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Tropical Storm John Discussion Number 2

Wiki-Hurricanes Forecast Center

8:00 PM PDT Sun Aug 5 2018

Over the last six hours, Tropical Depression Twelve-E has developed numerous spiral bands in an effort to wrap about its broad center of circulation. Infrared imagery shows a large expanse of various convective foci, with a curved band near the center that appears to be laying the seeds for an eventual central dense overcast. Microwave data from AMSR2 at 2023Z and SSMIS at 2218Z showed that the system lacked a coherent inner core, but indicated that the convergent boundaries that should eventually allow for more organized convection to develop were taking shape. A fresh SSMIS pass from 0128Z confirmed this evolution. SAB and TAFB gave a consensus T2.5/35kt estimate, and on this basis Twelve-E has been upgraded to Tropical Storm John and now takes form as a low-end tropical storm with maximum sustained winds of 35 kt. 

A mid-level inverted trough over Mexico northeast of John is alliowing the tropical storm to be steered towards the northwest. This trough is providing a weakness in the overall subtropical ridge pattern, pulling John away from the ordinary easterly flow. However, as this wave is closer to the ridging, it is expected to move longitudinally faster than John, eventually passing to its north and then northwest. In doing so, the weakness should pull John more towards the west-northwest around days 3 and 4. Afterwards, bolstered ridging over the continental United States should do away with this weakness, allowing John to complete a turn towards the west. Statistical, dynamical, and ensemble guidance are in good agreement in this track progression, but there is some variance in how north of west John tracks. GFS and GFS-derived guidance is comparatively north of ECMWF-IFS and its derived guidance.

Environmental parameters are all in the green for John over the next two days. The tropical northeastern Pacific this time of year is a nearly ideal nursery for tropical cyclones, so this comes as no surprise. Deep-layer thermal differentials are in excess of 80°C for 48 hours and mid-level relative humidity exceeds 70% over the entire forecast period, providing ideal thermodynamically unstable conditions for convective growth. Conditions are also supportive for John to foster its own upper-level anticyclone to keep wind shear low. The stage is set for quite the intensification episode, and things could get quite rapid. Global guidance from UKMO-G, ECMWF-IFS, and GFS all point towards a major hurricane, as do SHIPS/LGEM. HWRF and HMON are comparatively less enthused, perhaps through their treatment of dry air intrusion from the Pacific subtropics. The primary issue blocking certain rapid intensification is John's structure and size, which could increase the amount of time it needs to get going. Still, environmental parameters are expected to be favorable enough to support a major hurricane. In around three days, warming tropopause temperatures and cooling sea surface temperatures should begin to put a cap on the intensification window before rapidly falling sea surface temperatures around and after 96 hours precipitate a quick weakening process. A track shifted to the west in line with the EPS and its ensembles would keep John over warmer waters for a bit longer, delaying weakening by about a half-day to a day or so. SHIPS diagnostic values point to a somewhat middle ground between the two camps, so its environmental parameters are favored at this time.

FORECAST POSITIONS AND MAX WINDS

INIT 06/0300Z 35 KT 40 MPH

12H 06/1200Z 45 KT 50 MPH

24H 07/0000Z 65 KT 75 MPH

36H 07/1200Z 85 KT 100 MPH

48H 08/0000Z 100 KT 120 MPH

72H 09/0000Z 105 KT 125 MPH

96H 10/0000Z 75 KT 85 MPH

120H 11/0000Z 45 KT 50 MPH