Tropical weather analysis - September 30, 2011

By: KoritheMan , 5:08 AM GMT on September 30, 2011

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Ophelia became the season's fourth hurricane today when it underwent a bout of strengthening. In fact, the cyclone is still strengthening as of the latest NHC advisory:

Wind: 85 mph, with higher gusts
Location: 21.9°N 62.3°W
Movement: NNW at 9 mph
Pressure: 979 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Ophelia is well-organized on satellite images, and there is no reason to assume that the earlier deepening has ceased. The hurricane is sporting a well-defined central dense overcast and what appears to be improving spiral banding within the northern eyewall. An earlier SSMIS overpass revealed that Ophelia possesses a well-defined inner core, with a solid eyewall that is about 80% complete. However, the 85 GhZ channel is sensitive to the middle troposphere, meaning that this organization is primarily aloft. Recent satellite frames show what appears to be an eye.

Figure 1. Latest infrared satellite image of Hurricane Ophelia, courtesy of NOAA's Satellite Services Division (SSD).

Both CIMSS analysis and the SHIPS model diagnose about 20 kt of westerly shear over the hurricane, but upper-level outflow is spreading out within the western quadrant, albeit with impeded progress. This suggests that an upper high is building atop the hurricane, which should prevent this shear from permeating the core. This shear is forecast to decrease during the next few days, which should allow for continued intensification of the tropical cyclone. However, by Sunday morning, the hurricane will be crossing the 26C isotherm, which, in conjunction with an abrupt increase in southwesterly shear that is forecast by 72 hours, should bring about a quick weakening. Global models suggest that Ophelia should lose tropical characteristics on Monday, and become absorbed into a frontal system on Tuesday.

Given the favorable environment depicted by the GFS/SHIPS pair, it is possible that Ophelia could briefly become a major hurricane, especially if current organizational trends persist. Either way, a solid Category 2 is certainly well within the realm of possibility, and is what I am forecasting at this time.

Ophelia is riding the southwest periphery of the subtropical ridge. Global models weaken this ridge over the next day or so as an upper-level trough approaches the hurricane from the west. The ridge will be slow to budge, though, as this trough is still undergoing the amplification process. Ergo, as has been the typical progression this year, Ophelia's recurve will be gradual, not rapid. In about three days, Ophelia should begin moving north-northeast to northeast as it becomes embedded within southwesterly mid-latitude flow associated with the aforementioned trough. Model guidance suggests that Ophelia will pass about 125 miles east of Bermuda, but interests there should continue monitoring the progress of this tropical cyclone, especially if the National Hurricane Center's track continues shifting westward in subsequent advisories. Which, given the relatively zonal flow still lying north of Ophelia, is definitely possible.

Interests in Atlantic Canada, particularly Newfoundland, should also monitor the progress of Ophelia.


Tropical Storm Philippe continues to churn harmlessly in the Atlantic. As of the latest NHC advisory, here is the information on the cyclone:

Wind: 45 mph, with higher gusts
Location: 21.2°N 44.0°W
Movement: NW at 13 mph
Pressure: 1004 mb
Category: Tropical storm (Saffir-Simpson Hurricane Scale)

Philippe is not well organized. Convection is minimal near the center, which in itself is a challenge to identify.

Figure 2. Latest infrared satellite image of Tropical Storm Philippe, courtesy of NOAA's Satellite Services Division (SSD).

Outflow from intensifying Ophelia is imparting northwesterly shear over Philippe, as evidenced by the squashed outflow pattern to the northwest of the center. However, the cloud pattern suggests that this shear is not yet infiltrating the core; more likely, the disheveled appearance is due to mid-level dry air being advected into the system from an upper-level low several hundred miles southwest of Philippe.

Although the shear is forecast to remain low for about the next 24 hours, the continued influx of dry air associated with southerly flow from the upper low will likely continue to inhibit development. While Philippe could still intensify a little during this time, the most likely scenario in light of current trends is for the cyclone to maintain its intensity. Beyond 24 hours, an abrupt increase in vertical shear is forecast, in association with Ophelia's outflow. This shear and the resultant subsident flow associated with it should promote weakening of Philippe, and eventual dissipation.

It should be noted that although the models and the NHC official forecast take Philippe rather far to the west for this time of year, the synoptic pattern over the US is probably still a bit too progressive for Philippe to make it to the United States. However, should the cyclone dissipate sooner than 3 or so days, it could come a hair close to the southeastern United States as it comes underneath the low-level flow.

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1. sunlinepr
5:25 AM GMT on September 30, 2011
Thanks for the update Kori....
Philippe looks bad tonight....
Ophelia got healthy...

Member Since: August 2, 2010 Posts: 22 Comments: 10134

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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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