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Tropical weather analysis - September 26, 2012

By: KoritheMan , 5:08 AM GMT on September 26, 2012


Tropical Storm Nadine continues to be persistent. As of the latest NHC advisory, the following information was available on the storm:

Wind: 45 mph, with higher gusts
Location: 31.2°N 30.4°W
Movement: SSW at 5 mph
Pressure: 997 mb

An eye-like feature was evident at the 37 GhZ channel in an SSMIS overpass taken at 2140Z, surrounded by a ring of convection. While Nadine certainly retains a well-defined circulation, I question how much of this is actually an eye, or dry air. My hunch is that an eye did legitimately try and develop earlier, but has since been overtaken by the strong subsident flow that prevails in this part of the Atlantic.

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

Since there appears to be little vertical shear over Nadine, the primary reason why the storm has not strengthened is no doubt the very dry airmass in which it is embedded. The SHIPS analyzes the 700-500 mb ambient relative humidity values at a mere 33% as of 0z. This is not a favorable pattern for a tropical cyclone. While the warmer sea surface temperatures that Nadine is expected to move into could mitigate this factor, the overall effect should be negligible at best.

Nadine's future doesn't look too bright. Southwesterly shear is forecast to increase sharply in about 48 hours as the cyclone approaches an upper level low/trough zone over the central Atlantic. If Nadine is going to strengthen, it has only a little over a day to do so. I am not particularly optimistic. I am a little below the intensity consensus. An alternate scenario is that Nadine doesn't strengthen at all.

Recent satellite fixes indicate that the tropical storm is gradually making the long anticipated southward turn as an upper trough over the western Atlantic shunts the subtropical ridge eastward and northerly mid-level flow increases over the storm. A turn toward the west is forecast over the next couple of days, followed by a turn to the northwest on day three as Nadine comes under increasing southerly flow associated with the western Atlantic trough. The models, while still a little divergent, seem to be in better agreement than they were yesterday, although there is still a fairly sizable spread amongst the 18z GFS ensemble members.

My forecast track is close to the National Hurricane Center.

5-day intensity forecast

INITIAL 09/26 0300Z 40 KT 45 MPH
12 hour 09/26 1200Z 40 KT 45 MPH
24 hour 09/27 0000Z 45 KT 50 MPH
36 hour 09/27 1200Z 50 KT 60 MPH
48 hour 09/28 0000Z 50 KT 60 MPH
72 hour 09/29 0000Z 50 KT 60 MPH
96 hour 09/30 0000Z 45 KT 50 MPH
120 hour 10/01 0000Z 40 KT 45 MPH

5-day track forecast

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


Hurricane Miriam continues to steadily weaken. As of the most recent NHC advisory, the following information was available on the storm:

Wind: 80 mph, with higher gusts
Location: 19.1°N 115.2°W
Movement: NW at 5 mph
Pressure: 983 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Miriam is beginning to shear apart. While upper-level outflow remains fairly well-defined, the cloud pattern suggests shear. This shear is coming from the southwest and appears to be confined beneath the outflow layer judging by the convective pattern. Also, recent microwave fixes suggest that the low- and mid-level circulations have started to decouple.

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

Water vapor imagery shows a sharp upper trough axis over the central Pacific. The southwesterly flow on the east side of this feature is forecast to maintain some semblance of vertical shear over the hurricane. In about 72 hours, a vigorous upper low currently centered near 33N 144W is forecast by the global models to begin approaching Miriam from the west. The net result will be an abrupt and rather gruesome increase in westerly shear. That along with cool ocean temperatures is expected to significantly disrupt the hurricane; so much so that I am now expecting dissipation in 72 hours, which is in sharp contrast to yesterday. It could certainly occur sooner.

Miriam is moving through a substantial break in the Pacific subtropical high. This is being generated by a large trough over the western United States. As the central Pacific upper low pivots eastward over the next few days, this weakness is expected to become reinforced. The global models remain in excellent agreement on the track, which has greater certainty than normal. Given the increasingly shallow depth of the cyclone circulation, a significant poleward bend now seems unlikely. Instead, Miriam is expected to move pretty much uniformly northwestward in the lower tropospheric flow.

