Tropical Storm Iba Discussion Number 8
Wiki-Hurricanes Forecast Center
12:00 PM AST Mon Mar 25 2019
Iba continues to feel the effects of strong northwesterly winds this morning as the tropical storm's central convection has struggled to spread over the low-level center of circulation, elaving it exposed to the northwest. The somewhat unfavorable upper-level winds are being induced by both the subtropical jet to Iba's south and a local vorticity maximum in the broader trough to Iba's north that is both robbing Iba of some of its convergence and producing counteractive outflow aloft. Still, Iba is producing deep convection with some cloud tops cooler than -80C, which has continued on the heels of the overnight convective growth. Both SAB and independent Dvorak analyses have yielded a T3.0/45 kt intensity estimate, but the relative lack of improvement since the last advisory and the rather unconvincing scatterometer data overnight suggest fixing Iba's at 40 kt.
Iba is drifting slowly towards the south-southwest, tracing out the periphery of a distant 588 dm subtropical ridge over the central South Atlantic. While Iba is rather removed from the polar steering current, a shortwave and longwave trough cresting in phase later this week should be enough to begin pulling Iba towards the southeast. The GFS and ECMWF have swapped roles in the latest guidance suite, with the GFS depicting the storm filling aggressively and thus recurivng north Friday while the ECMWF transitions the system into a baroclinic cyclone towards Tristan da Cunha. The current track forecast is for Iba to accelerate southeast, in line with the ECMWF and most ensembles. Iba is expected to more fully detach from its parental trough over the course of the day today and will do so at the expense of moisture content, a potent mix when combined with the system's sheared state. Wind shear is expected to increase quite substantially as it intercepts the subtropical jet, resulting in column shear averages of over 25 kt. The current intensity forecast depicts a final window of possible slight intensification today before hostile conditions set in after day 2. Some reintensification is possible late in the forecast window as an extratropical system due to baroclinic processes.
FORECAST MAX WINDS
INIT 25/1500Z 40 KT 45 MPH
12H 26/0000Z 45 KT 50 MPH
24H 26/1200Z 45 KT 50 MPH
36H 27/0000Z 40 KT 45 MPH
48H 27/1200Z 35 KT 40 MPH
72H 28/1200Z 25 KT 30 MPH
96H 29/1200Z 25 KT 30 MPH...POST-TROP/REMNT LOW
120H 30/1200Z 30 KT 35 MPH...POST-TROP/REMNT LOW