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Tropical weather analysis - September 16, 2012

By: KoritheMan , 3:38 AM GMT on September 17, 2012


Nadine continues to move across the cenrtal Atlantic Ocean, and has weakened to a tropical storm. As of the latest NHC advisory, the following coordinates were available on the storm:

Wind: 70 mph, with higher gusts
Location: 31.4°N 38.1°W
Movement: ENE at 18 mph
Pressure: 987 mb

The convective pattern has continued to deteriorate this evening, with the shower activity taking on a more banded appearance. The center actually appears to be partially exposed to the south of the convection, suggesting that Nadine has finally succumbed to the strong upper tropospheric shear. A homogeneous comparison of various microwave fixes over the last 12-24 hours further attests to the erosion of the inner core convection.

Figure 1. Latest infrared satellite image of Tropical Storm Nadine. Image credit: NOAA

Since the shear is not forecast to abate over the next few days, Nadine may weaken a little more, especially given the disorganized appearance on satellite images. I have revised my intensity forecast downward from the last several days. In about 48 hours, Nadine is expected to cross the 26C isotherm, which should induce faster weakening beyond that time. It should be noted that the GFS has become more consistent that Nadine will ultimately interact with a powerful upper level low pressure system forecast to dive out of the north Atlantic in about three days. If true, this could significantly alter the structure of the tropical cyclone, and may actually cause extratropical transition at or just beyond the end of the forecast period, especially given the cold water in the vicinity of the Azores. Conversely, the ECMWF continues to insist that Nadine will not interact much with the low, which would of course keep Nadine tropical for longer. At this point it is physically impossible to pick sides.

The intensity forecast shows weakening through 72 hours. After that, Nadine may get a shot of baroclinic energy from the aforementioned upper low. While none of the guidance explicitly shows this, I feel it is the best course of action given the uncertainty in the future evolution of the cyclone.

Nadine continues to move eastward in the fast mid-latitude westerly flow associated with an upper tropospheric cold low rotating north of the Azores. Water vapor imagery shows a trough approaching the cyclone. This models suggest this trough should be sufficient to turn Nadine northward in the next 24-36 hours as the upper low lifts out. Thereafter, the trough is expected to bypass the storm, leaving Nadine in a region of weak steering. Thereafter, the northerly flow between another upper low and the subtropical ridge should force Nadine southward. While the model guidance generally agrees with this, there are still significant differences in forward speed, as well as how sharp of a southward turn Nadine makes. The best course of action for now still appears to be the order for a slow motion at the end of the forecast period.

5-day intensity forecast

INITIAL 09/17 0300Z 60 KT 70 MPH
12 hour 09/17 1200Z 60 KT 70 MPH
24 hour 09/18 0000Z 55 KT 65 MPH
36 hour 09/18 1200Z 55 KT 65 MPH
48 hour 09/19 0000Z 50 KT 60 MPH
72 hour 09/20 0000Z 45 KT 50 MPH
96 hour 09/21 0000Z 45 KT 50 MPH
120 hour 09/22 0000Z 45 KT 50 MPH

5-day track forecast

Figure 2. My 5-day forecast track for Nadine.

Invest 92L

A weak tropical wave continues its approach to the Lesser Antilles. Convection has virtually vanished near the low-level center (I use that term quite loosely here).

Figure 3. Latest infrared satellite image of Invest 92L. Image credit: NOAA

I am beginning to want to give up on this wave, at least in the short-term. It may still have a shot in the western Caribbean at longer ranges, but that too will be contingent on how much of a circulation remains with the wave by the time it approaches that particular longitude. The GFS still insists on the easterly shear abating, and an anticyclone building over the system. This has not happened yet, and the global models tend to perform poorly in a shearing environment, oftentimes weakening the shear too fast. Indeed, the GFS seems to be latching onto the idea of a weaker, more broad anticyclone as the system traverses the Caribbean. There is some subsident flow to contend with across the eastern and central Caribbean, which will also inhibit development. Even as the system reaches the western Caribbean in about five days, the GFS does not portray a shear pattern that is all that favorable for intensification, with strong southwesterly shear lying in very close proximity to the disturbance.

