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Tropical Storm Hilda Forecast Discussion Number 16

2021-08-03 16:36:40| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Tue Aug 03 2021 000 WTPZ43 KNHC 031436 TCDEP3 Tropical Storm Hilda Discussion Number 16 NWS National Hurricane Center Miami FL EP082021 800 AM PDT Tue Aug 03 2021 There's been little change with the structure of Hilda overnight as deep convection continues to be mostly south of the center due to persistent northeasterly shear. The low-level eye feature in 37 GHz microwave data remains on the latest passes, but it isn't very deep because of the shear. A blend of the T- and CI-numbers from TAFB/SAB, plus the UW-CIMSS SATCON, yields 60 kt as the initial wind speed. Hilda should weaken during the next several days, first primarily due to shear, then cool water temperatures and a more stable environment on Wed-Fri. Model guidance is in good agreement on this scenario, and the latest forecast is close to the previous NHC advisory and the model consensus. Hilda should decay into a remnant low by Friday and dissipate as a trough this weekend well east of the Hawaii. The initial motion estimate, 315/7 kt, is the same as the previous advisory. Hilda is maintaining this motion as it is steered by a mid-level ridge centered over southern California. The global models are consistently building this ridge westward over the eastern Pacific during the next several days as a shortwave trough lifts out along 135W, causing Hilda to turn west-northwestward by Thursday. The only significant forecast difference is how quickly the cyclone moves, mostly in the latter stages, with more of the guidance showing a faster motion, perhaps because the models are showing a weaker Hilda being steered by the quicker low-level flow. The new NHC prediction shows that acceleration at long-range as well, near or just behind the latest model consensus. FORECAST POSITIONS AND MAX WINDS INIT 03/1500Z 17.5N 124.5W 60 KT 70 MPH 12H 04/0000Z 18.2N 125.3W 50 KT 60 MPH 24H 04/1200Z 19.1N 126.4W 45 KT 50 MPH 36H 05/0000Z 19.9N 127.7W 40 KT 45 MPH 48H 05/1200Z 20.8N 129.4W 35 KT 40 MPH 60H 06/0000Z 21.6N 131.4W 30 KT 35 MPH 72H 06/1200Z 22.3N 133.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 07/1200Z 23.0N 138.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 08/1200Z...DISSIPATED $$ Forecaster Blake

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Tropical Storm Hilda Forecast Discussion Number 15

2021-08-03 11:00:05| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Tue Aug 03 2021 000 WTPZ43 KNHC 030900 TCDEP3 Tropical Storm Hilda Discussion Number 15 NWS National Hurricane Center Miami FL EP082021 200 AM PDT Tue Aug 03 2021 Overnight, Hilda's deep convection has continued to pulse, but primarily in the southern semicircle of the circulation as 15-20 kt of northeasterly vertical wind shear continues displacing this activity downshear. A helpful ASCAT-B pass at 0602 UTC indicated the center of Hilda was located on the northern edge of the convective cirrus canopy, with the instrument also indicating a peak wind retrieval of 53 kt. The most recent microwave imagery concurs with this assessment and also indicates the earlier eyewall is no longer well-defined. While the 0600 UTC subjective Dvorak estimates from TAFB and SAB were both T4.0/65 kt, the most recent objective Dvorak estimate from UW-CIMSS was down to T3.5/55 kt. Given the devolving convective structure seen on microwave, the initial intensity has been reduced to 60 kt, leaning towards the lower intensity estimates. Hilda continues to move slowly northwestward, at 315/7 kt as it remains positioned southwest of a mid-level ridge centered over southern California. Over the next several days, this ridge is forecast to build westward over the East Pacific as a short-wave trough offshore of California lifts out. This synoptic pattern should allow Hilda to gradually bend to the west-northwest and increase its forward motion slightly over the forecast period. Hilda will also become increasingly influenced by low-level ridging, steering the cyclone more westward as it becomes a weak and shallow cyclone. The latest NHC track forecast is just a touch south of the previous forecast track, blending the reliable TVCE and HCCA consensus aids. Northeasterly vertical wind shear, related to the remaining convective outflow of Ignacio, is expected to keep Hilda on a weakening trend over the next day or so. Even though this shear is expected to subside thereafter, Hilda will also be crossing the 25 degree Celsius sea-surface temperature isotherm in 24 hours. Thus, continued weakening is expected. The latest NHC intensity forecast follows the HCCA consensus aid closely, showing weakening throughout the forecast period. The tropical cyclone is now expected to degenerate to a remnant low by Friday, when model guidance suggests organized deep-convection will cease. The remnant low is then expected to open up into a trough by the weekend, well east of Hawaii. FORECAST POSITIONS AND MAX WINDS INIT 03/0900Z 17.0N 123.9W 60 KT 70 MPH 12H 03/1800Z 17.6N 124.6W 50 KT 60 MPH 24H 04/0600Z 18.6N 125.5W 45 KT 50 MPH 36H 04/1800Z 19.4N 126.7W 40 KT 45 MPH 48H 05/0600Z 20.2N 128.1W 35 KT 40 MPH 60H 05/1800Z 21.0N 129.9W 30 KT 35 MPH 72H 06/0600Z 21.7N 132.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 07/0600Z 22.4N 136.2W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 08/0600Z...DISSIPATED $$ Forecaster Papin/Brown

