Tropical Storm John Discussion Number 3
Wiki-Hurricanes Forecast Center
3:00 AM MDT Mon Aug 6 2018
John has become exceptionally better organized since the previous advisory, with numerous spiral bands and explosive convective growth near or over the low-level circulation. There have been no recent, direct microwave or ASCAT passes to assess the structural integrity of the tropical storm, but the 0433z ASCAT pass did show winds near tropical storm force far away from the center, highlighting John's monsoonal origins. The 6z satellite intensity estimate from SAB remained at T2.5/35kt. UW-CIMSS ADT, meanwhile, posted T2.7/39kt, with raw values above 55kt to account for the rapidly-improving presentation. Based on manual Dvorak analysis, the initial intensity is set to 45kt, a bit higher than the aforementioned values.
John is in an exceptionally favorable environment, with wind shear below 5kt, ocean temperatures above 30C, and mid-level relative humidity values near 80 percent. The only limiting factors continue to be its broad upbringings, as well as the amount of time it remains over warm waters before approaching the sharp gradient. Most model guidance continues to forecast rapid intensification, and this coincides with the rapid intensification forecasts from SHIPS, which indicate an 83% chance of 25kt/24hr RI and 71% chance of 55kt/48hr RI. The SHIPS itself indicates a peak of 130kt, with 113kt from LGEM. This is in line with the GFS but a bit stronger than recent forecasts from the ECMWF and HWRF. Nonetheless, given the current structure and forecast environment, the updated forecast has been upped again and now makes John a Category 4 hurricane. Rapid weakening should begin after 48 hours as the system enters a colder and drier environment.
John is moving northwest toward a weakness in the subtropical ridge to its north. Models are in good agreement that this motion should continue unabated for the next 72 hours. Thereafter, as John becomes a weaker tropical cyclone and the high pressure re-intensifies, it should begin to curl more west.
FORECAST POSITIONS AND MAX WINDS
INIT 06/0900Z 45 KT 50 MPH
12H 06/1800Z 60 KT 70 MPH
24H 07/0600Z 85 KT 100 MPH
36H 07/1800Z 105 KT 120 MPH
48H 08/0600Z 120 KT 140 MPH
72H 09/0600Z 105 KT 120 MPH
96H 10/0600Z 70 KT 80 MPH
120H 11/0600Z 40 KT 45 MPH