Tropical weather analysis - September 2, 2013

By: KoritheMan , 9:29 AM GMT on September 02, 2013

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Invest 97L

A tropical wave accompanied by a broad surface low pressure system is located just north of Barbados. After a significant increase in convection Sunday afternoon, the associated shower activity has dwindled significantly. A new burst of convection is affecting Barbados, but this is south of the analyzed low-level vortex, centered near 14.0N 59.9W; this is supported by doppler radar data from Barbados as well.



Figure 1. Latest infrared satellite imaged of Invest 97L. Image credit: NOAA's Satellite Services Division (SSD).

Surface observations suggest that the circulation associated with the tropical wave axis is not particularly well-defined, and the system still lacks significant vorticity above 700 mb, although I note there has been a substantial improvement in the 700 mb vorticity compared to 24 hours ago, likely due to the convective burst we saw Sunday helping to amplify the wave axis.

There doesn't appear to be a whole lot of shear in the near-storm environment, which would ordinarily portend strengthening. However, MIMIC-TPW from the University of Wisconsin CIMSS shows some mid-level dry air to the north of the system, associated with the backside of the mid-oceanic trough over the Greater Antilles region, which the wave could entrain over the next day or two. In addition, the system is headed to the eastern Caribbean, which has widely been dubbed as a hurricane graveyard, disrupting or destroying even well-developed tropical cyclones. So even though the large-scale environment around and ahead of the system is characterized by weak vertical shear, I expect the subsident flow aloft will continue to promote sinking air and discourage organized thunderstorms from forming along the wave axis.

The track forecast is of low confidence. There has been a progressive poleward shift in the guidance over the last several cycles, with the 6z dynamical guidance even farther east than the 0z envelope was, showing a northwestward motion across Puerto Rico, across the Dominican Republic, and into the central Bahamas. There will be some weak shortwave energy moving across the western Atlantic over the next few days, the first impulse being a trailing shortwave from an upper low moving through Toronto; in addition, two upper tropospheric cold lows are seen spinning to the north of the system on water vapor imagery; one southeast of Bermuda, and the other over eastern Haiti. In theory, this would promote at least a slight poleward component of motion to the tropical wave, but satellite images and official ATCF center fixes continue to indicate a due west motion, and the system has not gained latitude in about 24 hours. The more westward motion would make sense given the convectionless state of the system, which isn't doing particularly well even with the advent of the diurnal convective maximum. Given this, and the fact that I see no evidence of any large-scale weakness to the north of the system on UW-CIMSS tropospheric steering data or water vapor imagery, I am a little confused as to why the dynamical guidance turns the system almost immediately west-northwestward and then northwestward. A track across Puerto Rico seems extremely unlikely to me, and I would anticipate a significant westward shift in the guidance today. I normally don't diverge against a guidance consensus, but the center is still not particularly well-defined, and the forecast component of motion is clearly not realistic considering current happenings, so I am inclined to discount this consensus for now.

I still see a few possibilities for the ultimate track of this system:

- It heads westward into Central America or Mexico as a weak tropical wave

- It heads west-northwestward while passing near or over the south coast of Hispaniola, and eventually finds its way into the southern Gulf of Mexico, possibly as a hurricane

- It heads northwestward while passing over Hispaniola, then proceeds up through the Bahamas and possibly the United States east coast

Given current trends discouraging any significant development in the near-term, I favor the middle track, but the Bahamas solution could happen if the upper lows to the north of the system exert a greater poleward influence than anticipated, and also if convection reforms and persists. I would consider the Central America solution an enormous outlier with little chance of actually occurring.

The upper wind environment is forecast to be quite favorable for intensification when the wave is forecast to enter the western Caribbean Sea in about 5 or 6 days. Assuming things pan out that way, we would have to watch it very carefully. An alternate scenario is that the system becomes too disrupted by the Greater Antilles to ever redevelop, similar to Chantal.

Regardless of development, locally heavy rain and gusty winds -- perhaps to tropical storm force -- are expected over portions of the Windward Islands over the next day or so.

Probability of development in 48 hours: 30%



Kiko

Kiko weakens to a tropical depression as of the 0900Z NHC advisory:

Wind: 35 mph, with higher gusts
Location: 22.7°N 116.3°W
Movement: NNW at 5 mph
Pressure: 1005 mb

Satellite classifications are plummeting as fast as the Dvorak rules allow, and Kiko is a well-defined swirl of low- to mid-level clouds devoid of deep convection. Since the cyclone has lacked deep convection for many hours, it is presumed that it will degenerate into a remnant low later today.



Figure 2. Latest infrared satellite image of Tropical Depression Kiko. Image credit: NOAA's Satellite Services Division (SSD).

Kiko is already over cool waters, and is expected to continue moving across marginal waters; in combination with a meager thermodynamic environment, Kiko is expected to continue to weaken. Most of the guidance loses the circulation in the next 36-48 hours, and my forecast follows suit.

Kiko is on the south side of a low- to mid-level ridge over the southwestern United States. There has been no appreciable change to the steering philosophy or synoptic pattern, with the shallow cyclone forecast to continue moving slowly north-northwestward with a gradual decrease in forward speed. The GFS and ECMWF show Kiko remaining nearly stationary beyond 24 hours, so it is possible even this forecast is a little too enthusiastic on the forward speed spectrum.

Intensity forecast and positions

INITIAL 09/02 0900Z 22.7°N 116.3°W 30 KT 35 MPH
12 hour 09/02 1800Z 23.2°N 117.5°W 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
24 hour 09/03 0600Z 23.4°N 117.6°W 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
36 hour 09/03 1800Z 23.6°N 117.8°W 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
48 hour 09/04 0600Z...DISSIPATED

Track forecast



Figure 3. My forecast track for Kiko.

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4. WunderAlertBot (Admin)
6:51 AM GMT on September 04, 2013
KoritheMan has created a new entry.
3. Doppler22
6:02 PM GMT on September 02, 2013
Good blog Kori
Member Since: February 13, 2012 Posts: 11 Comments: 3659
2. Astrometeor
3:30 PM GMT on September 02, 2013
Are you going to do 5-day probabilities or just stick with the 48-hr forecast?
Member Since: July 2, 2012 Posts: 96 Comments: 9875
1. AtHomeInTX
9:38 AM GMT on September 02, 2013
Good job Kori. :) I admit I'm confused by the tracks of the models too. And the GFS not developing 97l at all. Not sure about the EURO. I guess we'll see. Thanks.
Member Since: August 24, 2010 Posts: 0 Comments: 679

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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.