Tropical weather analysis - November 2, 2012

By: KoritheMan , 5:52 AM GMT on November 02, 2012


Tropical Storm Rosa continues its crawl over the open Eastern Pacific. As of the latest NHC advisory, the following was available on the cyclone:

Wind: 50 mph, with higher gusts
Location: 13.7°N 118.4°W
Movement: Stationary
Pressure: 1003 mb

The satellite signature of the tropical cyclone has changed little over the past few hours. The central convection continues to pulsate, but remains quite cold.

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

Analysis of water vapor imagery suggests that the core of the westerlies remain to the north of Rosa, which is likely the reason the cyclone has not weakened as quickly as original prognostications indicated. In fact, the outflow pattern looks better defined than earlier. Since these unfavorable upper-level winds appear to be sagging only very slowly southward, and since Rosa has been losing latitude, an abrupt weakening is not anticipated in the short-term. In about 48 hours, the GFS and SHIPS still insist on sharply increasing the shear over Rosa as the cyclone begins to gain some latitude. Since the synoptic pattern does appear consistent with a slow increase in latitude by this time, and the fact that there is evidence of the shear moving slowly southward, this seems like a reasonable expectation. A quicker rate of weakening is anticipated beyond 48 hours, and Rosa is forecast to degenerate into a remnant low by 72 hours, although it is likely to maintain its identity throughout the forecast period. It should be noted that the SHIPS shows Rosa approaching cooler waters Tuesday evening, so termination of the system at that time may need to be considered in my next forecast.

Rosa remains entrenched within a weak steering regime between a large anticyclone over the central Pacific and weak cyclonic flow to the north. Recent satellite fixes and comparison of evening microwave data suggest that little movement has occurred. 0z upper air data and water vapor images suggests that a well-established weakness has begun to manifest across southern California and the desert southwest as a mid- to upper-level shortwave trough amplifies across that region. This trough is forecast to dig southward according to the global models, but they realize that it will be a slow process. Thus, Rosa's latitude gain will not be met without opposition; the central Pacific high appears to be the dominant steering mechanism at this time, and although that's not saying much, it is expected that this will prove sufficient to keep Rosa in a very weak steering environment, with perhaps a slow southwestward drift. This is consistent with a blend of the GFS and ECMWF, and in fact, the forecast track as a whole is pretty close to this.

5-day intensity forecast

INITIAL 11/02 0300Z 45 KT 50 MPH
12 hour 11/02 1200Z 45 KT 50 MPH
24 hour 11/03 0000Z 40 KT 45 MPH
36 hour 11/03 1200Z 40 KT 45 MPH
48 hour 11/04 0000Z 35 KT 40 MPH
72 hour 11/05 0000Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
96 hour 11/06 0000Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
120 hour 11/07 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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