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Hurricane RACHEL Forecast Discussion Number 17

2014-09-28 16:45:25| National Hurricane Center (East Pacific)

Issued at 800 AM PDT SUN SEP 28 2014 000 WTPZ43 KNHC 281445 TCDEP3 HURRICANE RACHEL DISCUSSION NUMBER 17 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182014 800 AM PDT SUN SEP 28 2014 Rachel's cloud pattern has become much less organized. The central dense overcast has taken on a distorted appearance, with cold-topped deep convection more asymmetrically distributed. An intermittent eye seen earlier in nighttime visible imagery disappeared several hours ago. Although Dvorak intensity estimates are unchanged from 0600 UTC, the initial intensity estimate is lowered to 70 kt, given the recent degradation of the cyclone's satellite signature. A considerable increase in southwesterly shear is expected during the next 24-48 hours in association with a deep mid-latitude trough passing through the western United States. The increase in shear, coupled with substantially less conducive thermodynamic factors, are expected to result in steady weakening that could become more rapid than indicated in the forecast. The NHC wind speed forecast shows a faster rate of weakening relative to the previous one, but remnant low status is still indicated by day 3. Rachel continues on a slow north-northwestward track, or 340/06, on the western periphery of a mid-level ridge near western Mexico. The cyclone's forward speed is expected to decrease further during the next day or so as it reaches a col area, and Rachel may come to a temporary halt in about 36 hours. With significant weakening occurring during this time, a much-shallower Rachel should begin to come under the influence of the low-level trade wind flow after this time and turn southwestward and westward with increasing forward speed. The NHC track forecast is similar to the previous one and the bulk of the track guidance but not as far south and west as the ECMWF solution. Only the GFS continues to insist on a track farther to the right and faster as a consequence of a stronger and more coupled vortex, responding to the deep-layer flow associated with the previously mentioned mid-latitude trough. FORECAST POSITIONS AND MAX WINDS INIT 28/1500Z 21.5N 117.2W 70 KT 80 MPH 12H 29/0000Z 22.0N 117.4W 65 KT 75 MPH 24H 29/1200Z 22.4N 117.5W 55 KT 65 MPH 36H 30/0000Z 22.3N 117.7W 45 KT 50 MPH 48H 30/1200Z 22.1N 117.9W 35 KT 40 MPH 72H 01/1200Z 21.3N 118.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 02/1200Z 21.0N 120.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 03/1200Z 20.5N 122.5W 15 KT 15 MPH...POST-TROP/REMNT LOW $$ Forecaster Kimberlain

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Hurricane RACHEL Forecast Discussion Number 16

2014-09-28 10:40:21| National Hurricane Center (East Pacific)

Issued at 200 AM PDT SUN SEP 28 2014 000 WTPZ43 KNHC 280840 TCDEP3 HURRICANE RACHEL DISCUSSION NUMBER 16 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182014 200 AM PDT SUN SEP 28 2014 Rachel's eye disappeared for a few hours, but it is now becoming distinct again. Final-T numbers from TAFB and SAB decreased at 0600 UTC, but the initial intensity is held at 75 kt based on CI numbers of 4.5/77 kt and an ADT estimate of T4.4/75 kt. If Rachel is going to strengthen further, it probably has another 6-12 hours to do so before vertical shear begins to increase out of the south and southwest. In addition, Rachel will be moving into a drier environment over the next couple of days, and upwelling of colder ocean water could have a greater-than-normal influence on the intensity since the cyclone will become nearly stationary in a day or two. The SHIPS and LGEM models show Rachel increasing in strength just a bit later today and then only gradually weaken the system through 48 hours. In contrast, the GFDL, HWRF, and FSU Superensemble show weakening beginning soon and predict a faster weakening rate during the next two days. Given the hurricane's improving appearance in satellite imagery, the official forecast still allows some strengthening in the short term. After 24 hours, the NHC forecast shows steady weakening and essentially splits the difference between the two model scenarios. Rachel is moving slowly north-northwestward--335 degrees at 5 kt-- through a break in the subtropical ridge caused by a deep-layer low over the western United States. The steering currents near Rachel are forecast to collapse during the next few days after the deep-layer low moves eastward, and the cyclone is expected to become nearly stationary in 36-48 hours. Once the vertical shear increases, the low-level circulation is expected to begin moving southwestward and west-southwestward in the trade wind flow. The track guidance, in particular the ECMWF, is not as far south on this cycle, and the new NHC track forecast is therefore shifted north and west during the remnant low phase. FORECAST POSITIONS AND MAX WINDS INIT 28/0900Z 21.1N 117.0W 75 KT 85 MPH 12H 28/1800Z 21.6N 117.2W 80 KT 90 MPH 24H 29/0600Z 22.2N 117.4W 70 KT 80 MPH 36H 29/1800Z 22.4N 117.5W 60 KT 70 MPH 48H 30/0600Z 22.2N 117.6W 45 KT 50 MPH 72H 01/0600Z 21.6N 118.2W 30 KT 35 MPH...POST-TROP/REMNT LOW 96H 02/0600Z 21.3N 119.2W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 03/0600Z 21.0N 121.0W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Berg

