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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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Tropical Storm Irwin Forecast Discussion Number 5

2017-07-23 16:56:47| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Sun Jul 23 2017 000 WTPZ45 KNHC 231456 TCDEP5 Tropical Storm Irwin Discussion Number 5 NWS National Hurricane Center Miami FL EP102017 800 AM PDT Sun Jul 23 2017 Irwin's center is exposed to the north and west of its associated deep convection due to about 15 kt of northerly shear. Subjective and objective Dvorak intensity estimates are a consensus T2.5, so 35 kt will remain the initial intensity for this advisory. The initial motion remains westward at 280/6 kt, with Irwin located south of a weak low-level ridge and almost midway between Tropical Storms Greg and Hilary. This orientation should cause Irwin to maintain a westward, albeit much slower, motion during the next several days. However, once Greg degenerates into a remnant low in about 4 days, a binary interaction between a stronger Irwin and Hilary is more likely to occur, which could force Irwin to turn west-southwestward at the end of the forecast period. There is some notable spread in the guidance, highlighted by the GFS and its ensemble mean on the southern end of the guidance envelope, and the ECMWF and its ensemble mean on the northern edge. The HWRF is also north of most models by the end of the forecast period, presumably because it is not capturing any interaction between Irwin and Hilary. The updated NHC forecast is nudged a little south on this advisory, and it is south of the model consensus after day 3 to account for an Irwin-Hilary track interaction. The shear over Irwin is expected to decrease within the next 12-24 hours, which should allow the cyclone to begin a sustained period of strengthening. Sea surface temperatures will be between 27-28 degrees Celsius, and the ambient environment appears sufficiently moist to foster intensification. The NHC intensity forecast favors a blend of the ICON intensity consensus, HCCA, and the Florida State Superensemble, showing Irwin reaching hurricane intensity in about 48 hours. This forecast is not too different from the previous advisory. FORECAST POSITIONS AND MAX WINDS INIT 23/1500Z 15.0N 115.8W 35 KT 40 MPH 12H 24/0000Z 15.0N 116.6W 40 KT 45 MPH 24H 24/1200Z 15.0N 117.5W 50 KT 60 MPH 36H 25/0000Z 15.0N 118.2W 60 KT 70 MPH 48H 25/1200Z 15.0N 119.1W 65 KT 75 MPH 72H 26/1200Z 14.8N 121.4W 65 KT 75 MPH 96H 27/1200Z 14.2N 123.9W 60 KT 70 MPH 120H 28/1200Z 14.0N 125.4W 55 KT 65 MPH $$ Forecaster Berg

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Tropical Storm HILARY Forecast Discussion Number 9

2017-07-23 16:35:48| National Hurricane Center (East Pacific)

Issued at 1000 AM CDT Sun Jul 23 2017 000 WTPZ44 KNHC 231435 TCDEP4 Tropical Storm Hilary Discussion Number 9 NWS National Hurricane Center Miami FL EP092017 1000 AM CDT Sun Jul 23 2017 Visible imagery indicates that Hilary is becoming better organized. A small central dense overcast has formed, along with a large curved band in the northern semicircle of the cyclone. The initial wind speed is set to 40 kt, a blend of the latest TAFB/SAB estimates. Hilary could be on the verge of starting its long-anticipated significant strengthening period. A 1059 UTC SSMIS microwave pass indicates more inner-core structure with the storm, which suggests that Hilary will be able to take advantage of the nearby conducive atmospheric and oceanic conditions. The SHIPS guidance is also indicating relatively high chances of rapid intensification, roughly a 50/50 chance during the next 48 hours. Given the factors above, the NHC forecast is raised from the previous one, best in line with a blend of the HWRF/DSHIP/HFIP corrected consensus models. An increase in northerly shear, possibly enhanced by outflow from TS Irwin, should cause some weakening late in the period, along with more marginal SSTs. Hilary is moving slower now to the west-northwest at about 9 kt. A ridge over Mexico should steer Hilary in that general direction for the next several days, at various speeds due to the strength of the ridge. Later in the period, there is a possibility of some binary interaction with Irwin, which adds some uncertainty to the forecast. Overall, there hasn't been a lot of change to the model guidance, and the latest NHC track prediction stays relatively close to the previous forecast and the consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 23/1500Z 12.7N 102.3W 40 KT 45 MPH 12H 24/0000Z 13.1N 103.5W 50 KT 60 MPH 24H 24/1200Z 13.8N 104.8W 65 KT 75 MPH 36H 25/0000Z 14.6N 106.2W 75 KT 85 MPH 48H 25/1200Z 15.4N 107.9W 90 KT 105 MPH 72H 26/1200Z 16.5N 112.0W 100 KT 115 MPH 96H 27/1200Z 17.6N 115.8W 90 KT 105 MPH 120H 28/1200Z 19.0N 119.0W 85 KT 100 MPH $$ Forecaster Blake

