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Tropical Storm Norman Forecast Discussion Number 3

2018-08-29 04:36:20| National Hurricane Center (East Pacific)

Issued at 900 PM MDT Tue Aug 28 2018 548 WTPZ41 KNHC 290236 TCDEP1 Tropical Storm Norman Discussion Number 3 NWS National Hurricane Center Miami FL EP162018 900 PM MDT Tue Aug 28 2018 The tropical cyclone has become significantly better organized over the past several hours. A band of very cold-topped convection now spirals well over halfway around the circulation. Dvorak T-numbers from both TAFB and SAB are up to 3.0, which corresponds to a current intensity of 45 kt. On this basis the system is being upgraded to a tropical storm, making Norman the fourteenth named tropical cyclone of this busy eastern North Pacific hurricane season. Since Norman will continue to move over warm waters and through an environment of low vertical shear and moist mid-level air, continued strengthening is likely. In fact, the various Rapid Intensification (RI) Indices show high probabilities of RI. Therefore the official forecast shows intensity changes of 30 and 35 kt per day from 0-24 hours and 24-48 hours respectively. This is also close to the simple intensity consensus but below the HFIP Corrected Consensus and the latest HWRF model run. The most recent geostationary and microwave satellite center fixes indicate that Norman is located a little south and southwest of the previous track, but the initial motion estimate remains about the same as before, 290/9 kt. The cyclone is moving along the southern periphery of a large mid-level ridge which is expanding westward. This should cause the system to turn toward the west very soon. Later in the period, the ridge builds a little to the southwest and this will likely cause Norman to turn toward the west-southwest. The official track forecast is a little south of the previous one, but close to the latest simple and corrected model consensus predictions. FORECAST POSITIONS AND MAX WINDS INIT 29/0300Z 17.5N 113.9W 45 KT 50 MPH 12H 29/1200Z 17.9N 115.2W 60 KT 70 MPH 24H 30/0000Z 18.2N 117.0W 75 KT 85 MPH 36H 30/1200Z 18.3N 118.6W 90 KT 105 MPH 48H 31/0000Z 18.1N 120.3W 110 KT 125 MPH 72H 01/0000Z 17.2N 124.1W 110 KT 125 MPH 96H 02/0000Z 16.0N 128.0W 100 KT 115 MPH 120H 03/0000Z 16.0N 132.0W 90 KT 105 MPH $$ Forecaster Pasch

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Tropical Storm Miriam Forecast Discussion Number 12

2018-08-29 04:33:18| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Tue Aug 28 2018 680 WTPZ45 KNHC 290233 TCDEP5 Tropical Storm Miriam Discussion Number 12 NWS National Hurricane Center Miami FL EP152018 800 PM PDT Tue Aug 28 2018 There's been little change in the overall cloud pattern of Miriam during the past several hours. An earlier Advanced Technology Microwave Sounder (ATMS) overpass and a 2053 UTC GCOM-W1 AMSR2 image indicate that the surface center is still sheared to the northwest of the deep convective banding feature encompassing the southern portion of the cyclone. Dvorak satellite intensity estimates from TAFB and SAB remain unchanged, and a 2213 UTC SATCON analysis showed 51 kt. Based on these data, the initial intensity is held at 50 kt. The intensity forecast is rather problematic this evening. Decay SHIPS guidance (both GFS and ECMWF background fields) shows the moderate northwesterly shear increasing after 48 hours, while indicating little change in strength up to that period. After that time, these statistical/dynamical guidance models quickly degenerate the cyclone into a remnant low in 4 days. The NOAA-HCCA, HWRF, and the IVCN consensus model, on the other hand, all show Miriam becoming a hurricane in 36-48 hours. For this advisory, I've elected to maintain continuity and show gradual strengthening to just below hurricane strength in 36 hours, with a weakening trend commencing in 3 days. All the deterministic models agree with Miriam becoming a remnant low in 5 days, and this is reflected in the NHC forecast. The initial motion is estimated to be westward, or 270/11 kt, within the easterly mid-level steering flow produced by a mid-tropospheric high to the north of Miriam. The cyclone is expected to turn toward the northwest in 36 hours, then rather abruptly north-northwestward in 2 days and continuing on through day 4, in response to a large mid- to upper level cutoff low digging southwestward toward the Hawaiian Islands. A large spread in the models still exists in the latter portion of the forecast, as mentioned in the previous advisory. The track forecast philosophy remains unchanged with a nudge more toward a blend of the TVCN consensus and the global models that show a gradual turn back toward the northwest due to a shallower vertical structure near the end of the period. FORECAST POSITIONS AND MAX WINDS INIT 29/0300Z 14.1N 137.4W 50 KT 60 MPH 12H 29/1200Z 14.2N 138.9W 50 KT 60 MPH 24H 30/0000Z 14.5N 140.4W 55 KT 65 MPH 36H 30/1200Z 15.4N 141.4W 60 KT 70 MPH 48H 31/0000Z 16.7N 141.8W 60 KT 70 MPH 72H 01/0000Z 20.2N 142.0W 55 KT 65 MPH 96H 02/0000Z 23.4N 143.6W 40 KT 45 MPH 120H 03/0000Z 26.5N 146.7W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Roberts

