Special tropical weather analysis - May 31, 2011

By: KoritheMan , 4:01 AM GMT on June 01, 2011

Invest 93L

A small area of low pressure (dubbed "Invest 93L") located several hundred miles east of Cape Fear, North Carolina, is a feature to watch over the next few days as it heads quickly toward the Bahamas, northern Florida, and eventually, the Gulf of Mexico, underneath a deep-layered subtropical ridge over the western Atlantic and the adjacent southeastern United States.

Even using infrared satellite loops, notoriously bad for detecting low-level circulations, one can see a small but well-defined lower cyclonic circulation associated with this feature.

Figure 1. Latest infrared satellite image of Invest 93L, courtesy of NOAA's Satellite Services Division (SSD).

This small low is currently experiencing some moderate northeasterly vertical wind shear, clearly evident on the aforementioned satellite frames. As per the 12z GFS, upper-level winds are forecast to steadily improve as the system speeds along toward the west-southwest or southwest over the next couple of days, eventually changing to a light easterly regime. None of the global models initialized this tiny circulation all that well, and as such, fail to intensify it. Nevertheless, atmospheric parameters are favorable for development of this small disturbance, as previously noted. Interests in the Bahamas, central and northern Florida, and southwestern Georgia should carefully monitor this system over the next two days.

Despite the poor initialization amongst the global model ensembles, there is a rather tight consensus aiming at the northern Florida coast near Jacksonville in about 24 hours, give or take.

The key player in where this system will go after crossing Florida/Georgia will be determined by an upper-level trough and associated cold front currently moving eastward across the central plains. Obviously, it is still a bit too early to ascertain the exact amplitude and orientation of this trough, but I feel the BAM shallow (suite shown below) is more or less correct in this case, with the system coming ashore eastern Louisiana in about 3 - 4 days.

Figure 2. The BAM model suite as of 8:00 PM EDT May 31, 2011.

We will know more about the amplitude, orientation, and strength of this trough by tomorrow.

There are certainly factors arguing against its development though, such as its fast forward motion, diagnosed at 20 to 25 kt on CIMSS synoptic steering analyses. As we've seen countless times in the past, such as with Tropical Storm Ana in 2009, even if the upper tropospheric flow is light, if a system is embedded within a swift lower- to middle tropospheric steering environment, it can essentially "outrun" itself, with result being a net increase in vertical shear. However, the upper and lower tropospheric flow are aligned in roughly the same direction, which may lessen the shear some. Nevertheless, with upper-level outflow restricted to the western semicircle, a pronounced shearing flow is certainly evident.

I give this system a 10% chance of developing, and unfortunately, given the abundant dry air in the vicinity, as well as the system's small size, drought-stricken Florida may not receive significant rains. One can hope, though.

Caribbean disturbance

A persistent, stationary area of low pressure in the southwest Caribbean Sea, has been consistent in generating convection in spite of strong southwesterly shear associated with the climatological subtropical jet, which sits just north of the system. Wind shear really hasn't changed much in the last 24 hours, although there was perhaps a slight decrease.

Figure 3. Latest infrared satellite image of the Caribbean disturbance.

Using a blend of shortwave infrared imagery, along with local surface observations, the low-level center appears to be centered about 150 miles east of the Costa Rica/Panama border, and remains practically stationary. This system is currently being aided by the diffluent region of the subtropical jet.

Upper-level winds should improve enough to allow potential development in about 36 hours, as the upper flow will transition from southwesterly to southerly, and also slacken some as the subtropical jet retreats northward. Indeed, by day four, the GFS has been insistent on developing an anticyclonic environment across the western Caribbean, characterized by low vertical wind shear. It should be noted that by days 5 - 7, regardless of where the system ends up, wind shear will increase substantially as the subtropical jet remains nearly stationary along 20-25N. Given the monsoonal nature of this system, it should take a bit to organize, and I do not expect rapid development. I give this system a 20% chance of becoming a tropical cyclone in the next 48 hours.

Regardless of development, heavy rains will continue to overspread portions of Costa Rica, Panama, and eventually, Jamaica, the Cayman Islands, Cuba, and possibly even the Yucatan Peninsula. Interests in these areas should continue to monitor this system due to the potential for flooding. This is especially true across Costa Rica and Panama, where this system has just been sitting for the last several days.

The track continues to be the crux of the storm. The global models continue to portray two different scenarios: one, the system gets ejected out to sea by the cold front currently moving across the central United States. Two, the system gets pulled more northwest underneath a rebuilding western Atlantic ridge in the wake of said trough. It is far too early to tell which of these is the more likely scenario. Also, depending upon the evolution of an upper low currently centered west of the Florida Keys, it is possible that tropical moisture from this system will eventually get drawn northward toward portions of drought-stricken Gulf Coastal areas. However, there is much uncertainty in this potential outcome. It is also the strength of this upper low that will at least partially determine the track for this system.

There is still a lot of uncertainty this far out, and interests along the Gulf Coast from Louisiana to south Florida should continue to monitor this system, as a sheared tropical depression or storm is certainly possible in these areas in about 6 - 7 days.

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4:12 AM GMT on June 01, 2011
nice little write up
Member Since: July 15, 2006 Posts: 193 Comments: 60433
1. MiamiHurricanes09
4:08 AM GMT on June 01, 2011
Thanks Kori, I appreciate the in-depth updates.
Member Since: September 2, 2009 Posts: 130 Comments: 21375

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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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