Tropical weather analysis - September 28, 2012

By: KoritheMan , 4:26 AM GMT on September 29, 2012


Tenacious Nadine became a hurricane again around midday today. As of the latest NHC advisory, the following information was available on the hurricane:

Wind: 75 mph, with higher gusts
Location: 31.0°N 35.5°W
Movement: NNW at 10 mph
Pressure: 988 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

There has been little change to the structure of Nadine this evening. The hurricane continues to generate a circular area of cold topped convection. Although an eye is not readily apparent on conventional satellite images, recent microwave data suggests the internal structure remains well-organized. However, the eyewall is open to the south, and the lower and middle-level circulations exhibit a northeastward displacement of about 25 miles. This is probably due to about 20 kt of southwesterly vertical wind shear as depicted by analysis from UW-CIMSS and the SHIPS model.

Figure 1. Latest infrared satellite image of Hurricane Nadine. Image credit: RAMMB Colorado State University (CSU).

The shear is anticipated to continue for another 48 hours or so. After that time, the both the SHIPS and GFS forecast a relative decrease in the shear. Given that the GFS has been indicating this for the last several days, it is a little difficult to ignore. While restrengthening is not being explicitly indicated at this time, if enough baroclinic forcing happens to develop, Nadine will probably strengthen a little from the 48-72 hour point. However, the models are not consistent in when or if this will occur. Beyond day three, northerly shear is forecast to abruptly increase over the storm, which should put an end to any possible intensification.

Nadine has made a turn north-northwestward, as water vapor imagery and UW-CIMSS steering data suggest that the nearby subtropical ridge continues to weaken with the approach of a vigorous upper low/baroclinic zone over the central Atlantic. This synoptic pattern is forecast to continue the north-northwestward motion for the next day or two. Beyond 48 hours, Nadine is expected to move north and northeast as it gradually becomes enraptured in mid-latitude westerly flow associated with a deep-layer trough that the global models are unanimous in bringing across the north Atlantic shipping lanes. It appears that, finally, that Nadine may have had her fun. With the models having come into better agreement over the past 24 hours, confidence in the long-range forecast track is a little higher, but not completely. And while a relatively slow motion is still shown at the end of the forecast period, I am considerably faster than the last several days. But I'm still prepared for surprises, and I'm not going to go too fast.

5-day intensity forecast

INITIAL 09/29 0300Z 65 KT 75 MPH
12 hour 09/29 1200Z 65 KT 75 MPH
24 hour 09/30 0000Z 65 KT 75 MPH
36 hour 09/30 1200Z 65 KT 75 MPH
48 hour 10/01 0000Z 65 KT 75 MPH
72 hour 10/02 0000Z 65 KT 75 MPH
96 hour 10/03 0000Z 55 KT 65 MPH
120 hour 10/04 0000Z 45 KT 50 MPH

5-day track forecast

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


Tropical Storm Norman quickly developed today off the southwest coast of Mexico, but has already weakened to a tropical depression. As of the latest NHC advisory, the following information was posted on the cyclone:

Wind: 35 mph, with higher gusts
Location: 25.2°N 109.0°W
Movement: NNW at 10 mph
Pressure: 1003 mb

Norman is a poorly-organized tropical cyclone, to say the least. Based on satellite images, surface observations from mainland Mexico and southern Baja, and finally, doppler radar data from Guasave, the center appears to be located to the north of an elongated band of relatively deep convection.

Figure 3. Latest infrared satellite image of Tropical Storm Norman. Image credit: NOAA

The highest sustained wind speed I could find overland was a 23 mph sustained wind at Los Mochis, located about 60 miles from the coast of Mexico. There is no indication that Norman is producing any sustained winds to tropical storm force, although 40 to 45 mph gusts will still be possible within the rather vigorous band to the east of the center. The previously cited wind report from Los Mochis was actually outside of this band, so people living near or underneath it should exercise caution.

Norman is located just off the coast, and it will not take much for it to make landfall. However, some of the more reliable guidance, such as the GFS, has come westward, and now show Norman moving parallel to, but just offshore, the coast before turning westward. Since the storm has recently turned north-northwest, a motion just off the coast is indeed within the realm of possibility. That being said, given that not all of the models agree with the GFS scenario, I will go ahead and pull Norman inland, but will more or less keep it between the coastal waters and the immediate coast, obviously out of respect for the GFS.

I should note that Norman's possible landfall is of little consequence, as all of the weather is in bands to the south and east of the center. These rains will be capable of causing flash flooding and mudslides, particularly in areas of higher terrain. This will be especially if true the farther Norman moves inland.

