Tropical Storm John Discussion Number 4

Wiki-Hurricanes Forecast Center

8:00 AM PDT Mon Aug 6 2018

John is making it clear this morning that the environmental conditions it finds itself in are exceptionally favorable for rapid intensification. Robust spiral band development continued throughout the night as the tropical storm has worked to condense its circulation and build an inner core. Cloud tops have retained their cold temperature throughout the night as sea surface temperatures are at or above 30°C throughout the region, allowing for large-scale covnective growth. Recent microwave images show organizational progression of John's internal structure, and a more permanent inner core structure appears to be in the works. A GMI pass from 0706Z showed numerous rainband and rainband fragments wrapping around the center of the monsoonal tropical cyclone--akin to a tropical skeleton--and by 1247Z and SSMIS microwave pass showed John appearing to have settled on a curved band-type core structure with hints of a mid-level eyewall beginning to take form. There are some hints that an eye feature or primordial warm spot could emerge as soon as the early afternoon. A quickly evolving tropical cyclone, John is well on its way to becoming a hurricane, and likely a major hurricane down the line. SAB evaluated a rather conservative T3.0/45kt while TAFB estimated T3.5/55kt at 12z. UW-CIMSS ADT final values are climbing as raw values remain in excess of T4.0/65kt. Taking a blend of these values, the TAFB estimate seems to offer good consensus and thus the intensity for John has been set at 55 kt, and this is liable to change on an hourly basis given the current intensification.

Statistical, dynamical, and ensemble guidance have clustered more tightly together in this morning's model suite, though the general steering idea has always remained the same, with a few variable fine details caused mostly by differences in the handling of John's large size and interaction with the nearby Tropical Storm Ileana. A weakness in the subtropical ridge carved out by a trough to John's north is steering John towards the northwest. Some abnormal wobbling in its track could occur over the next 48 hours as the nearby Ileana is expected to wrap into and become absorbed into John's circulation. Over time, the weakness should fill in as ridging over the US Rocky Mountains strengthens, materializing in a westerly curve with time.

Tropical Storm John is rapidly intensifying this morning and there is not much in the short term to stop that process from occuring. Sea surface temperatures are favorable for development over the next 2-3 days, and are exceptionally warm and favorable for rapid intensification over the next 2 days. Conditions are more than moist enough to support continued convection, and due to John's size and convective activity, an upper-level anticyclone has already developed to shape a favorable environment for the tropical storm, supporting good equatorward and poleward outflow channels. SHIPS and LGEM both observe John reaching Category 4 status in roughly two days, with the former nearly indicating Category 5 strength. The GFS shows rapid intensification to a Category 4 as well. Contrastingly, HWRF sees John having difficulty managing its large size, and compounded by the presence of Ileana, but nonetheless shows John very nearly reaching major hurricane strength, with ECMWF-IFS and UKMO-G suggesting similar conclusions. For now, the intensity philosophy from previous advisories has been maintained, and shows a Category 4 peak. After three days, rapidly cooling ocean waters should cause convective decay and quickly weaken the storm as it heads further north.


INIT 06/1500Z 55 KT 65 MPH

12H 07/0000Z 75 KT 85 MPH

24H 07/1200Z 95 KT 110 MPH

36H 08/0000Z 115 KT 130 MPH

48H 08/1200Z 125 KT 145 MPH

72H 09/1200Z 100 KT 115 MPH

96H 10/1200Z 70 KT 80 MPH

120H 11/1200Z 40 KT 50 MPH

Community content is available under CC-BY-SA unless otherwise noted.