Tropical weather analysis - September 7, 2012

By: KoritheMan , 7:10 AM GMT on September 07, 2012

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Leslie refuses to move. As of the latest NHC advisory, the following information was posted on the hurricane:

Wind: 75 mph, with higher gusts
Location: 26.5°N 62.2°W
Movement: Stationary
Pressure: 985 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Looks can certainly be deceiving. While Leslie has a well-organized appearance on satellite images, earlier microwave data suggested that the lower- and mid-level circulations continue to have an eastward displacement, which implies that the hurricane is still under some westerly shear.

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

The SHIPS and GFS still insist on a relaxation of the shear, but so far this has not materialized. An upper low north of the Bahamas is forecast to decay over the next 12-24 hours. There is some evidence of this on water vapor images. Assuming that this trend continues, the shear should decrease over the hurricane, and it would have an opportunity to strengthen. However, microwave data suggest that Leslie is a large hurricane that lacks an inner core. Such systems generally take awhile to spin up (and sometimes never do), as we saw with Isaac. In addition, NOAA buoy 41049 located very near the cyclone center indicates that water temperatures have dropped significantly under the hurricane -- as much as 3F -- over the last 24 hours, and are now below 26C. No doubt, this is due to the continued quasi-stationary movement of the storm. Until Leslie begins to move, it will probably not strengthen. The global models suggest this should happen over the next 24-36 hours as an upper trough over the central United States swings eastward. While not explicitly shown, I would not be surprised to see Leslie briefly drop below hurricane strength over the next 24 hours due to the cooling SSTs. After 72 hours, the GFS shows Leslie losing its anticyclone, and southwesterly shear increasing over the hurricane. That, in combination with progressively cooler sea surface temperatures should lead to weakening during the latter portion of the forecast period. Beyond day five, Leslie will likely become extratropical due to very cold SSTs.

Leslie remains in an environment of weak steering. UW-CIMSS analysis shows that the hurricane is sandwiched between two anticyclones, which is resulting in little movement. However, the models suggest that the easternmost anticyclone will remain stronger, which fits well with synoptic trends over the United States. The models respond to this pattern by forecasting a slow, mostly northward motion through the next day or so. Thereafter, the hurricane is forecast to slowly pick up speed as the aforementioned trough amplifies. A greater degree of acceleration is shown near the end of the forecast period as Leslie becomes fully embedded in the high-latitude westerlies. In general, the guidance has trended eastward over the last 24 hours, and it now appears that Leslie will miss Bermuda by a fairly wide margin. However, the wind field is large enough that tropical storm force winds may still occur on that island, particularly if the track shifts westward. For that reason, a tropical storm watch has been posted for that island.

In the meantime, Leslie's large wind field will continue to generate substantial waves and rip currents across much of the United States east coast, Bermuda, and Atlantic Canada over the next several days.

My forecast track is a little slower than the guidance.

5-day intensity forecast

INITIAL 09/07 0600Z 65 KT 75 MPH
12 hour 09/07 1800Z 65 KT 75 MPH
24 hour 09/08 0600Z 65 KT 75 MPH
36 hour 09/08 1800Z 70 KT 80 MPH
48 hour 09/09 0600Z 80 KT 90 MPH
72 hour 09/10 0600Z 90 KT 105 MPH
96 hour 09/11 0600Z 80 KT 90 MPH
120 hour 09/12 0600Z 65 KT 75 MPH

5-day track forecast

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


After briefly becoming the first major hurricane of the season early on Thursday, Michael has steadliy weakened. As of the latest NHC advisory, the following was posted on the hurricane:

Wind: 105 mph, with higher gusts
Location: 30.8°N 40.8°W
Movement: NNE at 5 mph
Pressure: 970 mb
Category: 2 (Saffir-Simpson Hurricane Scale)

The earlier erosion of the inner core convection appears to have stopped for now, with satellite images showing a more mature CDO. However, the eye has continued to contract, and is fairly ragged at the moment. Earlier microwave data also suggested that the eyewall was open to the northeast. All of this data points to a hurricane that is slowly weakening.

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

Water vapor imagery suggests that Michael is approaching a wall of northerly shear emanating from the backside of a mid- to upper-level trough located to the north of the hurricane. The GFS suggests that shear levels will oscillate between favorable and unfavorable over the next 72 hours, after which an abrupt increase in northerly shear is forecast as the tiny hurricane moves closer to the much larger circulation of Hurricane Leslie. Given the small size of the vortex, it is possible that Michael will weaken more quickly than indicated below.

Based on UW-CIMSS steering analysis and water vapor imagery, which shows the trough bypassing the hurricane, I agree with the National Hurricane Center forecast track, albeit a little to the east for the first 12 hours.

5-day intensity forecast

INITIAL 09/07 0600Z 90 KT 105 MPH
12 hour 09/07 1800Z 80 KT 90 MPH
24 hour 09/08 0600Z 75 KT 85 MPH
36 hour 09/08 1800Z 65 KT 75 MPH
48 hour 09/09 0600Z 60 KT 70 MPH
72 hour 09/10 0600Z 50 KT 60 MPH
96 hour 09/11 0600Z 40 KT 45 MPH
120 hour 09/12 0600Z 30 KT 35 MPH

5-day track forecast

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

Invest 90L

A weak area of low pressure in the northern Gulf of Mexico located about 60 miles southeast of the Mouth of the Mississippi River remains disorganized. While this low is producing shower activity, it is poorly-organized, and displaced well to the southwest of the low-level center due to persistent northeasterly shear. Surface observations still indicate a fairly robust surface circulation, however, and winds to about 30 knots are probably still occurring in the deep convection to the south based on earlier oil rig reports.

Upper-level winds could improve in about 36 hours, and the system still has the opportunity to develop before it reaches the Florida peninsula in about three days. The global models are in good agreement on this. While some of the statistical models have trended toward Louisiana in the last two cycles, the more reliable dynamical consensus continues to indicate a turn toward Florida. However, the statistical models could turn out to be right if the ridge -- which appears quite strong at the moment -- does not appreciably weaken over the next day or two, or if 90L moves more southwestward than indicated. Currently, it is not forecast to get below about 27.5N as per the global models.

There were some faint indications in the global models earlier on Thursday that the trough could bypass the system once it is in the western Atlantic, and the models responded by showing some 850 mb vorticity moving westward across Florida and into the Gulf of Mexico. However, the 0z camp has become less aggressive on this.

Probability of development in 48 hours: 30%

New African wave

A tropical wave has just emerged off the west coast of Africa. The GFS and ECMWF suggest this system could become a tropical depression over the next few days. Environmental conditions are favorable for gradual development of this wave as it moves generally westward over the next 48 hours.

Probability of development in 48 hours: 10%

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2. wxchaser97
12:10 PM GMT on September 08, 2012
Thanks Kori!
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1. GeorgiaStormz
1:29 PM GMT on September 07, 2012
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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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