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Tropical Storm Ileana Discussion Number 6

Wiki-Hurricanes Forecast Center

8:00 PM PDT Sun Aug 5 2018

Ileana is an interesting example of how a tropical cyclone fares when it relies on the sheer volume and rapidity of its convection to drive its intensity and internal organization in the absence of favorable upper-level winds. The fairly well-organized convective structure that Ileana possessed earlier this afternoon, apparently driven by a strong mid-level circulation, gave way to shearing northerlies. In its place, a new burst of convection is currently taking place, possibly over the actual low-level center of circulation. Poor alignment of the circulation vertically throughout the troposphere is leading to a poor focus of thunderstorms, and SSMIS and SSMI microwave data from 2217Z and 2100Z suggest that even despite all the convective noise, the poor internal organization is enough to leave the surface center exposed to subsident air at the lower levels. As recently as 0128Z per microwave data,  the center of circulation is only half stout, with the western half lacking actual deep-layer convection.The frequent convective bursts have thrown automated satellite intensity estimates in for a loop today, with even the normally change resistent CI value from UW-CIMSS ADT rising to an inflated T3.6/57kt. A more rational value of T2.5/35kt was estimated by both SAB and TAFB. Given the maintained potency of convection since the last advisory and the detection of 40 kt winds from the early afternoon, it seems fair to maintain Ileana's intensity at 45 kt.

The nearby Tropical Storm John has now materialized, and is the key component deciding Ileana's fate. For the moment, Ileana has some time and space for itself. The current west-northwesterly heading is being provided by a mid-level inverted trough tracking westward across Mexico. After about a day or so, there is some divergence in the models over whether Ileana will have tracked closely enough to John to become captured by its comparatively broader circulation. The absorption scenario stipulates a rapid curve westward and deterioration. In the other case, if John tracks more south and west, Ileana could avoid getting absorped, in which case the trough will continue to bring Ileana west-northwest or northwest at a brisk pace before reintensifying ridging curves Ileana west. The GEFS ensembles are entiretly in the former camp while the EPS ensembles are entirely in the latter. We'll have to keep tabs on track and circulatory deviations for John to more properly determine Ileana's fate.

Ileana's intensity is also dependent on Tropical Storm John's developments. Were it not for the nearby John, very warm sea surface temperatures and a highly moisturized environment would give Ileana plenty of rope to strengthen, with ample opportunity for organized convective development. Instead, the upper-level environment is quite chaotic, and strong 15-25 kt northerly wind shear is blowing at Ileana, disrupting developments of its core and forcing convection to play catch up with the true center. Circulatory misalignment should prevent considerable intensification as winds would be driven more by diurnal pulsing of the mid-level circulation rather than actual organization of the pressure depression or low-level structure. As Ileana passes by John, an uncertain low-level structure could make it quite difficult for the system to maintain a closed circulation within another tropical cyclone's wind flow. After this point, Ileana's days are over if it is absorbed by Ileana. If not, it may last slightly longer, but strong outflow from John should diffuse Ileana regardless, making the forecasting challenge more over when Ileana exactly dissipates. Given the poor inner core of the tropical storm, I cannot confidently buy into the persistent Ileana scenario, so for now, absorption by John continues to be depicted.

FORECAST POSITIONS AND MAX WINDS

INIT 06/0300Z 45 KT 50 MPH

12H 06/1200Z 45 KT 50 MPH

24H 07/0000Z 50 KT 60 MPH

36H 07/1200Z 45 KT 50 MPH

48H 08/0000Z...DISSIPATED