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Tropical Storm Six-E Discussion Number 4

Wiki-Hurricanes Forecast Center

800 AM PDT Thu Jun 28 2018

During the overnight hours, convection has maintained itself around the circulation of Six-E, with convection beginning to initiate more appreciably near the center of circulation at dawn, with some high cloud tops over 16 km in height. The system is being affected by some easterly shear originating from a nearby upper-level anticyclonic feature which has partially exposed the center of circulation to the east of convection; the deep-layer wind shear mean is 17 knots, but higher values are likely occuring in specific shallow-layers of the atmosphere. Still, the character of Six-E has improved in the presence of shear, and microwave imagery continues to show improved banding in the western semicircle. Microwave satellite intensity estimates are in the range of 35-41 kt with high confidence, while SAB and UW-CIMSS ADT give slightly lower figures at T2.0/30kt and T2.3/33kt, respectively. Given these factors and the increase of convection this morning, the initial intensity has been set to 35 kt, upgrading Six-E to a tropical storm.

Six-E continues to move briskly northwestward with dominant subtropical ridging to the north, with a motion assessed at 290/15. Not much has changed with the overarching track regime, with the elements mentioned in previous advisories still in play. At around day 3 the shortwave trough over the United States West Coast is expected to extend farther southward, providing a brief window wherein Six-E should gain latitude on a more northwesterly heading. A weaker storm by day 5 in concert with a temporary return to the overall easterly steering regime should curve Six-E back on a westerly path by that point.

Intensity guidance is slightly more enthused this morning, but in examining the specific parameters for storm intensity there has not been considerable change. Unless Six-E is able to make efficient use the moisture in its current moderately sheared environment, it will eventually struggle as sea surface temperatures quickly decline, with temperatures going below 26.5C in about 48 hours and nearing 23C at the end of the forecast window. At that point it is unlikely that developign convection will aid in increasing winds. The HWRF and LGEM indicate a moderate tropical storm while SHIPS projects a stronger tropical storm. The GFS, yielding a Category 1 hurricane, appears to be an outlier among the global guidance cohort with the ECMWF- IFS and UKMO-G indicating more or less the same business as the hurricane intensity guidance. Relatively quick motion in deteriorating oceanic conditions are not a particularly good recipe for quick intensification, and the current forecast shows a rather ordinary gradual intensification and weakening, with the intensification window coming to a close in about 2 days.



FORECAST POSITIONS AND MAX WINDS

INIT 28/1500Z 35 KT 40 MPH

12H 29/0000Z 40 KT 45 MPH

24H 29/1200Z 45 KT 50 MPH

36H 30/0000Z 50 KT 60 MPH

48H 30/1200Z 50 KT 60 MPH

72H 01/1200Z 45 KT 50 MPH

96H 02/1200Z 30 KT 35 MPH

120H 03/1200Z 20 KT 30 MPH...POST-TROP/REMNT LOW