Tropical weather analysis - September 26, 2011

By: KoritheMan , 3:13 AM GMT on September 27, 2011


Although Ophelia dissipated yesterday under the influence of strong vertical shear, she is showing signs of organization this evening, and could regenerate into a tropical cyclone over the next day or two. She will only be a threat to Bermuda and possibly Atlantic Canada (Newfoundland specifically). I will have a more detailed update when/if she regenerates.

It is possible that the northern Leeward Islands receive some squally weather from her, depending on exactly where the center is, and how quickly she organizes tonight.


Tropical Storm Philippe continues churning in the eastern Atlantic. As of the latest NHC advisory, here is the information on the system:

Wind: 50 mph, with higher gusts
Location: 15.9°N 34.8°W
Movement: NW at 8 mph
Pressure: 1003 mb
Category: Tropical storm (Saffir-Simpson Hurricane Scale)

Don't be deceived by the convection. Analysis of shortwave infrared satellite imagery indicates that Philippe's low-level center is partially exposed along the west edge of the thunderstorm complex due to moderate westerly shear.

Figure 1. Latest infrared satellite image of Tropical Storm Philippe, courtesy of NOAA's Satellite Services Division (SSD).

I assume the cyclone has already peaked. Water vapor imagery shows a large area of dry air lies to the west of the cyclone. With the aforementioned shearing regime present, there will be plenty of opportunity for this dry air to become entrained into the core. This shear is not forecast to abate by the reliable models, and high cloud motions derived from water vapor imagery shows westerly flow along and north of 20N. This, combined with passage over progressively cooler sea surface temperatures along the forecast track, lends credence to the likelihood that Philippe will begin a slow weakening trend by tomorrow. There is the possibility that Philippe could weaken a little quicker given the small size of the circulation envelope, as well as the large expanse of dry air that covers much of the central and eastern Atlantic. It should be noted that some of the models show a westward bend of Philippe beginning on Saturday. Should this happen, shear profiles are forecast to become more conducive, and the cyclone would also be moving into an area of warmer SSTs.

Philippe should keep moving generally northwestward over the next two or so days. Thereafter, the models diverge, with some taking Philippe more northward, presumably in response to a deeper vortex in the vertical. Others still, keep the tropical storm more northwestward, likely due to a weaker cyclone being depicted in the forecast grids. The typically reliable ECMWF continues to insist on a much weaker Philippe moving more westward than any the rest of the guidance is indicating. However, the initialization of Philippe's circulation within this model as of 12z was very poor. Thus, it is probably too quick to weaken it, the unfavorable environment notwithstanding.

I have opted to discount the ECMWF's forecast for now, regardless of how reliable it is under ordinary circumstances. My forecast is closer to the NHC official forecast, which also shows a westward bend at day five.


Hurricane Hilary regained Category 4 status today, but has recently weakened. As of the latest NHC advisory, here is the information on the hurricane:

Wind: 125 mph, with higher gusts
Location: 16.6°N 114.8°W
Movement: W at 9 mph
Pressure: 953 mb
Category: 3 (Saffir-Simpson Hurricane Scale)

Hilary remains well-organized on satellite imagery, although the eyewall convection has eroded a bit, and temperatures within the eye have also cooled.

Figure 2. Latest infrared satellite image of Hurricane Hilary, courtesy of NOAA's Satellite Services Division (SSD).

It is possible that Hilary is getting ready to begin another eyewall replacement cycle. A recent SSMIS overpass taken just after 0z shows an intense band of convection well to the east of eye, within the outer rainbands of the hurricane. This is also supported by satellite imagery, which shows cloud tops cooling within the eastern semicircle of the cyclone, and also to the south. Although Hilary strengthened after the previous such cycle, the hurricane will be passing over cooler sea surface temperatures from this point onward, with a more rapid cooling in a few days. Thus, reintensification after this next cycle, assuming it is starting, appears unlikely.

Instead, the most likely scenario is for Hilary to slowly weaken over the next 48 hours. Thereafter, a quicker pace of weakening is expected given the very cool SSTs the hurricane will encounter. As usual, these SSTs will be accompanied by a stable marine layer. Wind shear is also forecast to increase late on Thursday. These factors should be enough to bring Hilary back down to a tropical storm in about 60 hours.

Hilary is forecast to turn sharply to the northwest in about 36 hours as an upper low near 28N 131W moves eastward and increases the southerly flow over the hurricane. This evolution should cause Hilary to turn move generally northward until about 72 hours. Thereafter, the models diverge, with the GFS showing a landfall in central Baja on Friday. The GFDL turns Hilary westward, possibly due to the fragile nature of the vortex feeling the low-level flow by that time. The CMC also hints at a westward turn of Hilary late in the forecast period. I think the most likely scenario is for the vortex to continue moving generally northward, but given the perceived hostilities of the synoptic environment, it would not be the least bit surprising to see Hilary move more northwestward at days four and five.

As is common of eastern Pacific hurricanes moving into harsh environments at high latitudes, the mid-level vorticity maximum is likely to move northeastward into the southwestern United States and enhance the rainfall potential there, while the low-level vortex remains in the open water and drifts in a westward direction.

For now, my forecast track remains in pretty good agreement with the NHC official forecast, which is as follows:

Figure 3. Latest NHC 5-day forecast track for Hurricane Hilary.

Due to the uncertainty, interests in western and central Baja should continue monitoring the progress of Hilary.

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1. BaltimoreBrian
3:47 AM GMT on September 27, 2011
You write a good blog Koritheman.
Member Since: August 9, 2011 Posts: 26 Comments: 11628

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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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