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Tropical Storm Irwin Forecast Discussion Number 6
2017-07-23 22:32:21| National Hurricane Center (East Pacific)
Issued at 200 PM PDT Sun Jul 23 2017 000 WTPZ45 KNHC 232032 TCDEP5 Tropical Storm Irwin Discussion Number 6 NWS National Hurricane Center Miami FL EP102017 200 PM PDT Sun Jul 23 2017 Deep convection has covered up Irwin's center since the last advisory, a sign that the shear affecting the system continues to diminish. Dvorak intensity estimates have increased to T3.0/45 kt from TAFB and SAB, but objective numbers are still at T2.5/35 kt. Based on these estimates, and a 1748 UTC ASCAT-B pass, the initial intensity is raised to 40 kt. Irwin is moving westward, or 270/7 kt, between Tropical Storms Greg and Hilary and to the south of a weak low- to mid-level ridge extending west of the Baja California peninsula. Since the ridge is expected to remain weak, Irwin is forecast to move only slowly westward through 48 hours. After that time, its motion is likely to become increasingly influenced by Hilary, with the two beginning some degree of a binary interaction. The track guidance now indicates that Irwin will turn west-southwestward on days 3 and 4, and then possibly turn back to the northeast around Hilary's southern side on day 5. With a shift in the overall guidance envelope, the new NHC track forecast has been shifted southward toward HCCA and the TVCN multi-model consensus, and it also now shows Irwin stalling or drifting northward on day 5. Irwin is located over warm sea surface temperatures and in a relatively moist environment, and the northerly shear affecting the cyclone should continue to decrease over the next day or so. Therefore, additional gradual strengthening is anticipated, and Irwin has the possibility of reaching hurricane strength in about 48 hours. After that time, Hilary (possibly as a major hurricane) will be getting closer to Irwin, and its upper-level outflow could cause stronger shear to develop over Irwin. The intensity guidance has responded to this possibility by showing more pronounced weakening after 48 hours, and the new NHC intensity forecast now shows Irwin holding a steady intensity as a tropical storm on days 3 through 5. This forecast remains closest to HCCA and the ICON intensity consensus. FORECAST POSITIONS AND MAX WINDS INIT 23/2100Z 14.9N 116.6W 40 KT 45 MPH 12H 24/0600Z 14.8N 117.3W 45 KT 50 MPH 24H 24/1800Z 14.7N 118.1W 50 KT 60 MPH 36H 25/0600Z 14.6N 118.9W 60 KT 70 MPH 48H 25/1800Z 14.5N 119.8W 65 KT 75 MPH 72H 26/1800Z 14.0N 122.1W 55 KT 65 MPH 96H 27/1800Z 13.5N 123.5W 55 KT 65 MPH 120H 28/1800Z 14.0N 123.5W 55 KT 65 MPH $$ Forecaster Berg
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