Tropical weather analysis - September 24, 2012

By: KoritheMan , 5:41 AM GMT on September 24, 2012

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Due to issues with the GOES-13 satellite, I am physically incapable of doing a blog on Nadine. So I will be sticking with Miriam and Jelawat for now.


Miriam has rapidly strengthened to a hurricane this evening, the ninth of the season. As of the latest NHC advisory, the following information was available on the hurricane:

Wind: 90 mph, with higher gusts
Location: 16.8°N 111.3°W
Movement: NW at 12 mph
Pressure: 979 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Miriam is a well-organized hurricane. A small eye was evident in a recent microwave pass, and this feature has become apparent in conventional satellite images as well.

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

Miriam has no more than about 36 hours to intensify, most likely closer to 24. After that time, the hurricane is forecast to encounter cooler waters, a more stable airmass, and an increase in southwesterly shear beyond 48 hours. These factors should initiate a weakening trend, possibly fairly rapid near the end of the period. While the intensity consensus does not currently anticipate Miriam becoming a major hurricane, I think it's possible given current trends. Alas, it is difficult to tell how long these rapid intensification episodes will last. Given the nature of intense hurricanes, it is possible that Miriam will undergo an eyewall replacement cycle subsequent to assuming its peak intensity. That would likely result in more short-term weakening than indicated below, but it would also result in a larger circulation that is more fit to survive cold waters later in the period.

One fly in the ointment pertaining to the intensity of Miriam beyond day three is the point at which it recurves. Since the GFS and ECMWF are now in agreement that Miriam will get picked up by the west coast trough, I suppose the only real question is to the specific longitude that she will recurve. A cyclone that curves farther east, south of central Baja, would feel warmer sea surface temperatures and be less to prone to weakening. On the hand, a storm that recurves left -- or north -- will find itself in a harsher thermodynamic environment, with much cooler water temperatures.

The steering for Miriam is relatively straightforward, with the biggest uncertainty being precisely where the hurricane will recurve. Miriam is expected to begin recurving in about 48 hours as the mid-level ridge to the north weakens with the amplification of a large upper trough along the west coast of the United States. There are considerable differences in forward speed though, likely due to the influence of a secondary vortex noted in the various model forecast fields. This vortex has not been evident in either the GFS or the Euro, so it is most likely of a spurious nature. The GFS and Euro aren't that far apart in terms of speed, which leads me to believe the disagreements are outliers being falsely influenced by said vortex.

Interests in Baja California and southwestern Mexico should monitor the progress of the hurricane. Regardless of whether or not Miriam makes landfall, heavy rainfall associated with the mid-level vortmax will likely overspread portions of the southwestern United States 5 - 7 days from now.

5-day intensity forecast

INITIAL 09/24 0300Z 80 KT 90 MPH
12 hour 09/24 1200Z 90 KT 105 MPH
24 hour 09/25 0000Z 100 KT 115 MPH
36 hour 09/25 1200Z 95 KT 110 MPH
48 hour 09/26 0000Z 85 KT 100 MPH
72 hour 09/27 0000Z 65 KT 75 MPH
96 hour 09/28 0000Z 45 KT 50 MPH
120 hour 09/29 0000Z 30 KT 35 MPH

5-day track forecast

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


Jelawat became a super typhoon yesterday (Sunday). I mentioned this was possible, since western Pacific storms often accomplish this feat fairly easily. As of the latest JTWC advisory, the following was posted on the storm:

Wind: 150 mph, with higher gusts
Location: 13.6N 128.5E
Movement: N at 5 mph
Category: 4 (Saffir-Simpson Hurricane Scale)

There have been some structural changes with Jelawat this evening. The eye has shrunk a bit, at times becoming completely cloud filled. In addition, satellite images show considerable erosion of the northern eyewall. Microwave data throughout the day on Sunday, continuing up through 0z Monday, have shown a proclivity toward a concentric eyewall structure. This is not uncommon for western Pacific typhoons, which again, can become monstrous quite effortlessly.

Figure 3. Latest infrared satellite image of Super Typhoon Jelawat. Image credit: NOAA

Once Jelawat reaches the culminaton of its inner core fiasco, it should be able to reintensify a little. By day three, the typhoon is forecast to encounter gradually decreasing sea surface temperatures and underlying oceanic heat content. Increasing vertical shear will also be a problem at those forecast ranges. This pattern is expected to bring about a weakening of the system at that time, although it is expected to remain large and powerful as it bears down on Okinawa early this weekend. Interests there should carefully monitor the progress of Jelawat. However, structures on that island are generally built to withstand the force of powerful typhoons like Jelawat, so I do not anticipate a catastrophe. However, some very strong winds could occur on that island if Jelawat follows the intensity forecast, possibly upwards of over 140 mph in wind gusts. Beyond day five, Jelawat is expected to accelerate northeastward in the mid-latitude westerly flow and move toward mainland Japan.

My track forecast has come a little westward today, as there appears to be more ridging than I originally thought. A ridge is building in over mainland China in the wake of a shortwave trough, which is now lingering near the southwestern coast of Japan. This synoptic evolution favors a gradual turn to the north and northeast, but will allow for a more northweserly course than I was prognosticating yesterday. There does not appear to be enough forcing with the trough to produce a particularly sharp recurvature subsequent to the storm passing Okinawa. If I lived in Japan I would be preparing for a formidable typhoon hitting the southern coast early next week. I would also watch this if I lived in Taiwan, just in case Jelawat turn does turn as predicted.

5-day intensity forecast

INITIAL 09/24 0300Z 130 KT 150 MPH
12 hour 09/24 1200Z 120 KT 140 MPH
24 hour 09/25 0000Z 125 KT 145 MPH
36 hour 09/25 1200Z 130 KT 150 MPH
48 hour 09/26 0000Z 130 KT 150 MPH
72 hour 09/27 0000Z 125 KT 145 MPH
96 hour 09/28 0000Z 115 KT 135 MPH
120 hour 09/29 0000Z 105 KT 120 MPH

I should note that my intensity forecast carries greater than normal uncertainty due to not well understood inner core dynamics (namely eyewall replacement cycles) as well as the somewhat erratic behavior of storms in this region.

5-day track forecast

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

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3. GeorgiaStormz
12:18 PM GMT on September 24, 2012
Tha Blog Is Hot
Member Since: February 11, 2012 Posts: 0 Comments: 9727
2. wxchaser97
10:48 AM GMT on September 24, 2012
Thanks Kori, I like how you used a map for Jelawat.
Member Since: March 16, 2012 Posts: 127 Comments: 7942
1. Civicane49
5:53 AM GMT on September 24, 2012
Thanks Kori. I think you found a good tracking map of the western Pacific.
Member Since: July 21, 2011 Posts: 83 Comments: 7167

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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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