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Tropical Storm Miriam Forecast Discussion Number 11

2018-08-28 22:35:16| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Tue Aug 28 2018 991 WTPZ45 KNHC 282035 TCDEP5 Tropical Storm Miriam Discussion Number 11 NWS National Hurricane Center Miami FL EP152018 200 PM PDT Tue Aug 28 2018 Northwesterly shear has taken a toll on Miriam, with visible satellite imagery depicting an exposed low-level center located northwest of the deep convection. Subjective Dvorak T-numbers have decreased to 3.0 by both SAB and TAFB, and a blend of the T- and CI-numbers yields an initial intensity of 50 kt for this advisory. Model guidance suggests that the northwesterly shear that is currently affecting the cyclone will continue for at least the next 24 hours, but it could diminish slightly by late Wednesday, allowing for some modest strengthening in 36 to 48 hours. Thereafter, a mid- to upper-level trough to the northwest of the cyclone is expected to bring about increasing southwesterly shear over the system, which is expected to cause steady weakening in 3 to 4 days. The cyclone is then expected to become a post-tropical cyclone by 120 hours as shear increases even further, and the system crosses cooler SSTs. The official intensity forecast has been lowered for much of the forecast period, reflecting both the lowering of the initial intensity and the expected continuation of the shear. Although not explicitly shown in the official forecast, there is still some chance that Miriam could reach hurricane status in a couple of days, before the shear increases. The initial motion continues to be due west, or 270/11 kt, as Miriam continues to be steered around the southern periphery of a subtropical ridge. The track forecast philosophy for the next few days remains unchanged as the aforementioned mid- to upper-level trough northeast of Hawaii weakens the western periphery of the ridge, and Miriam is forecast to turn northwestward then north-northwestward between the trough and ridge. There continues to be a large spread in the guidance by day 5, with the ECMWF and UKMET taking a more vertically coherent system northward, while the GFS, HWRF, and HMON turn Miriam westward as the system weakens and becomes a more shallow system. The latter scenario is beginning to seem more plausible and the NHC track forecast was adjusted southward and westward, but additional changes may be required if future forecasts shown a faster rate of demise. FORECAST POSITIONS AND MAX WINDS INIT 28/2100Z 14.1N 136.4W 50 KT 60 MPH 12H 29/0600Z 14.2N 138.0W 50 KT 60 MPH 24H 29/1800Z 14.4N 139.8W 50 KT 60 MPH 36H 30/0600Z 14.9N 141.1W 55 KT 65 MPH 48H 30/1800Z 16.1N 141.7W 60 KT 70 MPH 72H 31/1800Z 19.3N 142.0W 55 KT 65 MPH 96H 01/1800Z 22.6N 143.1W 40 KT 45 MPH 120H 02/1800Z 26.0N 146.0W 30 KT 35 MPH...POST-TROP/REMNT LOW $$ Forecaster Brown/Latto

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Tropical Storm Miriam Forecast Discussion Number 10

