Tropical weather analysis - September 27, 2012

By: KoritheMan , 5:10 AM GMT on September 27, 2012

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Tropical Storm Nadine continues to slowly move across the eastern Atlantic. As of the latest NHC advisory, the following information was available on the storm:

Wind: 50 mph, with higher gusts
Location: 29.5°N 31.4°W
Movement: SW at 6 mph
Pressure: 993 mb

Nadine has become a little better organized, and the winds have come up today. Convection is limited to a curved band that wraps around the eastern semicircle.

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

Having said that, conditions are still not ideal for strengthening. The cyclone continues to be surrounded by a very dry airmass, as evidenced by an ambient stratocumulus cloud deck. How long will Nadine linger around? Will it live to see the light of Christmas Day, and then abruptly die as a nice Christmas gift for all to see? If enough children write Santa, it just might happen.

In about 48 hours, an increase in southwesterly shear is forecast as Nadine approaches an upper-level trough. Beyond that time, cooler waters, shear, and dry air will hopefully take their toll on this tropical cyclone, as I am tired of forecasting it. There is some indication that the shear could decrease a little at day five as suggested by the GFS, but there is too much uncertainty in long-range vertical shear forecasts to be particularly confident of this.

Following the expected motif, the future track of Nadine has become uncertain once again. After a little bit of mutuality yesterday, the models have become significantly divergent after 96 hours. The ECMWF suggests that Nadine will move southwestward under a building ridge over the central Atlantic. The CMC shows little motion, while the HWRF shows Nadine getting pushed back to the south. Finally, the GFS and GFDL indicate a slow eastward motion as the cyclone remains embedded in broad westerly mid-latitude flow. Since meteorologically -- my personal longings for the demise of this tropical cyclone notwithstanding -- there is little reason to favor one solution over the other, all I can do is show a quasi-stationary storm at the end of the period. This remains a low confidence forecast.

5-day intensity forecast

INITAL 09/27 0300Z 45 KT 50 MPH
12 hour 09/27 1200Z 45 KT 50 MPH
24 hour 09/28 0000Z 50 KT 60 MPH
36 hour 09/28 1200Z 50 KT 60 MPH
48 hour 09/29 0000Z 50 KT 60 MPH
72 hour 09/30 0000Z 45 KT 50 MPH
96 hour 10/01 0000Z 40 KT 45 MPH
120 hour 10/02 0000Z 40 KT 45 MPH

5-day track forecast

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


Miriam continues to weaken as of the most recent NHC advisory:

Wind: 45 mph, with higher gusts
Location: 21.5°N 115.7°W
Movement: NNW at 7 mph
Pressure: 999 mb

What was once a powerful hurricane seems to be quickly decaying. The low-level center is completely exposed, but ill-defined shower activity continues well north of the center, most likely associated with the mid-level remnants of the storm, rather than the actual low-level circulation.

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

There are probably still some tropical storm force winds occurring within the circulation though, as per Dvorak constraints. It generally takes a while for the surface winds to match the satellite signature of a tropical cyclone in its decaying stage. Oftentimes the circulation itself can help to maintain tropical storm force winds long after the absence of deep convection. This is why the National Hurricane Center continuously follows what seem to be naked swirls.

With no respite from the vertical shear anticipated for the next few days, the only future ahead of Miriam appears to be a short one. Dissipation is expected in about 36 hours, although it could obviously occur a little sooner, particularly if the core of strongest shear moves directly over the cyclone. Although the National Hurricane Center official forecast shows Miriam surviving as a distinct entity through 96 hours, I have serious doubts about this given the magnitude of shear forecast by the GFS/SHIPS. A forecast track out to 72 hours seems reasonable, so I'll go with that.

Water vapor imagery shows the upper trough slowly moving through the four corners region. In the wake of this trough, a building mid-level ridge is noted upstream from Tropical Storm Miriam. While another trough is seen amplifying offshore British Columbia, Miriam is expected to be too shallow to feel any significant poleward influence from this trough. This in good agreement with the global models, which show Miriam moving westward in the low-level flow south of a building ridge. The track forecast is relatively straightforward, unlike with Nadine.

If the remnants survive beyond three days, the models show them turning southwest. While this would take them into warmer water, strong vertical shear is expected to preclude regeneration.

Remnant high-level moisture is still expected to overspread portions of the southwestern United States through the weekend.

5-day intensity forecast

INITIAL 09/27 0300Z 40 KT 45 MPH
12 hour 09/27 1200Z 35 KT 40 MPH
24 hour 09/28 0000Z 30 KT 35 MPH
36 hour 09/28 1200Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
48 hour 09/29 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
72 hour 09/30 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
96 hour 10/01 0000Z...DISSIPATED

5-day track forecast

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

Invest 94E

An area of disturbed weather and associated surface low is located about 350 miles south of Manzanillo. Unfortunately, a 0230 UTC SSMI overpass missed the center of the disturbance, but satellite images suggest that the low is becoming better organized.