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Tropical Depression Ignacio Forecast Discussion Number 7

2021-08-03 10:47:42| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Tue Aug 03 2021 000 WTPZ45 KNHC 030847 TCDEP5 Tropical Depression Ignacio Discussion Number 7 NWS National Hurricane Center Miami FL EP102021 200 AM PDT Tue Aug 03 2021 Strong east-northeasterly shear has continued to take a toll on Ignacio overnight with a small cluster of remaining deep convection now displaced over 60 n mi to the southwest of the center. An ASCAT overpass that arrived shortly after the release of the previous advisory indicates that Ignacio has weakened to a tropical depression with peak winds of around 30 kt. That is also supported by a blend of the latest Dvorak T- and CI-numbers from TAFB and SAB. The strong shear and marginal SSTs should continue to cause gradual weakening over the next 12-24 hours, and Ignacio is expected to become a remnant low Tuesday night. The GFS, ECMWF, and UKMET all show the remnant low opening up into a trough of low pressure shortly thereafter, and so does the official forecast. Ignacio is moving slower toward the northwest than before, or about 315/5 kt. A further reduction in forward speed is predicted over the next 12-24 hours as Ignacio weakens and is steered by the weaker low-level flow. The track guidance again suggests that the remnant low will turn northeastward before dissipating, and the updated NHC track forecast has been adjusted accordingly. FORECAST POSITIONS AND MAX WINDS INIT 03/0900Z 20.1N 116.3W 30 KT 35 MPH 12H 03/1800Z 20.9N 116.9W 25 KT 30 MPH 24H 04/0600Z 21.2N 116.6W 20 KT 25 MPH...POST-TROP/REMNT LOW 36H 04/1800Z...DISSIPATED $$ Forecaster Brown

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Hurricane Hilda Forecast Discussion Number 14

