Tropical weather analysis - September 19, 2012
Nadine has weakened a little more this evening. As of the latest NHC advisory, the following information was available on the storm:
Wind: 50 mph, with higher gusts
Location: 36.2°N 32.1°W
Movement: NNE at 7 mph
Pressure: 993 mb
Nadine continues to lose organization. While no recent microwave passes are available, the satellite signature is starting to become reminiscent of a frontal low, although cyclone phase diagrams from FSU still showed at least a weak warm core in association with the system at 12 and 18z, respectively. While the satellite signature has continued to deterioriate since then, it takes time for extratropical transition, and Nadine probably still retains at least a weak mid- to upper-level warm core.
Figure 1. Latest infrared satellite image of Tropical Storm Nadine. Image credit: NOAA
Significant vertical shear and cool sea surface temperatures should continue throughout the next five days. While these factors would normally argue for significant weakening, Nadine is obviously not very tropical, and is deriving at least some of its energy from temperature differentials. This should preclude significant weakening. Indeed, the majority of the guidance continues to suggest little change in strength, but Nadine has continued to weaken over the last 24 hours. However, with a major storm system now approaching the storm from the west, it seems likely that the cyclone will garner a little bit of baroclinic energy, which should act to keep the winds up, or at least not allow them to experience much additional dwindling. The future evolution -- structurally -- of Nadine, remains highly speculative. I see three possibilities: number one is for the cyclone to continue to lose central convection and degenerate into a remnant low pressure area; this seems a bit unlikely to me given the frontal appearance of the convection on satellite imagery. The second is for Nadine to transition to an extratropical cyclone over the next day or two, which is supported by the current structure. Finally, another potential event is for the cyclone to become absorbed by the north Atlantic low pressure system. This has been inconsistently suggested by some of the global models over the last few days. I tend to favor the middle option.
The steering currents around Nadine are beginning to weaken as the large scale flow to the north of the tropical cyclone becomes more zonal. A large upper low centered between the southern tip of Greenland and Newfoundland continues moving southeastward well to the north of Nadine. This evolution will drive the aforementioned frontal boundary eastward, but the amplitude of the front and the zonal flow over that portion of the north Atlantic suggests that this front will be insufficient to fully pick up Nadine. The storm is forecast to decelerate over the next day or two until the upper low drops southward as suggested by the global models. Beyond 24 hours, the approach of the low is forecast to turn Nadine southeastward. The global models have come into much better agreement on the future evolution of Nadine over the last several cycles, although the ECMWF remains an outlier at 12z in calling for a clockwise loop around the subtropical ridge. However, I recall the GFS solidly outperforming that particular model in the case of Isaac, when the latter stubbornly insisted on a hit along the Alabama coast even as the system was approaching Louisiana.
Despite the eastward shift, there continues to be some rather large disagreement pertaining to the details of that turn, as well as its duration. The GFS, after showing a much faster northeastward motion at 18z, has come in significantly slower at 0z, and keeps Nadine meanding between the Azores and the Canary Islands.
Interests in the Azores, particularly the northwestern section, should monitor the progress of Nadine over the next couple of days. Tropical storm force winds, especially in areas of higher elevation, are possible. A tropical storm warning has been issued for the archipelago (see below).
Watches and warnings
A TROPICAL STORM WARNING IS IN EFFECT FOR...
* THE AZORES ISLANDS OF FLORES...CORVO...FAIAL...PICO...SAO JORGE...
A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.
FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.
5-day intensity forecast
INITIAL 09/19 0600Z 45 KT 50 MPH
12 hour 09/19 1800Z 45 KT 50 MPH
24 hour 09/20 0600Z 45 KT 50 MPH
36 hour 09/20 1800Z 45 KT 50 MPH
48 hour 09/21 0600Z 45 KT 50 MPH...EXTRATROPICAL
72 hour 09/22 0600Z 45 KT 50 MPH...EXTRATROPICAL
96 hour 09/23 0600Z 45 KT 50 MPH...EXTRATROPICAL
120 hour 09/24 0600Z 45 KT 50 MPH...EXTRATROPICAL
5-day track forecast
Figure 2. My 5-day forecast track for Nadine.
Central Atlantic low
A non-tropical area of low pressure has developed about 900 miles east of Bermuda. Satellite imagery suggests a well-defined surface circulation, and upper-level winds appear conducive for some development -- tropical or subtropical -- of this disturbance over the next couple of days. While a westward motion is anticipated in the interim, the models suggest recurvature ahead of a shortwave trough moving from North America by day three.
Probability of development in 48 hours: 20%
Lane has weakened to a tropical storm as of the latest NHC advisory, which is as follows:
Wind: 40 mph, with higher gusts
Location: 20.8°N 130.2°W
Movement: WNW at 9 mph
Pressure: 1006 mb
There is no deep convection associated with the circulation of Lane, which remains quite vigorous. There is only a small shower located about 150 miles northeast of the center. This suggests that Lane is quickly becoming a remnant low as it succumbs to southwesterly shear and cold waters. In fact, the cyclone may actually be a remnant low as of this writing.
Figure 3. Latest infrared satellite image of Tropical Storm Lane. Image credit: NOAA
Regardless of the technical status of the system, the low should dissipate in a few days and move westward in the low-level flow. A more definitive turn to the west should begin shortly as Lane has lost convection.
The track and intensity forecast philosophies remain unchanged. My track is similar to the National Hurricane Center 11:00 PM one, albeit faster and farther south based on the current motion relative to the 1200Z forecast point.
5-day intensity forecast
INITIAL 09/19 0600Z 35 KT 40 MPH
12 hour 09/19 1800Z 30 KT 35 MPH...POST-TROPICAL/REMNANT LOW
24 hour 09/20 0600Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
36 hour 09/20 1800Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
48 hour 09/21 0600Z 15 KT 15 MPH...POST-TROPICAL/REMNANT LOW
72 hour 09/22 0600Z...DISSIPATED
5-day track forecast
Figure 4. My 5-day forecast track for Lane.