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Hurricane RACHEL Forecast Discussion Number 15

2014-09-28 04:31:55| National Hurricane Center (East Pacific)

Issued at 800 PM PDT SAT SEP 27 2014 000 WTPZ43 KNHC 280231 TCDEP3 HURRICANE RACHEL DISCUSSION NUMBER 15 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182014 800 PM PDT SAT SEP 27 2014 Satellite imagery shows that the eye of Rachel has become somewhat better defined during the past several hours, and that the cloud tops in the eyewall have cooled. Satellite intensity estimates are now 77 kt from TAFB and SAB, and an objective estimate from the CIMSS ADT is 75 kt. Based on these data, the initial intensity is increased to 75 kt. The initial motion is now 340/6. Rachel is moving into a weakness in the subtropical ridge caused by a large deep-layer trough over the western United States. The large-scale models forecast the trough to move eastward after 12-24 hours, leaving Rachel stuck in an area of weak steering currents for a day or two. Beyond that time, Rachel or its remnants should be steered southwestward by a low-level ridge over the northeastern Pacific. There is little change in the guidance since the last advisory, and the new forecast track is almost identical to the previous forecast. Some additional strengthening is possible tonight. After that, a combination of increasing shear and the entrainment of a dry and stable airmass should cause a quick weakening. The new intensity forecast follows the previous forecast in calling for Rachel to degenerate to a remnant low in about three days time. The new forecast is a little stronger than the previous forecast from 12-48 hours based on the initial intensity, and it is in best agreement with the SHIPS model. FORECAST POSITIONS AND MAX WINDS INIT 28/0300Z 20.7N 116.6W 75 KT 85 MPH 12H 28/1200Z 21.4N 116.9W 80 KT 90 MPH 24H 29/0000Z 22.1N 117.1W 70 KT 80 MPH 36H 29/1200Z 22.2N 117.2W 60 KT 70 MPH 48H 30/0000Z 22.0N 117.3W 50 KT 60 MPH 72H 01/0000Z 21.0N 118.0W 30 KT 35 MPH...POST-TROP/REMNT LOW 96H 02/0000Z 20.5N 118.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 03/0000Z 19.5N 119.0W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Beven

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Tropical Storm RACHEL Forecast Discussion Number 13

2014-09-27 16:34:18| National Hurricane Center (East Pacific)

