Tropical weather analysis - September 28, 2012

By: KoritheMan , 7:29 AM GMT on September 28, 2012


Nadine continues as a tropical storm. The following information was available on the storm as of the latest National Hurricane Center forecast package:

Wind: 65 mph, with higher gusts
Location: 28.8°N 33.6°W
Movement: W at 7 mph
Pressure: 993 mb

Nadine looks more tropical than it has in millenia. Convection is actually near the center for once, and a large curved band wraps around the western semicircle. Upper-level outflow is becoming established as well, something Nadine has lacked for awhile. An eye feature briefly tried to appear around 4z as well. A well-timed microwave pass at 0327 UTC suggested this feature also, but the associated convection was more reminiscent of banding rather than a formative eyewall.

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

Environmental conditions are forecast to remain marginally conducive for the next 24 hours or so, with relatively warm sea surface temperatures and weak vertical shear. However, satellite data and water vapor imagery indicate strong upper-level southwesterly flow a couple hundred miles southwest of the storm. This shear is probably going to interfere with development before long, but then again, Nadine has shielded herself well against the shear thus far, which was forecast to pick up by this time. It should be noted that some of the guidance now calls for Nadine to regain hurricane strength. This is a distinct possibility, although acknowledging the aforementioned vertical shear pattern, I am not ready to call for that at this time. In addition to the shear, Nadine will be moving over slowly decreasing sea surface temperatures subsequent to this point. Hopefully this will be her final hurrah, but I wouldn't count on it.

Nadine is approaching the southwestern side of a deep-layer ridge over the central Atlantic, which is being pushed slowly eastward by an approaching shortwave trough. Recent satellite fixes and earlier microwave fixes suggest that the tropical cyclone may have begun to turn toward the west-northwest; it's certainly not moving west anymore. Nadine is expected to turn northwestward over the course of the day today as it rounds the axis of the subtropical ridge. A turn toward the north is anticipated in about 36 hours. The models agree up to the 72 hour point, and then begin to diverge. I would say I see signs of Nadine's departure in the global model fields, but I've said this before only to get burned. Hence, I fully intend to keep my mouth shut this time.

To err on the side of caution, and because the models have been anything but consistent with this storm, I will continue to forecast a quasi-stationary storm beyond the 72 hour point. This remains a low-confidence forecast, as per usual.

5-day intensity forecast

INITIAL 09/28 0600Z 55 KT 65 MPH
12 hour 09/28 1800Z 60 KT 70 MPH
24 hour 09/29 0600Z 60 KT 70 MPH
36 hour 09/29 1800Z 60 KT 70 MPH
48 hour 09/30 0600Z 55 KT 65 MPH
72 hour 10/01 0600Z 50 KT 60 MPH
96 hour 10/02 0600Z 50 KT 60 MPH
120 hour 10/03 0600Z 45 KT 50 MPH

5-day track forecast

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

Invest 94E

A broad area of low pressure centered off the Mexican coast about 125 miles southeast of the southern tip of the Baja Peninsula ("94E") continues to produce a large area of showers and thunderstorms.

Figure 3. Latest infrared satellite image of Invest 94E. Image credit: NOAA

Looks can be deceiving. Unlike yesterday, I was fortunate enough to be witness to a 0430 UTC AMSU microwave pass, which depicted the inner structure of the disturbance rather well. It suggested that the low-level center remains broad, and the surface wind field is probably not closed. Based on the microwave data as well as shortwave infrared satellite fixes, the center appears to be on the southwest side of a large ball of cold-topper convection.

While environmental conditions are generally not unfavorable for development, global model guidance suggests the low will move inland in no more than 24 hours. If I am correct in thinking that the system is moving even faster now (which is admittedly very hard to tell using nighttime satellite imagery), it could be as early as 18 hours. Notwithstanding, as long as the circulation remains over water, the low could become a tropical cyclone prior to landfall along the southwest coast of Mexico.

The low is rounding the western extent of the subtropical ridge, and is expected to move generally northward at about 15 to 20 mph until landfall occurs.

Regardless of whether or not this system becomes a tropical cyclone, heavy rainfall and winds to tropical storm force -- primarily in gusts -- can be anticipated in heavy squalls associated with the disturbance. These rains could cause flash flooding and mudslides in areas of mountainous terrain along southwestern Mexico. A recent ASCAT pass indicated that the low is already producing winds to near tropical storm force in the deep convection.

