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Tropical Depression Seven-E Discussion Number 2

Wiki-Hurricanes Forecast Center

8:00 AM CDT Sat Jun 30  2018

Seven-E remains a sprawling system this morning with numerous rainbands continuing both northern and southern semicircles of the center of circulation. The mid-level preference of convective activity isn't particularly oriented over the center yet, with showers and thunderstorms for the most part generating cirrus debris over where the center likely is, but this will likely change as the tropical depression continues to evolve. Microwave imagery from 0758Z and 1033Z show a tightening circulation from a broad and elongated one yesterday, with the overall convective configuration adapting by also beginning to coalesce in a focal region northwest of the center. SAB estimated T1.5/25kt with TAFB estimating T2.0/30kt. A relatively old ScatSat pass from 0324Z had a fair amount of suspect values but did indicate 30 kt winds were occuring north of the center. Given no significant changes in convective strength and cohesion in that time, the intensity for this advisory is set to 30 kt, in agreement with TAFB.

Seven-E continues to track westward, with the dominant steering feature being the 591 dm subtropical ridge centered off of Port Arthur which is producing easterly winds across the eastern East Pacific this morning. Enhanced heights caused by the evacuation of air in the wake of Emilia has also produced a weak anticyclone just south of Baja California Sur which is helping to extend these easterlies westward. Heights across the United States are expected to increase this week as a stable anticyclonic pattern emerges. Seven-E is expected to reach the southwestern periphery of this large ridge around day 4-5, which should gradually inflect a more northwesterly bearing. However, this is expected to be a very gradual transition, and for the most part Seven-E's trajectory will be fairly linear.

As discussed earlier, latent heat release from convective activity, now more associated with the center of circulation, is helping to even out the upper-air windflow around Seven-E by generating a nascent upper-level anticyclone which is assessed in the latest upper-air analyses. This is still a developing feature, however, and for now water vapor imagery suggests that the dominant outflow-level wind is still easterly across most of the circulation for Seven-E. Cirrus banding does indicate that this is sufficient to work against some of the inhibitory flow from a weak potential vorticity anomaly cenetered around Zihuantenejo this morning. In the short range, environmental parameters look favorable for intensification. The broad nature of Seven-E's genesis should provide a sizeable field of divergent flow aloft from the scattershot convection which will decrease local wind shear over the circulation. Sea surface temperatures and atmospheric moisture content will also be supportive of intensification in the short-range before a sharp temperature gradient arrives in the day 4-5 range. Although the broad nature of Seven-E will help it undercut some of the unfavorable atmospheric conditions, it will also make it more difficult for the cyclone to gain in strength and organization, complicating the intensity forecast as internal mechanisms are often difficult to predict. Most statistical guidance suggests a solid Category 1, though SHIPS prognosis has indicated a major hurricane. The GFS and ECMWF-IFS each indicate a minimal hurricane, while the HWRF and HMON are slightly stronger. The UKMO-G has a system digging into the 950 mb range, though this aggressiveness comes from a more southerly track which would keep it over warmer sea surface temperatures for longer. For now, gradual intensification is expected over the first two days, with more accelerated strengthening occuring in the day 2-4 window before weakening occurs over cooler waters at the end of the forecast timeframe.



FORECAST POSITIONS AND MAX WINDS

INIT 30/1500Z 30 KT 35 MPH

12H 01/0000Z 35 KT 40 MPH

24H 01/1200Z 40 KT 45 MPH

36H 02/0000Z 50 KT 60 MPH

48H 02/1200Z 60 KT 70 MPH

72H 03/1200Z 85 KT 100 MPH

96H 04/1200Z 100 KT 115 MPH

120H 05/1200Z 90 KT 105 MPH

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