Issued at 800 AM PDT SAT SEP 27 2014 000 WTPZ43 KNHC 271434 TCDEP3 TROPICAL STORM RACHEL DISCUSSION NUMBER 13 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182014 800 AM PDT SAT SEP 27 2014 Rachel continues to exhibit an occasional eye-like feature in infrared satellite imagery, and this was also seen in an AMSR2 pass just after 0900 UTC. However, the overall convective pattern is not terribly impressive, with only a small area of deep convection near the center and little in the way of banding. The initial intensity remains 55 kt based on the latest Dvorak estimates from TAFB and SAB. With the inner core trying to organize, there is the potential for Rachel to reach hurricane intensity during in the next 12 to 18 hours before the thermodynamic environment becomes more hostile. After that time, Rachel will be moving over marginal SSTs and into an increasingly dry and stable atmosphere, which should result in steady weakening. The new NHC forecast shows Rachel peaking in 12 hours, and after that time is close to the IVCN intensity consensus while the cyclone decays. The initial motion estimate is 325/07. Rachel should continue turning poleward while it moves into a weakness in the subtropical ridge during the next 24 hours. After that time, the decaying cyclone will be caught in a region of weak steering currents, and a slow north-northwestward motion is shown at 36 to 48 hours. Later in the period, the shallow cyclone should begin moving southwestward as it comes under the influence of a low-level ridge over the northeastern Pacific. The track model guidance still shows a fair amount of spread, but the GFS and GFDL models have continued to trend southward this cycle. As a result, the TVCE multi-model consensus has shifted to the left, and the latest NHC track has been adjusted in that direction at 36 to 48 hours. Otherwise, the new NHC forecast is largely an update of the previous advisory when accounting for the initial position and motion. FORECAST POSITIONS AND MAX WINDS INIT 27/1500Z 19.6N 116.1W 55 KT 65 MPH 12H 28/0000Z 20.3N 116.5W 65 KT 75 MPH 24H 28/1200Z 21.2N 116.9W 60 KT 70 MPH 36H 29/0000Z 21.7N 117.1W 55 KT 65 MPH 48H 29/1200Z 21.9N 117.2W 50 KT 60 MPH 72H 30/1200Z 21.7N 117.5W 35 KT 40 MPH 96H 01/1200Z 21.0N 118.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 02/1200Z 20.0N 118.5W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Brennan

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Tropical Storm RACHEL Forecast Discussion Number 12

2014-09-27 10:38:48| National Hurricane Center (East Pacific)

Issued at 200 AM PDT SAT SEP 27 2014 000 WTPZ43 KNHC 270838 TCDEP3 TROPICAL STORM RACHEL DISCUSSION NUMBER 12 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182014 200 AM PDT SAT SEP 27 2014 Rachel's convective structure has evolved since the last advisory. The long curved band noted earlier has become broken and is separated from a small, compact area of deep convection which has developed over the low-level center. An eye-like feature has also occasionally appeared in infrared satellite imagery. Maximum winds are held at 55 kt based on a consensus of T3.5/55 kt Dvorak estimates from TAFB and SAB. Rachel is forecast to be in a low-shear environment for the next 36 hours or so, and the atmosphere should be sufficiently moist and unstable to support some strengthening in the short term. After 48 hours, southwesterly vertical shear of 15-20 kt is forecast to develop, and Rachel will be moving into a much drier and stable environment. The intensity models are in good agreement that Rachel should be at or just below hurricane intensity in 24 hours, which is indicated in the official forecast. Thereafter, the dynamical models show a faster rate of weakening than the statistical models, and the NHC forecast closely follows the multi-model consensus ICON. Rachel has turned northwestward, or 315/7 kt, into a weakness in the subtropical ridge caused by two deep-layer lows over California and the Rio Grande Valley. As the low over California moves eastward, steering currents are expected to collapse, and Rachel should turn north-northwestward but slow down and become nearly stationary in 2-3 days. Once vertical shear increases on day 3, a less-vertically-deep Rachel is expected to begin drifting southwestward in the prevailing low-level flow. The GFS, GFDL, and GFDL ensemble mean have begun to pull back a bit on their previous trend of showing Rachel turning northeastward toward the Baja California peninsula, although there is still considerable spread in the track guidance by days 4 and 5. The updated NHC track forecast essentially maintains continuity with the previous forecast and is not too far from the multi-model consensus TVCE. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 19.1N 115.8W 55 KT 65 MPH 12H 27/1800Z 19.8N 116.3W 60 KT 70 MPH 24H 28/0600Z 20.7N 116.6W 65 KT 75 MPH 36H 28/1800Z 21.5N 116.8W 60 KT 70 MPH 48H 29/0600Z 21.9N 116.9W 50 KT 60 MPH 72H 30/0600Z 22.1N 117.0W 35 KT 40 MPH 96H 01/0600Z 21.5N 117.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 02/0600Z 21.0N 118.0W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Berg

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