Tropical weather analysis - September 19, 2012
Nadine continues meandering near the Azores. As of the latest NHC advisory, the following was posted on the system:
Wind: 50 mph, with higher gusts
Location: 37.1°N 31.4°W
Movement: ENE at 3 mph
Pressure: 990 mb
A band of deep convection has developed in the northwest quadrant a little closer to the low-level center. A glance at buoy and ship reports in this area suggest that Nadine will encounter SSTs of 24-25C as it moves southeastward, which is a little warmer than what it has been experiencing. This could be why convection appears to have developed closer to the low-level center this evening.
Figure 1. Latest infrared satellite image of Tropical Storm Nadine. Image credit: NOAA
Water vapor imagery suggests that an upper low is orbiting around the south side of the low-level center. Depending on how close the low stays relative to the cyclone center, this may not allow for much in the way of truly tropical organization from Nadine despite the system moving into warmer waters. If the upper low moves directly above the center, however, Nadine will find itself in an environment of minimal shear. In combination with the relatively warm underlying waters, that could allow for a steep temperature gradient, enabling Nadine's low-level warm core to gradually warm the upper troposphere, eliminate the upper low, and finally maintain some persistent convection. A similar evolution occurred with Tropical Storm Beryl earlier in the year as it was approaching the southeast United States coast.
There is simply too much uncertainty in the forecast evolution of Nadine and the upper low to say definitively what the technical status of the tropical cyclone will be. It's possible that Nadine will lose convection and degenerate into a remnant low, get enough baroclinic forcing to become an extratropical cyclone, or become absorbed by the large frontal system over the north Atlantic. While all three scenarios remain possible, I still favor the middle option. One other option, as alluded to above, is for Nadine to maintain tropical characteristics throughout the next several days. This could occur with slow movement over warm water, especially if the current convective signature persists or expands to other parts of the circulation. It is virtually impossible to tell which one of them will verify at this point.
The upper trough north of Nadine has reached its point of closest to the tropical cyclone. With the flow becoming more zonal in that direction, Nadine is expected to begin moving southeastward between a strengthening mid-level ridge over the central Atlantic and the northerly flow on the back side of the north Atlantic upper low. The models still disagree substantially on the future trajectory of Nadine, but it is mostly after 48 hours, as they have come into better agreement that Nadine will move southeastward at a faster forward speed before 72 hours. My forecast will go along with this, but given the continued inherent uncertainty beyond that time, I am slower than the National Hurricane Center. This continues to be a low-confidence forecast.
Beyond 48 hours, the upper low is forecast to be just north of Nadine. This will be the make it or break it pitch. Will Nadine throw a curveball, avoid the low and move southwestward around the subtropical ridge, or will it throw a fastball and move quickly northeastward with the low? Unfortunately, there is no reason to favor either scenario over the other at this time.
Interests in the Azores should continue following Nadine's progress.
5-day intensity forecast
INITIAL 09/20 0300Z 45 KT 50 MPH
12 hour 09/20 1200Z 45 KT 50 MPH
24 hour 09/21 0000Z 45 KT 50 MPH
36 hour 09/21 1200Z 45 KT 50 MPH...EXTRATROPICAL
48 hour 09/22 0000Z 45 KT 50 MPH...EXTRATROPICAL
72 hour 09/23 0000Z 45 KT 50 MPH...EXTRATROPICAL
96 hour 09/24 0000Z 45 KT 50 MPH...EXTRATROPICAL
120 hour 09/25 0000Z 45 KT 50 MPH...EXTRATROPICAL
5-day track forecast
Figure 2. My 5-day forecast track for Nadine.
Watches and warnings
A TROPICAL STORM WARNING IS IN EFFECT FOR...
* THE AZORES ISLANDS OF FLORES...CORVO...FAIAL...PICO...SAO JORGE...
GRACIOSA...TERCEIRA...SAO MIGUEL...AND SANTA MARIA.
FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.
A non-tropical area of low pressure over the central Atlantic about 700 miles east-southeast of Bermuda ("94L") is showing signs of organization. Low cloud lines on satellite imagery indicates that the surface wind field associated with this low is not as broad as yesterday. In addition, shower activity is wrapping closer to the center.
Figure 3. Latest infrared satellite image of Invest 94L. Image credit: NOAA
Water vapor imagery and UW-CIMSS 200 mb vorticity data suggests that an upper low is entangled with 94L, pretty much over the low-level center. The GFS suggests that this upper low will more or less move in tandem with 94L for the next three days before becoming dismantled thereafter. By that time, however, the system is forecast to enter a weakness in the subtropical ridge and recurve into cooler waters. At this point, any organization would probably be subtropical, although I cannot rule out this system briefly becoming a tropical storm. A subtropical or tropical storm could form on Thursday, but the system would probably not get much stronger than 50 kt.
Interests in Atlantic Canada should monitor the progress of this system over the next few days.
Probability of development in 48 hours: 50%
A broad area of low pressure centered about 350 miles south-southeast of Acapulco ("93E" is producing disorganized showers and thunderstorms. Based on analysis of satellite imagery this evening, the center is devoid of deep convection. Recent microwave data suggested that the center was not at all well-defined, which is confirmed by low cloud movements on satellite photos as well.
Figure 4. Lastest infrared satellite image of Invest 93E. Image credit: NOAA
Upper-level winds are forecast to be of at least a diffluent nature, which should allow for steady development, particularly if the system can get convection. This system does not pose a threat to any landmasses.
Probability of development in 48 hours: 40%