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Tropical Depression Thirteen-E Special Discussion Number 1

Wiki-Hurricanes Forecast Center

8:45 PM PDT Mon Aug 6 2018

The area of disturbed weather located over 1,000 miles southwest of Baja California is now a tropical depression. Deep convection has been increasing and consolidating over the low-level circulation, and a brand new 0257z microwave pass indicates this center is more vigorous and defined than anticipated. Dvorak estimates from SAB and TAFB were only T1.0, but this predated the increase in organization on satellite. In addition, earlier ASCAT passes along the eastern fringes of the circulation have measured winds of 30kt. The initial intensity has therefore been set to that value.

Thirteen-E is located to the south of an expansive mid-level ridge that stretches from the subtropical Pacific into the Southwestern United States. This ridge should remain sturdy for the next 48 hours or so. Thereafter, as Hurricane John to our storm's east moves northwest, it should provide an outlet for Thirteen-E to turn north and maybe even north-northeast. The degree of curvature will depend on the strength of Thirteen-E, with a stronger storm more apt to feel changes in the mid-level flow to its north. This is indicated by the GFS and HWRF, which indicate significant northward movement and eventual interaction with Hurricane John. The ECMWF, on the other hand, keeps Thirteen-E weak and on a west-northwest heading as a result. For now, the newly-formed storm is expected to take an "L"-shaped track, remaining on a northward heading on days three to five. This is very likely to change as models shuffle in coming hours and days.

The environment around Thirteen-E is not exceptionally favorable, with moderate wind shear of 10-15kt expected to persist during much of the next 5 days. Ocean temperatures are currently around 28C but should fall to 26C within 36 hours before leveling off thereafter. Mid-level relative humidity values, however, are expected to remain conducive over the next 5 days. The SHIPS and LGEM are interested in modest intensification, indicating peaks of 48kt and 40kt, respectively, within the forecast window. The HWRF and GFS are more bullish and indicate the potential for Thirteen-E to become a Category 1 hurricane, albeit at various times. The ECMWF is generally weak and with a dramatically different forecast track than other models. On its own island, the FV3-GFS shows absolutely no interest in this tropical cyclone. The intensity forecast is complicated by a wide array of track forecasts, with some models indicating interaction with Hurricane John and others remaining far apart. For now, the first forecast shows slow development over the next few days. This is a very low confidence forecast, however.

FORECAST POSITIONS AND MAX WINDS

INIT 07/0300Z 30 KT 35 MPH

12H 07/1200Z 35 KT 40 MPH

24H 08/0000Z 35 KT 40 MPH

36H 08/1200Z 40 KT 45 MPH

48H 09/0000Z 45 KT 50 MPH

72H 10/0000Z 50 KT 60 MPH

96H 11/0000Z 50 KT 60 MPH

120H 12/0000Z 45 KT 50 MPH