Figure 5. Latest infrared satellite image of Invest 94E. Image credit: NOAA

Based on shortwave infrared imagery, the center appears to be just north of a ball of very cold convection. However, there is little evidence of curved banding at this time. The northerly shear that had been afflicting the system appears to be gradually relaxing as the large anticyclone over the Gulf of Mexico moves eastward.

I expect this low to move northwestward between the southerly flow associated with Miriam and the trough over the central United States. This system poses a threat to Baja over the next 4-5 days, and interests there should monitor it.

The SHIPS suggests the system will run into cool waters in about 48 hours, but on my projected path, this seems a little dubious. Upper-level winds are forecast to remain light throughout the next several days, and may even attempt to become anticyclonic at times.

This system could become a tropical depression during the next couple of days.

Probability of development in 48 hours: 60%


Jelawat remains a super typhoon, but continues to weaken as of the latest JTWC advisory:

Wind: 150 mph, with higher gusts
Location: 19.7N 124.7E
Movement: NW at 9 mph
Category: 4 (Saffir-Simpson Hurricane Scale)

There have been no significant change to the cloud pattern this evening. Jelawat remains a large and extremely well-organized typhoon, with a large eye amidst a vigorous central dense overcast that is well-encompassing. In addition, a large curved band exists north of the center.

Figure 6. Latest infrared satellite image of Super Typhoon Jelawat. Image credit: NOAA

While upper-level winds should soon increase over the typhoon, they clearly haven't just yet. In fact, the storm has become even more organized compared to six hours ago. While I doubt this will translate to a sudden spike in the winds, it should at least prevent any significant weakening for the next 24 hours. Beyond that point, increasing southwesterly shear -- already evident on water vapor imagery just north of Taiwan should begin affecting the system. In combination with steadily decreasing sea surface temperatures and underlying oceanic heat content, the net should result be weakening, with more rapid weakening anticipated beyond 48 hours. However, Jelawat is expected to remain a formidable typhoon throughout the duration of its passage over Okinawa. A weaker storm is expected to impact southern Japan.

The track forecast remains straightforward. Water vapor imagery and UW-CIMSS data suggest that Jelawat is moving through a well-established weakness in the subtropical ridge caused by a combination of a shortwave trough developing between China and South Korea and accelerating Tropical Storm Ewiniar. Consequently, I expect the typhoon to turn northward over the next 24 hours, followed by recurvature toward the northeast as it moves closer to the trough axis and comes under increasing southwesterly flow. After some disagreement, the computer models have come into better agreement on the future track of the typhoon, with a path through Okinawa and into southern Japan early next week. Interests in those areas should carefully monitor the progress of this typhoon. Winds could be tremendously high on Okinawa.

Rainfall and storm surge potential with this system will be limited due to the fast forward motion of the system, which is only forecast to get faster throughout the next five days. My forecast track has not changed much, and represents an update of the previous one.

5-day intensity forecast

INITIAL 09/27 0300Z 130 KT 150 MPH
12 hour 09/27 1200Z 130 KT 150 MPH
24 hour 09/28 0000Z 125 KT 145 MPH
36 hour 09/28 1200Z 115 KT 135 MPH
48 hour 09/29 0000Z 100 KT 115 MPH
72 hour 09/30 0000Z 70 KT 80 MPH
96 hour 10/01 0000Z 55 KT 65 MPH...ON THE COAST OF SOUTHERN JAPAN
120 hour 10/02 0000Z 40 KT 45 MPH...OVER WATER

5-day track forecast

Figure 7. My 5-day forecast track for Jelawat.

Gulf of Mexico development possible

Over the last several days, the GFS has been consistent in developing some sort of lower tropospheric vorticity maximum/area of low pressure in the western Gulf of Mexico in about 3 to 5 days. The cold front now moving into the Rockies appears to be what generates this. Any system here would likely be more baroclinic than tropical given that the subtropical jet has made its seasonal migration southward.

Synoptic steering favors any system getting drawn northeast and hitting areas between Louisiana and the western Florida panhandle. Given where the GFS has been insisting that the low will develop, Louisiana or Mississippi seems like the best bet at the moment. I want to reiterate that I do not anticipate a strong system. In fact, should this system actually become extant at any point over the next five days, I would not expect much in the way of tropical development either.

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3. WunderAlertBot (Admin)
7:29 AM GMT on September 28, 2012
KoritheMan has created a new entry.
2. wxchaser97
10:42 AM GMT on September 27, 2012
Thanks for the blog Kori.
Member Since: March 16, 2012 Posts: 127 Comments: 7948
1. stormwatcherCI
10:41 AM GMT on September 27, 2012
Excellent post as usual Kori.
Member Since: October 9, 2008 Posts: 0 Comments: 8392

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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.