I guess in 1-2 weeks, the dust will have settled?

Posted by: JerryFloyd1 | July 14, 2010 11:17 AM | Report abuse

It appears the MJO or Kelvin wave tends to drift eastward sort of like the temperate-zone westerlies.

Meanwhile the tropical waves drift westward from Africa along the ITCZ...what appears to happen seems to be that MJO and the easterly waves can collide and interact, and when water temperatures are warm enough and there's no westerly wind shear at upper levels, cyclogenesis may occur. It also seems that westward-moving MCS' may form in these situations, and can be strengthened by high-level outflow, thus becoming tropical cyclones.

I've always wondered what might happen if a strong extratropical MCS moves southeastward out of Texas or elsewhere into the Gulf into an area where conditions are favorable for tropical development. Could such an MCS stall and redevelop as a tropical cyclone? The same process could conceivably occur between now and October if a strong MCS moves over the Atlantic off Virginia or the Carolinas and stalls over the 80+ degree water flowing northeastward in the Gulf Stream. This probably explains the "dying front" mechanism whereby a few tropical cyclones form in the Atlantic and Gulf coast areas. Late in the season, dying fronts can also trigger cyclogenesis around the Yucatan/Central America regions.

Posted by: Bombo47jea | July 14, 2010 1:00 PM | Report abuse

Hi JerryFloyd1,

You have the right idea. The dust observed across the Atlantic right now would be problematic for tropical cyclone development there if there were precursors for it to disrupt. But I don't see any (not unusual, since the easterly wave season really hasn't kicked in yet).
My attention is directed further west across the western Caribbean ... in 1-2 weeks. At that time, some of the MJO forecats are suggesting that an upward branch of the circulation will be nearby. This would tend to increase the likelihood for widespread convection (thunderstorms) and humidify the local atmosphere. Dust impacts (if there is any dust there and then) would be reduced in that scenario.

Posted by: gregpostel | July 14, 2010 1:08 PM | Report abuse

Hi Bombo47jea,

"Could such an MCS stall and redevelop as a tropical cyclone?"

yes. But it would take a set of circumstances wherein (at least) the vertical structure of the temperature and humidity in the midlatitude convective system were able to morph into one in which the system were not outflow dominant.

the "dying front" mechanism, as you suggest, has been one to produce TCs in the past. The key here is "dying", i.e., one in which vertical wind shear -not to mention humidity contrasts- associated with the front (and therefore the horizontal temperature variations) have nearly dissipated. Then, the shallow horizontal gradients of wind can act to promote a rotary precursor in the already convectively prone environment.

Good stuff.

Posted by: gregpostel | July 14, 2010 1:26 PM | Report abuse

Greg, Bombo

What if any of the many MCS's will develop into a TC, regardless of origin, is highly problematic.

I believe the major theoretical and practical issue is lack of understanding what governs the following: with an MCS vertical airflow consists of up and compensating downdrafts over a fairly large region. To become a warm core TC, the downdrafts must somehow become concentrated into a single center with the convective updrafts (core and rainbands) organized about it. How and under what conditions that occurs, to the best of my knowledge, is not adequately understood.

Posted by: SteveT-CapitalWeatherGang | July 15, 2010 9:51 AM | Report abuse

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