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Tropical weather analysis - November 1, 2012

By: KoritheMan , 6:11 AM GMT on November 01, 2012


Tropical Storm Rosa continues moving across the open Pacific. As of the latest NHC, the following information was available on the storm:

Wind: 60 mph, with higher gusts
Location: 14.1°N 118.1°W
Movement: W at 2 mph
Pressure: 1000 mb

The cloud pattern associated with Rosa has changed very little, and consists of an amorphous mass of very deep convection. Satellite estimates have not appreciably changed, and the surface winds are probably still about 50 kt. Earlier microwave data suggested that the center was on the far western edge of the convection, and this signature is confirmed by analysis of nighttime satellite imagery. Upper-level outflow has become restricted to the west due to about 15 kt of westerly shear, as diagnosed by UW-CIMSS and the SHIPS model.

Figure 1. Latest infrared satellite image of Tropical Storm Rosa. Image credit: NOAA

Since the shear appears to be increasing, Rosa's brief intensification on Wednesday is likely to be its last. Beyond 48 hours, the shear is forecast to increase even further as an upper-level trough forces the westerlies to move southward toward the tropical cyclone. Ergo, the expected symptom of all this is a steady decay of the tropical storm subsequent to this point. Remnant low status is shown by day four, although it is possible that this could occur a little sooner if the shear is quick to increase. A rapid demise is not anticipated since Rosa is expected to remain over warm waters throughout the forecast period.

The track forecast remains tricky, owing to the inherent complexity of the synoptic pattern over the Pacific. While the models are in relatively good agreement on the first 48 hours of the track, there continues to be significant diversity after that. After reviewing the National Hurricane Center forecast discussion for the storm yesterday (Wednesday), and carefully analyzing water vapor imagery and the 500 mb forecast fields of the global models, it appears that the primary reason for the disagreement is that they seemingly cannot agree on the evolution and amplitude of a deep-layer trough developing off the western United States. The models that show recurvature show a more negatively-tilted trough, similar to what we saw with Sandy. Conversely, the models that show a more positively-tilted regime suggest a more zonal flow to the north of Rosa that is more apt to shunt the system westward, well south of the mean mid-latitude westerly flow. Since water vapor images show a trough that is more on the positively-tilted spectrum, my forecast track continues to be south of the GFS/GFDL/HWRF trio, and closer to the ECMWF, which appears to be handling the evolution of the synoptic pattern better.

My forecast is a little to the right of the NHC forecast track in the latter portion of the forecast period, in anticipation that the trough will cause at least some poleward bend, especially given the sizable weakness that exists north of the storm.

5-day intensity forecast

INITIAL 11/01 0300Z 50 KT 60 MPH
12 hour 11/01 1200Z 45 KT 50 MPH
24 hour 11/02 0000Z 45 KT 50 MPH
36 hour 11/02 1200Z 40 KT 45 MPH
48 hour 11/03 0000Z 35 KT 40 MPH
72 hour 11/04 0000Z 30 KT 35 MPH
96 hour 11/05 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
120 hour 11/06 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW

5-day track forecast

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

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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