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Tropical Storm RACHEL Forecast Discussion Number 23

2014-09-30 04:37:35| National Hurricane Center (East Pacific)

Issued at 800 PM PDT MON SEP 29 2014 000 WTPZ43 KNHC 300237 TCDEP3 TROPICAL STORM RACHEL DISCUSSION NUMBER 23 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182014 800 PM PDT MON SEP 29 2014 A burst of deep convection with cloud tops of -70C to -75C developed during the late afternoon in the same region where earlier ASCAT overpasses indicated several surface wind vectors of 40-43 kt. As a result, the intensity at 0000 UTC synoptic time was maintained at 45 kt. However, over the past couple of hours, cloud tops have warmed and decreased in areal coverage significantly, so the advisory intensity has been lowered to 40 kt. The initial motion estimate is 360/02 kt. Steering currents are collapsing as Rachel moves into a large break in the subtropical ridge created by a mid-latitude trough to the north of the cyclone. As a result, Rachel is expected to become nearly stationary fairly soon and meander in the same general area for the next 24 hours or so. After that, the ridge to the north of what should be a significantly weakened tropical cyclone is expected to build back in as the mid-latitude trough lifts out, driving Rachel or its remnants slowly toward the west or west-southwest until dissipation occurs in 72-96 hours. The new NHC forecast track is similar to the previous advisory track and the consensus model TVCE. Rachel is not long for this world due to the combination of strong southwesterly vertical wind shear, sea-surface temperatures less than 26C, and a cooler and drier airmass working its way into the inner core region of the cyclone. As a result, the storm is expected to weaken to a remnant low pressure system within 24 hours and dissipate by 96 hours. The official intensity forecast closely follows a blend of the Decay-SHIPS model and the consensus intensity model IVCN. FORECAST POSITIONS AND MAX WINDS INIT 30/0300Z 23.3N 117.5W 40 KT 45 MPH 12H 30/1200Z 23.3N 117.8W 35 KT 40 MPH 24H 01/0000Z 23.3N 118.1W 30 KT 35 MPH...POST-TROP/REMNT LOW 36H 01/1200Z 23.2N 118.4W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 02/0000Z 23.1N 118.7W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 03/0000Z 23.0N 120.3W 15 KT 15 MPH...POST-TROP/REMNT LOW 96H 04/0000Z...DISSIPATED $$ Forecaster Stewart

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Tropical Storm RACHEL Forecast Discussion Number 22

2014-09-29 22:36:34| National Hurricane Center (East Pacific)

Issued at 200 PM PDT MON SEP 29 2014 000 WTPZ43 KNHC 292036 TCDEP3 TROPICAL STORM RACHEL DISCUSSION NUMBER 22 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182014 200 PM PDT MON SEP 29 2014 Rachel continues to slowly weaken. Visible satellite imagery shows its exposed low-level center located on the south side of a small patch of deep convection, with the remnant mid-level circulation displaced well to the northeast. The cyclone's current ragged structure is partly a result of persistent south-southwesterly shear of around 25 kt as diagnosed by UW-CIMSS analyses. A 1715 UTC AMSU microwave pass also suggested that dry and stable air has been wrapping around the circulation of the storm. The initial intensity estimate is lowered to 45 kt based on peak winds of 43 kt from a 1714 UTC ASCAT-B pass. With strong southwesterly shear expected to continue and thermodynamic conditions likely to become even more unfavorable, further steady weakening is expected. These hostile environmental factors should cause Rachel to weaken to a remnant low in about 24 hours and dissipate in 2-3 days, as shown in global model guidance. The new NHC intensity forecast represents an update of the previous one and is in good agreement with the multi-model consensus. Rachel has been drifting steadily northward, and the initial motion estimate is 360/02. In the next 12 hours or so, a slow northward motion is likely to continue, so long as the cyclone maintains enough vertical integrity. After that time, Rachel should transition into a shallower vortex and be carried generally west-southwestward with a modest increase in forward speed. The NHC track forecast is adjusted a bit to the right and is somewhat south and west of the ECMWF, GFS, and multi-model consensus model, TVCE. FORECAST POSITIONS AND MAX WINDS INIT 29/2100Z 23.0N 117.5W 45 KT 50 MPH 12H 30/0600Z 23.2N 117.6W 35 KT 40 MPH 24H 30/1800Z 23.1N 118.0W 30 KT 35 MPH...POST-TROP/REMNT LOW 36H 01/0600Z 23.0N 118.2W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 01/1800Z 22.8N 118.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 02/1800Z 22.6N 120.0W 15 KT 15 MPH...POST-TROP/REMNT LOW 96H 03/1800Z...DISSIPATED $$ Forecaster Kimberlain/Lewitsky

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Tropical Storm RACHEL Forecast Discussion Number 21

2014-09-29 16:51:02| National Hurricane Center (East Pacific)