Probability of development in 48 hours: 50%


Jelawat is slowly weakening while heading for Okinawa. As of the latest JTWC advisory, the following information was available on the typhoon:

Wind: 145 mph, with higher gusts
Location: 22.4N 124.1E
Movement: NNE at 7 mph
Category: 4 (Saffir-Simpson Hurricane Scale)

The ragged eye has degraded significantly, and is now completely cloud filled.

Figure 4. Latest infrared satellite image of Typhoon Jelawat. Image credit: NOAA

Microwave data throughout the last 12 hours have suggested the formation of a concentric eyewall, which suggests Jelawat is experiencing its third eyewall replacement cycle in three days. This is not uncommon in strong typhoons/hurricanes. This one is fortuitously timed, as the collapse of the inner eyewall likely heralds a weaker storm for Okinawa. It will probably take the cyclone another 12-18 hours to reach the climax of this cycle. Seeing as though it is already moving over progressively cooler water as well as encountering strong upper tropospheric vertical shear, restrengthening is not anticipated subsequent to this particular cycle. However, the outer wind field may expand as a result. I still anticipate winds of 100 to 130 mph on Okinawa as the typhoon passes by. I also expect the storm to be at or near the threshold of minimal typhoon intensity as it moves across Tokyo late Sunday or early Monday local time.

The system is forecast to lose tropical characteristics at the end of the period, although it could occur sooner given the highly baroclinic environment that is typically found north of Japan.

I don't really have an issue with the track. It remains straightforward, and the models remain in good agreement. I am merely updating the previous few.

5-day intensity forecast

INITIAL 09/28 0600Z 125 KT 145 MPH
12 hour 09/28 1800Z 115 KT 135 MPH
24 hour 09/29 0600Z 105 KT 120 MPH
36 hour 09/29 1800Z 90 KT 105 MPH
48 hour 09/30 0600Z 80 KT 90 MPH
72 hour 10/01 0600Z 55 KT 65 MPH...INLAND
96 hour 10/02 0600Z 45 KT 50 MPH...OVER WATER
120 hour 10/03 0600Z 30 KT 35 MPH...POST TROPICAL/EXTRATROPICAL

5-day track forecast

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

Gulf of Mexico development possible

Not only does the GFS continue to suggest the formation of a non-tropical area of low pressure in the northwestern Gulf of Mexico, but so do the rest of the global models, as well as the regional models like the NAM. This low is forecast to form in about 48 hours from the vigorous shortwave trough moving across west Texas. Given the strong model agreement on this evolving scenario, as well as the fact that it falls within the window of short-range weather forecasting, I see little reason to doubt it.

Yesterday I noted that upper-level winds looked unfavorable for development. Upon analysis tonight, however, I have noticed that the GFS keeps the upper flow just a little bit lighter near where the surface low is purported to develop. The placement of the low will be the critical player in determining whether or nor tropical or subtropical cyclogenesis occurs. The farther offshore the low forms, the likelihood of development lessens, since it will move inland quicker (and farther west, I might add). Conversely, a system that originates farther south in the Gulf will have more time over water, and naturally will have greater potential for development.

This system could move inland anywhere from Louisiana to the western Florida panhandle. The most likely areas still seem to be southeast Louisiana or southern Mississippi to me, although there is still time to evaluate things.

Regardless of development, it appears that widespread heavy rainfall and gusty winds along the coastal waters will affect a large section of the northern Gulf Coast through the weekend into early next week. These rains could be rather prodigious at times.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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8. WunderAlertBot (Admin)
4:26 AM GMT on September 29, 2012
KoritheMan has created a new entry.
7. Grothar
3:39 AM GMT on September 29, 2012
Got all the bases covered on this one Kori. I have always admired the concise way in which you present your material. You give good samples and comparisons and explain the fine points. I would like to see you do some in depth writing of a particular subject sometime. I want to see you develop your writing ability further. A lot of talent there.
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6. KoritheMan
4:04 PM GMT on September 28, 2012
Quoting GeorgiaStormz:
If i'm reading this in the daytime, then i don't count as the dead right? ;)

Right! :D
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5. GeorgiaStormz
2:09 PM GMT on September 28, 2012
If i'm reading this in the daytime, then i don't count as the dead right? ;)
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4. StormPro
12:56 PM GMT on September 28, 2012
Nice, thanks!
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3. pcola57
10:50 AM GMT on September 28, 2012
Well done...again.. :)
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2. wxchaser97
10:11 AM GMT on September 28, 2012
Thanks for the good update Kori.
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1. indianrivguy
9:12 AM GMT on September 28, 2012
well done Kori, thanks!
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