2018-08-28 16:56:12| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Tue Aug 28 2018 674 WTPZ45 KNHC 281456 TCDEP5 Tropical Storm Miriam Discussion Number 10 NWS National Hurricane Center Miami FL EP152018 800 AM PDT Tue Aug 28 2018 A very recently arriving high-resolution microwave overpass indicates that Miriam is a sheared tropical cyclone. The GPM pass shows that the center of Miriam is located well northwest of the primary mass of deep convection due to moderate northwesterly shear. The center of the cyclone is located well west of the earlier position estimates, which has also required a re-location. The most recent Dvorak estimates from TAFB and SAB, as well as UW-CIMSS/ADT, support an initial intensity, however, this could be generous given the recent microwave data. Environmental conditions are not expected to change over the next 24 hours, with possibly a slight reduction in the shear in 24 to 48 hours. This would suggest that Miriam should only gradually strengthen over the next couple of days and it is likely to reach its peak intensity in 2-3 days. After that time, increasing southwesterly shear and cooler waters are expected to initiate weakening. By 96 hours, the shear is forecast to become quite strong and Miriam will be moving over sub 26C SSTs. These progressively hostile conditions should cause Miriam to become a post-tropical cyclone by 120 hours. The latest intensity forecast is close to the FSU Superensemble, and generally in between the ICON and HCCA consensus models. The initial motion continues to be due westward, or 270/12 kt, as Miriam continues to be steered around the southern periphery of a subtropical ridge. A mid- to upper-level trough northeast of Hawaii will weaken the western periphery of the ridge over the next couple of days, and Miriam is forecast to turn northwest then north-northwest, between the ridge and the aforementioned trough. Forecast models diverge regarding the forward speed of Miriam at days 3-5, depending how they handle the interaction of Miriam with the the trough to its northwest. The ECMWF continues to favor an accelerating northward track as Miriam gets caught in strong southerly flow on the eastern side of the trough. The GFS is much farther southwestward, as it appears to take a more shallow Miriam westward later in the forecast period. Since the consensus aids remain clustered between these two scenarios, the latest forecast NHC track is once again near the various consensus models. FORECAST POSITIONS AND MAX WINDS INIT 28/1500Z 14.2N 135.4W 55 KT 65 MPH 12H 29/0000Z 14.2N 136.9W 55 KT 65 MPH 24H 29/1200Z 14.3N 138.9W 60 KT 70 MPH 36H 30/0000Z 14.7N 140.4W 65 KT 75 MPH 48H 30/1200Z 15.6N 141.3W 75 KT 85 MPH 72H 31/1200Z 18.5N 141.8W 75 KT 85 MPH 96H 01/1200Z 22.0N 142.6W 50 KT 60 MPH 120H 02/1200Z 26.0N 144.8W 35 KT 40 MPH...POST-TROPICAL $$ Forecaster Brown/Latto

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Tropical Depression Sixteen-E Forecast Discussion Number 1

2018-08-28 16:40:45| National Hurricane Center (East Pacific)

Issued at 900 AM MDT Tue Aug 28 2018 968 WTPZ41 KNHC 281440 TCDEP1 Tropical Depression Sixteen-E Discussion Number 1 NWS National Hurricane Center Miami FL EP162018 900 AM MDT Tue Aug 28 2018 Since the previous scatterometer pass around 0430 UTC, the satellite presentation has improved significantly. The cloud pattern now consists of a couple of cyclonically curved convective bands, with the center of the system located on the northeastern edge of a circular area of thunderstorms, as indicated by a GMI microwave pass at 1201 UTC. Satellite intensity estimates from TAFB and SAB are T2.5 and T2.0, respectively, on the Dvorak scale, and on this basis, advisories are initiated on Tropical Depression Sixteen-E with an initial intensity of 30 kt. The depression has plenty of time to intensify given that the environmental conditions of warm ocean and light shear are expected to prevail during the next 5 days. The NHC forecast calls for the depression to become a hurricane in about 2 days with additional strengthening thereafter. The forecast is very close to the intensity consensus model, and follows the trend of most of the guidance. Since genesis has just occurred, the initial motion is somewhat uncertain. The best estimate is toward the west-northwest or 295 degrees at 9 kt. The depression is currently located on the southwestern edge of a subtropical high, and this flow pattern will continue to steer the depression on the same general track during the next day or two. After that time, the nose of the ridge is expected to amplify, and will force the cyclone to turn toward the west or even west-southwest. This is very consistent with the track guidance, and the NHC forecast lies in between the HCCA corrected consensus model and the other consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 28/1500Z 17.1N 111.8W 30 KT 35 MPH 12H 29/0000Z 17.7N 112.9W 35 KT 40 MPH 24H 29/1200Z 18.4N 114.6W 45 KT 50 MPH 36H 30/0000Z 18.8N 116.4W 55 KT 65 MPH 48H 30/1200Z 18.9N 118.1W 65 KT 75 MPH 72H 31/1200Z 18.5N 121.8W 80 KT 90 MPH 96H 01/1200Z 17.5N 126.0W 85 KT 100 MPH 120H 02/1200Z 17.0N 130.0W 85 KT 100 MPH $$ Forecaster Avila

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