After landfall, or even if not, the cyclone is forecast to quickly wind down. I am showing dissipation in 36 hours, but it could certainly occur sooner.

It should be noted that some of Norman's mid-level energy is expected to get caught up in deep-layer southwesterly flow ahead of a cold front and help to initiate a surface low over south Texas over the weekend, which will likely be very heavy rains to the northern Gulf Coast.

5-day intensity forecast

24 hour 09/30 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
36 hour 09/30 1200Z...DISSIPATED

5-day track forecast

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


Typhoon Jelawat continues to steadily weaken as it plows through Okinawa. As of the 0300Z JTWC advisory, the following information was available on the typhoon:

Wind: 115 mph, with higher gusts
Location: 26.1N 127.4E
Movement: NE at 15 mph
Category: 3 (Saffir-Simpson Hurricane Scale)

Recent satellite fixes indicate that Jelawat is slightly west of the 0z forecast point from the JTWC. Other than that, there has been no significant deviation from the projected track.

Satellite images still show a formidable typhoon, but the eye is extremely ragged.

Figure 5. Latest infrared satellite image of Typhoon Jelawat. Image credit: NOAA

There is not much to say about the storm. The long-anticipated southwesterly shear has arrived, as a 0100Z microwave pass indicated the southern eyewall had fragmented. In addition, some drier air has periodically filtered into the circulation. Unlike the episodes of the last several days, there is no indication that Jelawat is going through another eyewall replacement cycle. In fact, considering that it is forecast to quickly weaken subsequent to this point, it is likely that there will be no additional cycles. All of that being said, Jelawat is still expected to approach the southern coast of Japan as a strong tropical storm. It could pass directly over the heavily populated city of Tokyo, and residents there should be taking it seriously. Rainfall potential will be limited, since the cyclone will be moving quite rapidly as it moves across the country.

Jelawat is expected to lose tropical characteristics in about 72 hours as it moves over cold water and encounters even stronger upper-level shear.

The track forecast remains straightforward. The typhoon is well-embedded within a southwesterly steering current between a strong ridge to the east and a deep-layer trough to the west. A continued acceleration to the northeast is expected over the next few days. Tonight's track merely updates the previous few.

Okinawa reported a maximum wind gust of 112 mph, along with a central pressure of 955 mb as the eye passed over. This was along the southern end of the archipelago, on Naha.

5-day intensity forecast

INITIAL 09/29 0300Z 100 KT 115 MPH
12 hour 09/29 1200Z 90 KT 105 MPH
24 hour 09/30 0000Z 70 KT 80 MPH
48 hour 10/01 0000Z 30 KT 35 MPH...OVER WATER
72 hour 10/02 0000Z 25 KT 30 MPH...POST-TROPICAL/EXTRATROPICAL
96 hour 10/03 0000Z...ABSORBED BY FRONT

5-day track forecast

Figure 6. My 5-day forecast track for Jelawat.

Gulf of Mexico disturbance taking shape

The much-advertised Gulf of Mexico disturbance is beginning to manifest over southern Texas, as evidenced by a recently-formed cluster of convection. This is occurring at the southern end of a rather potent shortwave trough currently moving across central Texas. The global models have generally trended toward a more inland/less time over water solution. However, there could still be a narrow corridor of lighter upper-level winds that could assist in development if the low moves over water longer than predicted. As previously mentioned, this low is being fed in part by Eastern Pacific Tropical Depression Norman. This is a rare synoptic setup.

Regardless of development, heavy rainfall, gusty winds along the coast, as well as possible coastal flooding will occur over portions of the northern Gulf Coast through about Tuesday. These rains could be capable of causing flooding. Some severe weather will also be possible with this activity, although large-scale cloud cover may inhibit instability somewhat.

This system currently looks like it will move across Louisiana based on the model runs.

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3. WunderAlertBot (Admin)
7:37 AM GMT on September 30, 2012
KoritheMan has created a new entry.
2. KoritheMan
4:44 AM GMT on September 29, 2012
Quoting Grothar:
That Gulf disturbance looks quite strong. I hope it moves a little south and we get a little. Looks like you are about to get wet.

Wet indeed. I really think we could see 2 to 4 inches here, even factoring in the relatively quick forward motion.

Although I do have to wonder just how the low is going to stay primarily inland like the models are suggesting given the already southerly position of it. As always, time will tell.
Member Since: March 7, 2007 Posts: 617 Comments: 22395
1. Grothar
4:31 AM GMT on September 29, 2012
That Gulf disturbance looks quite strong. I hope it moves a little south and we get a little. Looks like you are about to get wet.
Member Since: July 17, 2009 Posts: 75 Comments: 30313

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