Tropical weather analysis - July 22, 2011

By: KoritheMan , 3:52 AM GMT on July 23, 2011

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A well-defined tropical wave (90L) is centered a few hundred miles east of the Lesser Antilles. This system has become better organized today, with deep convection on the increase from earlier.

Figure 1. Latest infrared satellite image of Invest 90L, courtesy of RAMMB.

A hint of upper-level outflow has recently manifested on the western side as well, as the system is experiencing upper divergence associated with an anticyclone displaced about 75 miles southeast of the wave axis. While this divergent upper flow will no doubt assist the system, a couple of negative factors do exist in the near-term.

For one thing, dry air still surrounds the wave, which will probably not allow for solid establishment of deep convection in the near-term. Also, the wave currently lacks low-level convergence, likely because the system is embedded within about 15 to 20 kt of lower tropospheric flow associated with the trade wind belt characteristic of the tropical regions. Without sufficient low-level convergence, a closed wind circulation cannot develop. However, the models do relax this flow somewhat over the next several days, perhaps just enough to allow for possible development in the Caribbean. However, the eastern Caribbean is a notorious graveyard for healthy tropical waves, and sometimes, even tropical cyclones, as dry continental air from South America gets entrained northward into the cyclonic circulation associated with such entities. And of course, lower tropospheric flow is usually quite fast there as well, which disallows low-level convergence and also typically results in the surface circulation outrunning the associated convective mass.

On the positive side, select surface observations indicate that the surface reflection associated with this wave is gradually becoming better defined. Upper-level winds are forecast to be generally favorable for slow development, but may not be ideal, as the ambient upper tropospheric flow could be somewhat zonal in the Caribbean.

In my opinion, 90L will not develop significantly until reaching the western Caribbean on Tuesday, when upper-level winds will be fairly weak. However, even with that arrangement, it would have some sizable shearing to overcome in the Gulf of Mexico, as the GFS paints strong northeasterly to easterly flow (in excess of 20 to 30 kt) over the system, associated with a deep-layer ridge over the eastern tier of the nation. This shearing should quickly lift out, however.

In short, the intensity forecast is highly uncertain, but because of the long-range threat it poses to the United States, it will need to be watched carefully if it can survive the climatologically unfavorable eastern Caribbean.

The track is just as uncertain, unfortunately. The model guidance has shifted significantly southward from yesterday, when they were showing the system going up the east coast, or even out to sea. However, they are still quite useless even with this shift, as most of them appear to have initialized the vortex too far to the north.

Given the persistent westward bias with this system, I do believe this system will continue through the Caribbean on a W to WNW track. When it reaches the western Caribbean, it could continue going westward, or it could bend to the northwest. All of this will depend upon the strength and orientation of the ridge over the eastern United States.

A weak trough is forecast to develop over the western Atlantic on Sunday/Monday, which, if strong enough, could tug the storm northward toward Hispaniola. In this scenario, development chances would substantially decrease. Some models are stronger with this trough than others. Given this uncertainty, one should keep in mind that the long-range track of this system is still up in the air.

It should be noted that despite how conservative I'm being, the SHIPS brings the system to just under hurricane strength on Monday as it nears Hispaniola. Even if this model is too far north (which it likely is), if the upper flow relaxes a bit more than currently indicated, that is certainly a possibility.

Finally, if this system develops quicker than anticipated, heat generated by the deep convection could develop an anticyclone aloft over the system. Were that to happen, both the track and intensity forecasts would likely be considerably different than presently.

Interests in the Lesser Antilles, Puerto Rico, and Hispaniola should monitor the progress of this system over the next few days. Regardless of development, strong gusty winds and periods of heavy rain in squalls will accompany this system as it moves through the Leeward Islands.


Tropical Storm Dora has continued to weaken today as it battles persistent northerly shear and traverses rapidly cooling sea surface temperatures. As of the latest NHC advisory, here is the information on Dora:

Wind: 70 mph, with higher gusts
Location: 20.4°N 110.5°W
Movement: NW at 8 mph
Pressure: 990 mb
Category: Tropical storm (Saffir-Simpson Hurricane Scale)

Fairly cold convection continues over Dora, and an earlier SSMIS overpass showed that the cyclone still possesses a distinct eye in the upper levels, though no real eyewall is present, and this eye is exposed to the northwest.

Figure 2. Latest infrared satellite image of Tropical Storm Dora, courtesy of NOAA's Satellite Services Division (SSD).

Though the current northerly to northeasterly shear is forecast to abate and give way to a more favorable synoptic environment, Dora is rapidly approaching the 26C isotherm, and quick weakening should commence by tomorrow morning. This, along with a more stable airmass, should cause dissipation of Dora as a tropical cyclone by early Sunday afternoon, if not sooner.

Dora has not made its closest approach to Baja yet, but it will overnight. Sustained tropical storm force winds will likely stay offshore, but occasional gust to 35 or 40 kt cannot be ruled out, especially along the southwest tip of the peninsula.

Dora should continue to moving generally NW. I remain on the western side of the guidance envelope, and agree quite strongly with the NHC's current projected track:

Figure 3. Latest NHC 5 day forecast track for Tropical Storm Dora.

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5. TomTaylor
4:36 AM GMT on July 23, 2011
Nice blog kori!
Member Since: August 24, 2010 Posts: 19 Comments: 4358
4. ajcamsmom2
4:10 AM GMT on July 23, 2011
awesome update...thanks
Member Since: March 15, 2008 Posts: 4 Comments: 2492
3. MiamiHurricanes09
4:05 AM GMT on July 23, 2011
Damn, absolutely excellent blog post. Thanks a lot.
Member Since: September 2, 2009 Posts: 130 Comments: 21195
2. sarahjola
4:04 AM GMT on July 23, 2011
nice update! very informative
Member Since: September 10, 2008 Posts: 0 Comments: 1299
1. LemieT
3:58 AM GMT on July 23, 2011
Good summary of the factors influencing Invest 90L. Great read.
Member Since: August 15, 2009 Posts: 0 Comments: 238

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About KoritheMan

I'm just a 23 year old with an ardent passion for weather. I first became aware of this interest after Tropical Storm Isidore struck my area in 2002.

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