Tropical weather analysis - September 12, 2012
Tropical Storm Nadine formed this evening over the central Atlantic. As of the most recent NHC advisory, the following information was available on the tropical storm:
Wind: 40 mph, with higher gusts
Location: 17.8°N 45.2°W
Movement: WNW at 15 mph
Pressure: 1004 mb
Satellite images show an improvement in the cloud pattern of the tropical cyclone. Central convection has increased significantly, and convective bands are noted to the north and south of the center, respectively. Upper-level outflow is well-defined in all quadrants except the east.
Figure 1. Latest infrared satellite image of Tropical Storm Nadine. Image credit: NOAA
I see no obvious reason why Nadine should not continue to strengthen. The shear is low, the water will be getting progressively warmer, and water vapor imagery does not currently show much dry air affecting the cyclone. Indeed, the SHIPS rapid intensification parameter at 0z shows a 44% chance of a 25 kt increase in the next 24 hours. Given the much improved convective structure, vertical structure, and outflow pattern, this could happen. However, tropical cyclones do not always rapidly intensify in seemingly favorable environments. In fact, they sometimes don't intensify at all. And since there is little skill in predicting these sort of events, I will not explicitly indicate rapid intensification. But again, given the environment, it's certainly possible, especially given the rapidity in which the central core has organized over the last several hours. I'll admit my intensity forecast could be conserative, and is likely too low if current trends continue.
By Friday, the GFS and SHIPS suggest an increase in westerly shear as an upper-level trough amplifies to the west of the tropical cyclone. This should cause Nadine to weaken at that time. There are indications in the GFS vertical shear fields that the upper flow could gradually become more favorable after that time as Nadine moves over high latitudes. This could allow for some reintensification beyond day five, although this is uncertain since the cold front that recurved Leslie and Michael was quite strong, and strong subsident flow is noted behind it. This could catch up with Nadine at longer ranges.
Recent satellite fixes indicate that Nadine might have turned back to the northwest. This is consistent with the global and dynamical models, which move Nadine through a weakness in the subtropical ridge. In about three days, an upper low is forecast to develop over the central Atlantic near Bermuda, which should reinforce the weakness and move Nadine out to sea. After 72 hours, there are large disagreements amongst the various model members as to the extent in which Nadine interacts with the low. The GFS, GFDL, and HWRF are faster, while the ECMWF and CMC are slower. Given the splendid performance of the GFS this year, as well as the fact that the synoptic pattern is generally less variable over the open Atlantic than closer to the United States (as we saw with Isaac), I am going to side with the faster solution.
5-day intensity forecast
INITIAL 09/12 0300Z 35 KT 40 MPH
12 hour 09/12 1200Z 40 KT 45 MPH
24 hour 09/13 0000Z 50 KT 60 MPH
36 hour 09/13 1200Z 60 KT 70 MPH
48 hour 09/14 0000Z 70 KT 80 MPH
72 hour 09/15 0000Z 75 KT 85 MPH
96 hour 09/16 0000Z 70 KT 80 MPH
120 hour 09/17 0000Z 65 KT 75 MPH
5-day track forecast
Figure 2. My 5-day forecast track for Nadine.
A well-defined area of low pressure in the eastern Pacific located several hundred miles south of Acapulco continue to show signs of organization. A ball of convection encompasses the center, but low cloud lines suggest that the central gyre remains displaced a little to the east of the deepest thunderstorms, possibly due to a little bit of easterly shear, as evidenced by the squashed outflow pattern in the eastern semicircle.
Figure 3. Latest infrared satellite image of Invest 90E. Image credit: NOAA
Only a slight increase in organization would result in the formation of a tropical depression.
Upper-level winds are expected to remain favorable for the next three days (although the GFS suggests that might not quite be anticyclonic). After that, the system is forecast to cross the 26C isotherm, encounter drier air, and come under increasing northwesterly shear due to a strong upper-level high over the central Pacific. This should induce weakening beyond that time.
The system is expected to continue moving generally toward the west-northwest. A turn to the northwest is expected in about three to four days as a weak upper trough develops over California. While the GFDL suggests a threat to Baja, this solution has been discontinued since that model is an outlier, and tends to perform poorly with eastern Pacific tropical cyclones.
Probability of development in 48 hours: 90%
Elsewhere in the Atlantic, the ECMWF has continued to suggest the possibility of a broad area of low pressure forming in the western or central Gulf of Mexico in about three to four days. I am unsure what this is supposed to come from -- there are no tropical waves apparent on satellite imagery, or the National Hurricane Center surface map. Looking at the 850 mb vorticity fields in that model, it seems to come from the cold front now moving across the upper midwest. The parent upper low is centered over Manitoba, and this trough appears quite vigorous. In fact, it may be trying to take on a negative (northeast-southwest) tilt. This suggests that the Euro's prognostication of homegrown development may not be too far off the mark. However, it lacks support from the GFS, and the other models have been scanty with it at best.
The synoptic pattern favors a threat to the northern Gulf Coast with any system that does develop. The upper tropospheric flow on the GFS does not look especially favorable, and this system will likely be a sheared one if it does develop, with the associated precipitation shield well east or northeast of the center.