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Tropical Storm IRWIN Forecast Discussion Number 8
2017-07-24 10:49:50| National Hurricane Center (East Pacific)
Issued at 200 AM PDT Mon Jul 24 2017 000 WTPZ45 KNHC 240849 TCDEP5 Tropical Storm Irwin Discussion Number 8 NWS National Hurricane Center Miami FL EP102017 200 AM PDT Mon Jul 24 2017 The convective structure of Irwin has improved quite a bit this evening. A recent AMSU microwave pass at 0541 UTC suggests that deep convection is wrapping nearly all the way around a developing mid-level eye. It isn't yet clear if this feature is aligned with the low-level center. Current intensity estimates from TAFB, SAB, and the UW-CIMSS ADT at 0600 UTC were all 45 kt, so that value has been set as the initial intensity. Determining the initial motion of Irwin has been tricky, since recent microwave fixes of the center between 2100 UTC yesterday and 0600 UTC today suggest that the center has barely moved during that time. It is assumed that at least some westward motion is still occuring, and the initial motion is a rather uncertain 270/5 kt. In the short term, Irwin should continue slowly westward to the south of a low- to mid-level ridge. At days 4 and 5, there remains a fair amount of uncertainty in the degree to which Irwin's track will be affected by Hurricane Hilary to the east, with the global models still generally showing more and the regional models less interaction. Although the NHC track forecast has been adjusted slightly to the north at day 5, it assumes that only a little interaction will take place, and remains south of the GFS and ECMWF. The environment in the short-term still appears favorable for intensification and the guidance is a little higher for this advisory. The NHC forecast now calls for Irwin to become a hurricane within 24 hours. After about 2 days, outflow from Hilary should begin to reach Irwin, resulting in an increase in vertical shear. The ECMWF model in particular shows a large increase in shear at days 3-5, and the NHC forecast therefore begins to show gradual weakening at that time. The intensity forecast is near the HFIP Corrected Consensus, HCCA. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 14.8N 117.4W 45 KT 50 MPH 12H 24/1800Z 14.8N 117.9W 55 KT 65 MPH 24H 25/0600Z 14.8N 118.6W 65 KT 75 MPH 36H 25/1800Z 14.8N 119.4W 70 KT 80 MPH 48H 26/0600Z 14.8N 120.5W 70 KT 80 MPH 72H 27/0600Z 14.4N 122.0W 65 KT 75 MPH 96H 28/0600Z 14.5N 123.1W 60 KT 70 MPH 120H 29/0600Z 15.7N 124.1W 60 KT 70 MPH $$ Forecaster Zelinsky
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