Tropical Storm Iba Discussion Number 4

Wiki-Hurricanes Forecast Center

12:00 PM AST Sun Mar 24 2019

Iba has continued to organize over the late-morning hours and has transitioned into a classical curved band system. Compared to yesterday, the storm's center of circulation has become better embedded within the convective envelope, and a convective bulb of -70C and cooler cloud tops has persisted over the center and continues to do so. The prominent spiral band from the overnight hours has remained in place, and additional banding is developing in the storm's northeastern and southeastern quadrants. Cirrus-band satellite imagery from GOES-16 shows a strengthening equatorward outflow channel while the subtropical jet stream to Iba's south is aiding with poleward upper-air transport. This favorable ventilative environment is supported by a co-aligned upper-level anticyclone resulting in modest wind shear of 10 knots. At 12Z, SAB estimated T1.5/30 kt for the intensity, but an 1130z ASCAT-B pass showed a small region of 35 kt winds and my own Dvorak analysis on a 0.4 spiral band is in concurrence. An ASCAT-A pass showed a region of 40 kt winds at the same time, though the circulation was somewhat elongated along a NW-SE axis. Given the size of the maximum wind region, the intensity of Iba is estimated at 40 kt with a central pressure of 1007 mb derived from KZC estimates.

Conditions remain locally favorable for continued intensification over warm ocean waters in excess of 28C. The GFS and ECMWF-IFS both show gradual intensification within these conditions for at least the next two days to a moderate tropical storm, though Iba has thus far exceeded the global intensity guidance. The upper-level anticyclone incubating Iba is expected to move westward over Brazil which could expose Iba to a rather dry layer maximized around 400 mb evident on water vapor imagery to the system's southwest and as depicted on the GFS beginning Tuesday. This should bring column-averaged relative humidity values down to a marginal 60%. Still, anomalously warm sea surface temperatures and divegent flow aloft could attenuate inhibatory effects the system out to about the day 4 timeframe. The current forecast shows continued intensification over the next two days before stabilizing and weakening thereafter as the system encounters a breaking trough, increasing wind shear as the system is pulled towards the southeast. The intensification phase has been increased though the weakening phase retains the pace from the previous advisory.


INIT 24/1500Z 40 KT 45 MPH

12H 25/0000Z 45 KT 50 MPH

24H 25/1200Z 50 KT 60 MPH

36H 26/0000Z 50 KT 60 MPH

48H 26/1200Z 40 KT 45 MPH

72H 27/1200Z 35 KT 40 MPH

96H 28/1200Z 30 KT 35 MPH...POST-TROP/REMNT LOW

120H 29/1200Z 25 KT 30 MPH...POST-TROP/REMNT LOW