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Tropical Storm Iba Discussion Number 6

Wiki-Hurricanes Forecast Center

12:00 AM AST Mon Mar 25 2019

Iba is not a particularly healthy tropical cyclone this morning. While deep convection has increased in intensity and coverage over the past few hours, coincident with the approach of the diurnal maximum, this thunderstorm activity remains focused mainly east of the low-level circulation. The culprit for this discombobulation appears to be a moderate 10-15kt of westerly wind shear above 500mb as indicated by GFS area-averaged soundings. In addition, a 23:42Z ASCAT MetOp-B pass indicates that Iba's center has become elongated toward the northwest, interacting with a weak surface trough that formed to the north earlier today. That same pass indicated the extent of tropical storm-force winds has decreased, with only one 35kt barb evident. That being said, a classic shear pattern has allowed the satellite intensity estimate from SAB to increase from T2.0/30kt to T2.5/35kt. So while it is tempting to temporarily lower Iba's strength, the combination of SAB's analysis plus the increase in convection over the past few hours leads me to assess the storm at a probably generous 40kt.

The intensity forecast is complicated. While Iba is experiencing moderate wind shear now -- thought to be the best conditions it will ever experience -- it remains positioned in a narrow axis between anticyclonic flow and a strong westerly jet stream just to its south. At any point, Iba may find itself in slightly less upper-level winds and begin to intensify, as indicated by the GFS and ECMWF. The latter model continues to be less bullish than the GFS, which continues to show a strong tropical storm. It should be noted, though, that this peak occurs in about 3 days, where point soundings indicate the storm should be experiencing strong wind shear. Therefore, it may be best to assume the GFS is outputting an erroneous peak strength. Ocean temperatures remain more than adequate for intensification, and dry air is expected to remain at bay until wind shear increases after 36 hours. I have, for now, left the forecast largely untouched; there are no changes to the forecast track reasoning either. A drift toward the south and then acceleration toward the southeast continues to appear likely.

FORECAST MAX WINDS

INIT 25/0300Z 40 KT 45 MPH

12H 25/1200Z 45 KT 50 MPH

24H 26/0000Z 50 KT 60 MPH

36H 26/1200Z 50 KT 60 MPH

48H 27/0000Z 40 KT 45 MPH

72H 28/0000Z 35 KT 40 MPH

96H 29/0000Z 30 KT 35 MPH...POST-TROP/REMNT LOW

120H 30/0000Z 20 KT 25 MPH...POST-TROP/REMNT LOW