Tropical weather analysis - September 8, 2012

By: KoritheMan , 2:42 AM GMT on September 09, 2012


Leslie continues to meander, but appears to be slowly moving away from its cold wake. As of the latest NHC advisory, the following was available on the cyclone:

Wind: 65 mph, with higher gusts
Location: 30.1°N 62.6°W
Movement: N at 8 mph
Pressure: 988 mb

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

There is still little evidence of an inner core with Leslie, and based on the satellite signature the radius of maximum winds are probably still on the order of about 75 nautical miles.

Analysis of water vapor imagery suggests that the shear over the system has decreased. Additionally, buoy data suggests that offshore water temperatures near Bermuda are around 84F. The low shear combined with the warm waters should favor intensification, although ambient dry air may still be a problem, especially considering how large Leslie's circulation is. It no longer appears likely, even given a remote chance, that Leslie will become a major hurricane. Based on SST analyses and the global model isobaric fields, Leslie is expected to lose tropical characteristics in about three days. While the models do not suggest reintensification as a post-tropical cyclone, they suggest that Leslie could be a rather vigorous extratropical low over the north Atlantic for a few days subsequent to transition.

As a large-scale trough amplifies over the western Atlantic, the surrounding steering flow has become better defined. Consequently, the tropical storm has responded by turning north with some acceleration. The global models are in good agreement on the track, taking the system well east of Bermuda tomorrow, and accelerating Leslie northeastward after that. It is possible that Leslie will make landfall along the southeastern tip of Nova Scotia as an extratropical cyclone in about four days.

My forecast is similar to but a little east of the National Hurricane Center.

5-day intensity forecast

INITIAL 09/09 0300Z 55 KT 65 MPH
12 hour 09/09 1200Z 60 KT 70 MPH
24 hour 09/10 0000Z 65 KT 75 MPH
36 hour 09/10 1200Z 70 KT 80 MPH
48 hour 09/11 0000Z 75 KT 85 MPH
72 hour 09/12 0000Z 70 KT 80 MPH...EXTRATROPICAL
96 hour 09/13 0000Z 60 KT 70 MPH...EXTRATROPICAL
120 hour 09/14 0000Z 45 KT 50 MPH...EXTRATROPICAL

5-day track forecast

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

A tropical storm warning remains in effect for Bermuda, although I doubt wind gusts above 40 mph will be felt on that island.


Michael continues as a hurricane. As of the latest NHC advisory, the following was available on the hurricane:

Wind: 100 mph, with higher gusts
Location: 33.4°N 42.4°W
Movement: NNW at 6 mph
Pressure: 975 mb
Category: 2 (Saffir-Simpson Hurricane Scale)

Michael hasn't changed much this evening, except that the central dense overcast has shrunk. The eyewall convection continues to fluctuate, but in general the northwestern eyewall appears to be opened.

Figure 3. Latest infrared satellite image of Hurricane Michael. Image credit: NOAA

Water vapor imagery shows northwesterly shear impinging on the western side of the small hurricane, associated with the backside of a mid- to upper-level trough passing to the north. However, the flow to the north of the hurricane is becoming zonal, signifying departure of the trough. As this feature lifts out, so will the shear. In about 72 hours, however, the GFS/SHIPS forecast a marked decrease in northerly shear as Michael begins to feel outflow from what should be an intensifying Hurricane Leslie. The shear is eventually forecast to transition to a more eastward direction and increase even further. The end result should be rapid weakening, especially given Michael's small size.

Most of the models are now suggesting that the hurricane will become absorbed by the much larger circulation of Leslie later in the forecast period, and as a course of least regret, I now will as well.

The hurricane should soon turn to the west-northwest under the influence of a building mid-level ridge. By late Monday, a turn back to the northwest and north is expected as Michael begins to interact with Leslie. This track is in good agreement with the global models.

5-day intensity forecast

INITIAL 09/09 0300Z 85 KT 100 MPH
12 hour 09/09 1200Z 85 KT 100 MPH
24 hour 09/10 0000Z 80 KT 90 MPH
36 hour 09/10 1200Z 75 KT 85 MPH
48 hour 09/11 0000Z 70 KT 80 MPH
72 hour 09/12 0000Z 55 KT 65 MPH

5-day track forecast

Figure 4. My 5-day forecast track for Michael.

Invest 91L

A tropical wave in the eastern Atlantic centered in the vicinity of the southern Cape Verde Islands ("91L") is producing disorganized showers and thunderstorms. Microwave and satellite data suggest that the system isn't too well-organized yet. However, upper-level winds appear favorable enough -- at least diffluent -- to allow for gradual development of this wave as it moves westward. In a few days, the system is expected to turn toward the west-northwest admist a break in the subtropical ridge. This wave looks like another one for the fishes. However, the recurve pattern this year seems to be a little weaker than the last few years, and I don't think the United States is quite done yet. We will need to watch any system that comes out of the Caribbean or Gulf.

Probability of development in 48 hours: 40%

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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2. GeorgiaStormz
3:45 AM GMT on September 09, 2012
Thx Kori
Member Since: December 31, 1969 Posts: Comments:
1. wxchaser97
2:52 AM GMT on September 09, 2012
Thanks Kori, I'm working on mine right now.
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Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.

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