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Tropical Storm GREG Forecast Discussion Number 25

2017-07-23 16:33:46| Tropical Depression LIDIA

Issued at 800 AM PDT Sun Jul 23 2017

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Tropical Storm Irwin Forecast Discussion Number 4

2017-07-23 10:53:43| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Sun Jul 23 2017 000 WTPZ45 KNHC 230853 TCDEP5 Tropical Storm Irwin Discussion Number 4 NWS National Hurricane Center Miami FL EP102017 200 AM PDT Sun Jul 23 2017 An ASCAT-B overpass at 0516 UTC showed several vectors of 32-33 kt winds and a max value of 34 kt in the northeast quadrant of the tropical cyclone. Based on that data, the system has been named Tropical Storm Irwin. Irwin is the fifth system to be named in the eastern North Pacific so far in July. Although the system has been upgraded based on the ASCAT data, the structure has not improved. Shortwave-IR imagery shows that the low-level center is clearly exposed to the northwest of the deep convection, and Dvorak intensity estimates from TAFB and SAB have not increased. The global models suggest that the shear should decrease quite a bit after about 12 to 24 hours, which should allow for some intensification. The regional dynamical models HWRF and COAMPS-TC depict Irwin quickly recovering from its current sheared state and intensifying to a category 2 or 3 hurricane. On the other hand, the statistical models still only show modest strengthening, and keep Irwin below hurricane strength. As a course of least regret, the intensity forecast splits these two scenarios, but slightly favors the lower statistical guidance since the dynamical models may show the structure of Irwin recovering from the current shear too quickly. Near the end of the forecast period, Irwin is expected to encounter a higher shear environment, due in part to outflow from Hilary to the east, so the forecast depicts gradual weakening. The initial motion is 280/6 kt. Irwin continues to be steered by a low- to mid-level ridge to the northwest. Although no significant changes have been made to the track forecast, confidence is still somewhat low since there is a fair amount of speed-spread in the models. The GFS remains an outlier in being the only model that shows substantial binary interaction with Hilary, and is thus the farthest east, though the 00 UTC ECMWF is also a little slower than before. The official forecast is a little slower at the end of forecast period than the previous advisory, but still assumes that any significant track deviation due to interactions with Hilary will occur beyond 120 h. FORECAST POSITIONS AND MAX WINDS INIT 23/0900Z 14.7N 115.2W 35 KT 40 MPH 12H 23/1800Z 14.8N 116.0W 40 KT 45 MPH 24H 24/0600Z 14.9N 117.0W 45 KT 50 MPH 36H 24/1800Z 15.0N 118.0W 50 KT 60 MPH 48H 25/0600Z 15.0N 118.8W 60 KT 70 MPH 72H 26/0600Z 14.7N 121.0W 65 KT 75 MPH 96H 27/0600Z 14.5N 123.1W 60 KT 70 MPH 120H 28/0600Z 14.0N 125.5W 55 KT 65 MPH $$ Forecaster Zelinsky

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