2021-08-03 04:36:34| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Mon Aug 02 2021 000 WTPZ43 KNHC 030236 TCDEP3 Hurricane Hilda Discussion Number 14 NWS National Hurricane Center Miami FL EP082021 800 PM PDT Mon Aug 02 2021 Moderate northeasterly wind shear continues to take a toll on Hilda this evening. The upper-level outflow on the northern and eastern sides of the circulation has a sharp edge in satellite imagery. Recent SSMIS microwave data show the structure of Hilda has changed little since earlier today, with a weakness in the upshear portion of the eyewall and a slight displacement of the low- and mid-level centers. The various objective intensity estimates and subjective Dvorak classifications from TAFB and SAB support holding the initial intensity at 65 kt for this advisory. Hilda is moving northwestward, or 315/6 kt, under the influence of a subtropical ridge to its northeast. This general motion is expected to continue through midweek, and the track guidance remains tightly clustered during this period. As Hilda continues weakening and the ridge builds westward, Hilda is forecast to move west-northwestward and then westward within the low-level flow through the rest of the period. However, a complicating factor is the potential for some interaction with the remnants of TD Nine-E, which now have a high chance of redevelopment during the next couple of days. The official NHC track forecast has been adjusted slightly poleward once again at day 3 and beyond, bringing it closer to the reliable consensus aids TVCE and HCCA and accounting for the aforementioned possibility of some interaction between the two systems. Moderate northeasterly shear will persist for the next day or so, and afterwards the cooler sea-surface temperatures along Hilda's forecast track will offset any reduction in shear over the system. Therefore, weakening is expected during the next several days. The official NHC intensity forecast lies slightly above the multi-model consensus aids in the near-term, but then closely follows IVCN and HCCA once Hilda reaches the cooler waters. The system is forecast to degenerate into a remnant low by 96 h, but this transition could occur soon after 72 h based on some of the latest GFS and ECMWF simulated satellite imagery. FORECAST POSITIONS AND MAX WINDS INIT 03/0300Z 16.4N 123.3W 65 KT 75 MPH 12H 03/1200Z 17.1N 123.9W 60 KT 70 MPH 24H 04/0000Z 18.0N 124.8W 50 KT 60 MPH 36H 04/1200Z 18.9N 125.9W 45 KT 50 MPH 48H 05/0000Z 19.8N 127.2W 40 KT 45 MPH 60H 05/1200Z 20.8N 128.9W 35 KT 40 MPH 72H 06/0000Z 21.6N 130.9W 30 KT 35 MPH 96H 07/0000Z 22.5N 135.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 08/0000Z 22.6N 139.5W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Reinhart/Pasch

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Hurricane Hilda Forecast Discussion Number 13

2021-08-02 22:36:04| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Mon Aug 02 2021 000 WTPZ43 KNHC 022035 TCDEP3 Hurricane Hilda Discussion Number 13 NWS National Hurricane Center Miami FL EP082021 200 PM PDT Mon Aug 02 2021 Recent satellite images show that Hilda's cloud pattern has become less organized, with the low-level center on the northeast edge of the central dense overcast. Microwave data also show a less distinct eye feature, along with a more broken eyewall. The initial wind speed is lowered to 65 kt, which is closest to the latest CIMSS-SATCON value. Hilda continues moving northwestward and should remain on that heading for the next day or two while it is steered by the subtropical ridge located near and west of Mexico. No changes were made to the forecast during that time. After that, the ridge builds farther to the west and a weakened Hilda should respond by taking west-northwestward and westward turns by late week. However, exactly when that happens is difficult to tell due to potential interaction at long range with the remnants of former TD 9-E. A fair number of the models are showing that system regenerating and getting closer to Hilda, which would have the tendency to steer Hilda farther northward. While the new forecast responds to this scenario by shifting the official track about half a degree poleward on day 3 and beyond, a lot of the latest guidance is even farther north. It seems like Hilda might finally be on a steadier weakening trend with the recent degradation noted in satellite data. With moderate northeasterly shear and decreasing water temperatures in the path of the cyclone, Hilda should continue to lose strength during the next few days. Somewhat surprisingly, guidance is a little higher than the last cycle, which doesn't feel like the correct forecast move at this time, especially as the forecast shifts northward over cooler waters faster. The new NHC forecast maintains the weakening trend from the last advisory, and now ends up lower than the bulk of the guidance. Remnant-low status is anticipated just after day 3. FORECAST POSITIONS AND MAX WINDS INIT 02/2100Z 15.9N 122.8W 65 KT 75 MPH 12H 03/0600Z 16.5N 123.3W 60 KT 70 MPH 24H 03/1800Z 17.4N 124.2W 55 KT 65 MPH 36H 04/0600Z 18.3N 125.2W 50 KT 60 MPH 48H 04/1800Z 19.2N 126.5W 40 KT 45 MPH 60H 05/0600Z 20.1N 128.0W 35 KT 40 MPH 72H 05/1800Z 21.1N 129.9W 30 KT 35 MPH 96H 06/1800Z 22.0N 134.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 07/1800Z 22.0N 139.0W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Blake

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