Tropical Storm Ileana Discussion Number 5
Wiki-Hurricanes Forecast Center
2:00 PM PDT Sun Aug 5 2018
The cloud pattern associated with Illeana has become much better organized since earlier this morning, with an expansive mass of convection cooler than -75C persisting over the center. A 1751z microwave pass did not show any signs of an inner core, so the potential for explosive intensification in the immediate future seems unlikely. Nonetheless, a 16z ASCAT pass showed a few 40kt barbs located to the north of Illeana's center. Given the persistent nature of convection since that time, as well as a satellite estimate of T3.0/45kt from SAB, the initial intensity has been raised to 45kt.
The intensity forecast started off complicated, and it hasn't become any clearer since yesterday. SHIPS analyzes high wind shear near 20kt in the vicinity of Illeana, which would tend to curtail any further development. Water vapor imagery from earlier today showed upper-level outflow propagating toward the center of the storm, indicating that upper-level winds are indeed hostile nearby. The problem is that given the degree of convection that continues to fire over Illeana's center, the sheer volume of latent heat release may be providing an immediate bubble of lower shear that is allowing it to thrive. Water temperatures remain 30C and mid-level relative humidity values remain above 70 percent, so such an outcome would be not surprising. Model guidance is much more bullish compared to this time yesterday, with the SHIPS, LGEM, HWRF, and UKMET all forecasting Illeana to attain hurricane strength; the ECMWF depicts a strong tropical storm. A simple blend of these forecasts would lend some confidence in Illeana attaining hurricane strength over the next 2 days. Conventional wisdom, however, suggests that with Twelve-E forecast to rapidly intensify just to Illeana's west, the shear imparted on the storm would not allow such predictions to verify. For now, the updated forecast has been adjusted upward, but not as high as some models suggest.
The track forecast for Illeana is also complicated. The system is moving west-northwest as dictated by a mid-level ridge to its north, and this should continue for at least the next 36 hours. Thereafter, models diverge significantly. The HWRF slingshots Illeana around Twelve-E (which is a rapidly-intensifying hurricane) before being absorbed. The ECMWF maintains Illeana on a constant west-northwest track with little interaction between the two tropical cyclones. The GFS and its ensembles, which only initialized Illeana as a vorticity maximum, shows absorption. Meanwhile, the statistical and dynamical guidance sides with ECMWF in showing a constant west-northwest trajectory. It is difficult to know which model has a better handle, although with Illeana forecast to become a stronger entity than originally believed, it would not be surprising if ECMWF was performing best here. Dissipation has been left unchanged at day 3, but this may need to be pushed back in later advisories.
FORECAST POSITIONS AND MAX WINDS
INIT 05/2100Z 45 KT 50 MPH
12H 06/0600Z 45 KT 50 MPH
24H 06/1800Z 50 KT 60 MPH
36H 07/0600Z 55 KT 65 MPH
48H 07/1800Z 45 KT 50 MPH