Tropical weather analysis - September 23, 2011
Tropical Storm Ophelia has weakened. As of the latest NHC advisory, here is the information on this system:
Wind: 50 mph, with higher gusts
Location: 14.1°N 49.9°W
Movement: WNW at 12 mph
Pressure: 1000 mb
Category: Tropical storm (Saffir-Simpson Hurricane Scale)
Infrared satellite loops indicate that Ophelia is highly disorganized; the low-level center is completely exposed to view, with the only remaining convection confined to a poorly-defined convective band about 125 miles northeast of the central gyre. However, low cloud motions suggest that the center is a bit better defined than it was earlier today.
Figure 1. Latest infrared satellite image of Tropical Storm Ophelia, courtesy of NOAA's Satellite Services Division (SSD).
UW-CIMSS wind shear analyses shows about 25 kt of southwesterly shear encroaching upon the cyclone. This is consistent with the SHIPS model, which shows 24 knots of shear from the same direction.
In about two to three days, there are still some indications within the GFS forecast fields that the upper tropospheric shear over Ophelia will relax, potentially allowing for a window of restrengthening. However, the GFS/SHIPS combo is less enthusiastic about this than they were 24 hours ago. The reasons for this are not quite clear, but water vapor imagery shows an upper low trying to close off near 24N 54W, to the northwest of the sheared tropical cyclone. It is possible that this feature, as depicted by the GFS, will move westward in tandem with Ophelia throughout the next several days, effectively keeping a strong shearing regime over the storm. Given that the trend of a less favorable upper wind environment later in the period has not been forecast over the last several days by the models, I am not apt to deviate from my previous forecast just yet. Thus, I am still looking for reintensification of Ophelia beginning on Saturday, although given the dismal appearance of the cyclone, as well as the relentless shear, it is entirely possible that Ophelia will degenerate into a sharp trough of low pressure tomorrow, in which case regeneration would be less likely since the upper flow will not be perfect even if it does let up some.
The track forecast remains practically unchanged. Ophelia is moving west-northwest under the influence of strong ridging to the north. Over the next 24 hours or so, however, the model consensus is for the cyclone to turn northwest as the ridge weakens with the approach of a cold front currently moving into the eastern United States. However, this front is also forecast to be a painstakingly slow-moving mid-latitude weather system, so the ridge will be slow to weaken. Essentially, this means that Ophelia's eventual recurvature will be gradual, not rapid. This is also a familiar theme this year. In about three to four days, Ophelia should begin moving more northerly, although I would not be surprised to see it continue with a more westerly component during that time, given how long the trough is forecast to stick around.
My forecast track remains pretty much the same as yesterday, calling for Ophelia to recurve near 70W. Some of the models suggest that Ophelia could be uncomfortably close to Bermuda in about five days, and since I feel the model consensus is a bit too far east, it is possible that Ophelia comes even closer. Thus, interests in and around that island should continue to monitor the progress of the cyclone over the next several days.
Ophelia is forecast to pass well to the north of the Leeward Islands, and since these islands will be on the equatorial (weaker) side of the storm, aside from some cloud cover, light rain showers, and a local disruption of the ordinary easterly flow, impacts will be minimal in this region.
It should be noted that if Ophelia dissipates, it could come a little farther southward than currently indicated, potentially giving the northern Leeward Island some gusty winds and perhaps a brief bout of heavy rain.
Hurricane Hilary continues to intensify off the coast of southwestern Mexico. As of the latest NHC advisory, here is the information on this system:
Wind: 145 mph, with higher gusts
Location: 16.0°N 101.4°W
Movement: WNW at 10 mph
Pressure: 944 mb
Category: 4 (Saffir-Simpson Hurricane Scale)
Hilary is a small but very impressive hurricane on satellite images, with a symmetrical central dense overcast surrounded by cloud tops within the eyewall which exceed -80C. This is an indication that Hilary's thunderstorms are reaching high into the troposphere, which is a sign of a mature hurricane. In addition, a small but well-defined eye is apparent within the middle of the CDO.
Figure 2. Latest infrared satellite image of Hurricane Hilary, courtesy of NOAA's Satellite Services Division (SSD).
With no obvious signs of a concentric eyewall on satellite and microwave images, there is little reason to assume that Hilary will not continue intensifying over the next 24-36 hours. One thing that could put a wrench into Hilary attaining Category 5 status is the possibility of a drier airmass over the next day or so, as indicated by the SHIPS model. Water vapor imagery also hints at that Hilary has a dry future ahead of her. I will continue to forecast strengthening, and call for Hilary to reach a peak intensity of 135 kt/155 mph over the next day, with a slow weakening thereafter. Of course, it is also possible that Hilary briefly reaches Category 5 strength, but I am not going to explicitly indicate this at this time due to the possibility of drier air and trademark eyewall replacement cycles. Also, Hilary has been rapidly intensifying for nearly 24 hours, and all good things must come to an end.
As for the track forecast, Hilary should continue moving generally west-northwest under the influence of a mid-level ridge over the eastern Pacific. This ridge is forecast to strengthen in a few days, imparting a more westerly component accompanied by an increase in forward speed. In about four days, the hurricane is forecast to decelerate as a trough amplifies over the west coast. This is when the models begin to diverge. The GFDL and HWRF continue to bring Hilary toward Baja California and into the Sea of Cortez as it comes under the influence of this trough. Indeed, even the reliable ECMWF predicts that Hilary will run into the trough. The remainder of the guidance indicates that Hilary will remain well offshore Baja and continue moving westward on an innocuous trajectory out to sea, as most storms in this basin do. This is what I will continue to insinuate at this time, although this forecast may need to be reevaluated if the ECMWF continues the northward trend.
Watches and warnings
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* WEST OF ACAPULCO MEXICO WESTWARD TO PUNTA SAN TELMO
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