While Miriam is forecast to dissipate before reaching Baja, as the vortex shears apart in the vertical, the 500 mb circulation should accelerate northeastward ahead of the decadent low-level center under the influence of the upper low/trough and bring rain to portions of the southwestern United States this weekend into early next week.

5-day intensity forecast

INITIAL 09/26 0300Z 70 KT 80 MPH
12 hour 09/26 1200Z 60 KT 70 MPH
24 hour 09/27 0000Z 50 KT 60 MPH
36 hour 09/27 1200Z 40 KT 45 MPH
48 hour 09/28 0000Z 30 KT 35 MPH
72 hour 09/29 0000Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
96 hour 09/30 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
120 hour 10/01...DISSIPATED

5-day track forecast

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


Jelawat has weakened a bit this evening, but remains a rather formidable typhoon. As of the latest JTWC advisory, the following information was posted on the storm:

Wind: 155 mph, with higher gusts
Location: 17.3N 126.6E
Movement: NW at 3 mph
Category: 4 (Saffir-Simpson Hurricane Scale)

The typhoon has jogged a little to the right of the 0300Z JTWC forecast track. Smoothing out the fits and wobbles yields a northwesterly motion as advertised in the advisory. Also, the storm appears to have accelerated a bit.

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

A slew of microwave fixes suggest that Jelawat may be undergoing another concentric eyewall cycle, as a large outer band was becoming prominent in the data, while the inner eyewall -- especially to the north -- was decaying. This is somewhat supported by conventional satellite fixes as well, which clearly imply a weakening of the inner eyewall, once again most recognizable to the north.

I see a flattening of the outflow on the northern side by looking at water vapor imagery. This is likely due to an upper cold low that has formed to the northwest of Tropical Storm Ewiniar. As this low moves southward, it will likely begin affecting Jelawat with southwest to westerly shear. Steadily decreasing sea surface temperatures should also help to initiate weakening. However, Jelawat is expected to remain a powerful typhoon as it bears down on Okinawa this weekend.

The synoptic situation over the western Pacific seems a little different tonight. Rather than a trough I now see a ridge over China. This is confirmed by analyses from UW-CIMSS and water vapor imagery, which show a mostly zonal flow over the country. There is very clearly some ridging going on upstream of the typhoon, which wasn't expected yesterday. Because of this, I have shifted my track a bit to the left for the next 48 hours, but the storm is still expected to remain well offshore from Taiwan. It should be noted that if the ridging does not give way as predicted, the eye could come a little closer to -- but still safely offshore -- the coast of Taiwan. Locally heavy rains and gusty winds in showery bands are still possible there, although they will be on the weaker side. Okinawa will face much harsher conditions, with wind gusts anywhere from 100 to 140 mph as Jelawat accelerates.

Subsequent to passing Okinawa, the storm is forecast to become fully embedded within the mid-latitude southwesterly flow ahead of a developing trough. This pattern favors a landfall in southern Japan, possibly near Tokyo, Monday morning local time. Interests there should begin monitoring the progress of Jelawat at this time.

5-day intensity forecast

INITIAL 09/26 0300Z 135 KT 155 MPH
12 hour 09/26 1200Z 130 KT 150 MPH
24 hour 09/27 0000Z 125 KT 145 MPH
36 hour 09/27 1200Z 115 KT 135 MPH
48 hour 09/28 0000Z 100 KT 115 MPH
72 hour 09/29 0000Z 90 KT 105 MPH
96 hour 09/30 0000Z 75 KT 85 MPH
120 hour 10/01 0000Z 60 KT 70 MPH...APPROACHING SOUTHERN JAPAN

5-day track forecast

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

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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2. WunderAlertBot (Admin)
5:10 AM GMT on September 27, 2012
KoritheMan has created a new entry.
1. LynnMelton
11:22 AM GMT on September 26, 2012
I am not a meteorologist but a hobbyist and learning as much as possible. I just wanted to let you know how much I enjoy your blog and leaarn daily. Thank you for your efforts.
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