Given these parameters, I do not expect significant development of 92L in the next few days. At this point a track through the length of the Caribbean into -- you guessed it ladies and gentlemen -- Mexico -- seems a little more likely barring some unlikely strengthening. A stronger system would still have the potential to move toward peninsular Florida.

Probability of development in 48 hours: 10%

Invest 93L

Although officially a low pressure area for now, the area of disturbed weather in the western Gulf of Mexico is highly disorganized. Based on satellite images, a circulation center is still evident near 26.2N 95.0W, and surface and buoy/oil rig observations suggest it remains well-defined.

Figure 4. Latest infrared satellite image of Invest 93L. Image credit: NOAA

However, surface pressures are not falling, and if there is a low, it remains very weak and completely devoid of convection. Water vapor imagery shows a well-defined upper trough is approaching the system from Texas. This trough/attendant frontal zone is producing strong southwesterly vertical shear over the disturbance, analyzed at 25 kt by UW-CIMSS. While 93L is moving northeast parallel to the upper flow, the shear vector is blowing faster than 93L is moving, which will counteract the general rule that systems can still intensify in shearing environments. Dry air covering the western Gulf should also preclude significant development before the system moves over the central or southern Louisiana coast sometime late Monday evening. The GFS and ECMWF are in good agreement on the timing, and I trend toward the faster side of the guidance given the depth of the aforementioned trough/low.

Regardless of development, episodic heavy rains are likely to accompany this low as it moves quickly northeastward. These rains could produce localized flooding in short bursts.

It should be noted that the global models show the circulation becoming better defined just prior to crossing the coast.

Probability of development in 48 hours: 10%


Lane is just under hurricane strength. As of the latest NHC advisory, the following was posted on the tropical storm:

Wind: 70 mph, with higher gusts
Location: 15.0°N 125.8°W
Movement: NW at 9 mph
Pressure: 995 mb

Satellite and microwave data suggests that Lane continues to become better organized, with a small but well-developed central dense overcast. Tropical cyclones with tight inner cores like Lane tend to intensify quicker than the climatological mean, but they also weaken just as fast once the environment becomes hostile. A warm spot has not yet become apparent on conventional satellite pictures, but a small eye-like feature surrounded by a developing eyewall was present in an earlier microwave taken shortly after 0z.

Figure 5. Latest infrared satellite image of Tropical Storm Lane. Image credit: NOAA

Lane has only a very narrow window of opportunity for further strengthening, 24 hours at the very most. After that time, the cyclone is forecast to move across cooler waters and encounter increasing westerly shear to the central Pacific trough. Weakening should begin by that time, with degeneration into a remnant low expected in about three days. This is consistent with forecasts from the global models and the National Hurricane Center.

Water vapor imagery shows a large amplitude trough approaching Lane from the west. Consequently, the storm is moving northwestward, and this motion is expected to continue for the next 48-72 hours. After that time, Lane is forecast to weaken and turn west then west-southwest in the low-level flow. My track remains very similar to that from the National Hurricane Center.

5-day intensity forecast

INITIAL 09/17 0300Z 60 KT 70 MPH
12 hour 09/17 1200Z 65 KT 75 MPH
24 hour 09/18 0000Z 65 KT 75 MPH
36 hour 09/18 1200Z 55 KT 65 MPH
48 hour 09/19 0000Z 45 KT 50 MPH
72 hour 09/20 0000Z 30 KT 35 MPH...POST-TROPICAL/REMNANT LOW
96 hour 09/21 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
120 hour 09/22 0000Z...DISSIPATED

5-day track forecast

Figure 6. My 5-day forecast track for Lane.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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3. GeorgiaStormz
1:24 PM GMT on September 17, 2012
Tha blog is Hot
Member Since: December 31, 1969 Posts: Comments:
2. Civicane49
3:56 AM GMT on September 17, 2012
Member Since: December 31, 1969 Posts: Comments:
1. wxchaser97
3:50 AM GMT on September 17, 2012
Thanks Kori, nice forecast and I agree with everything.
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Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.

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