Issued at 800 AM PDT MON SEP 29 2014 000 WTPZ43 KNHC 291450 TCDEP3 TROPICAL STORM RACHEL DISCUSSION NUMBER 21 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182014 800 AM PDT MON SEP 29 2014 Rachel's cloud pattern has become considerably less organized. Deep convection associated with the cyclone has significantly decreased in coverage and intensity during the past several hours. First-light visible images also indicate that the center has become exposed on the south side of the nearest convection, the remnants of what was a central dense overcast 6 to 12 hours earlier. This change in structure has resulted from strong upper-level south- southwesterly winds, which is confirmed by UW-CIMSS analyses and SHIPS model output diagnosing 20-25 kt of shear. Satellite classifications are decreasing, and were T3.0/45 kt and 3.5/55 kt at 1200 UTC from SAB and TAFB, respectively. A blend of these yields an intensity of 50 kt, which is in line with data from an overnight ASCAT pass. Even though Rachel will be over marginally warm waters during the next several days, strong south-southwesterly vertical wind shear associated with a trough moving through the western United States should cause the cyclone to decouple within 24 hours. The shear, in combination with increasingly less conducive thermodynamic factors, should cause steady or even rapid weakening. Rachel is likely to become a remnant low in about 24 hours, and dissipation is indicated after 72 hours in agreement with the latest global model guidance. Rachel's forward motion has decreased further, and the initial motion estimate is 360/01. While still a vertically coupled vortex, the cyclone could inch northward or become stationary in a col area during the next 24 hours. Rachel should transition into a shallower cyclone after that time and be carried southwestward and westward with some increase in forward speed by the low-level trade wind flow. The new NHC track forecast is largely an update of the previous one but is adjusted slightly southward in light of the cyclone's reduction in forward speed. FORECAST POSITIONS AND MAX WINDS INIT 29/1500Z 22.8N 117.5W 50 KT 60 MPH 12H 30/0000Z 23.0N 117.5W 40 KT 45 MPH 24H 30/1200Z 23.1N 117.6W 30 KT 35 MPH...POST-TROP/REMNT LOW 36H 01/0000Z 22.9N 117.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 01/1200Z 22.7N 118.1W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 02/1200Z 22.0N 119.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 03/1200Z...DISSIPATED $$ Forecaster Kimberlain/Lewitsky

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Hurricane RACHEL Forecast Discussion Number 19

2014-09-29 04:58:01| National Hurricane Center (East Pacific)

Issued at 800 PM PDT SUN SEP 28 2014 000 WTPZ43 KNHC 290257 TCDEP3 HURRICANE RACHEL DISCUSSION NUMBER 19 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182014 800 PM PDT SUN SEP 28 2014 Rachel has made a little bit of a resurgence since the previous advisory, with a well-defined 15-20 nmi diameter low- to mid-level eye apparent in various microwave satellite images. Visible satellite imagery during the past few hours also indicates a small ring of overshooting cloud tops beginning to encircle a ragged, cloud-filled eye. The only reason the initial intensity is being maintained at 65 kt is due to the lack of persistence in the recent increase in the inner-core convection. The initial motion estimate is 355/4 kt, which is based primarily on microwave satellite fix positions. The most recent NHC track model guidance has become less divergent since the previous advisory, with the 18Z GFS model now taking Rachel more westward after 24 hours. Overall, the models are in pretty good agreement on steering currents collapsing during the next 48 hours, resulting in Rachel becoming nearly stationary during that time, followed by a westward or southwestward drift as a remnant low pressure system. The official forecast track is similar to the previous advisory advisory track and the consensus model TVCE. Rachel is currently over 26C sea-surface temperatures, and the small cyclone will be moving over slightly cooler water during the next 48 hours. The cooler water, in combination with increasing southwesterly vertical wind shear in excess of 25 kt by 36 hours, should induce gradual weakening throughout the forecast period with Rachel degenerating into a remnant low pressure system by 48 hours or so. This intensity forecast is consistent with a blend of the intensity consensus model IVCN and the Decay-SHIPS model. FORECAST POSITIONS AND MAX WINDS INIT 29/0300Z 22.5N 117.6W 65 KT 75 MPH 12H 29/1200Z 22.8N 117.6W 55 KT 65 MPH 24H 30/0000Z 23.0N 117.6W 45 KT 50 MPH 36H 30/1200Z 22.9N 117.8W 35 KT 40 MPH 48H 01/0000Z 22.8N 117.9W 30 KT 35 MPH...POST-TROP/REMNT LOW 72H 02/0000Z 22.5N 118.4W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 03/0000Z 22.4N 119.3W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 04/0000Z...DISSIPATED $$ Forecaster Stewart

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Hurricane RACHEL Forecast Discussion Number 18

2014-09-28 22:40:28| National Hurricane Center (East Pacific)

Issued at 200 PM PDT SUN SEP 28 2014 000 WTPZ43 KNHC 282040 TCDEP3 HURRICANE RACHEL DISCUSSION NUMBER 18 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182014 200 PM PDT SUN SEP 28 2014 The cloud pattern of Rachel has not changed very much in visible satellite imagery during the past several hours. A ragged banded-type eye has occasionally been evident in visible imagery and recent microwave data has shown a similar feature. The latest Dvorak intensity estimates have decreased slightly so the initial wind speed has been reduced to 65 kt. Increasing southwesterly vertical wind shear and a less favorable thermodynamic environment are expected to cause steady weakening during the next couple of days, and Rachel should weaken to a tropical depression in about 48 hours. The cyclone is forecast to degenerate to a remnant low shortly thereafter. The updated NHC intensity forecast is close to the multi-model consensus. Rachel appears to be slowing down, with an initial motion estimate of 340/4 kt. The latest track guidance has become more divergent with the GFS and HWRF taking Rachel more northward, then northeastward in southwesterly flow between a mid-level ridge over Mexico and a trough moving across the western United States. On the other hand, the ECMWF and GFS ensemble mean show less of a poleward motion as the cyclone weakens and become a more shallow system. The NHC forecast leans toward the latter solution and shows Rachel moving slowly northward during the next 24 hours, then turning west-southwestward in the low-level flow after weakening occurs. The new forecast is similar to, but a little slower than the previous NHC advisory. FORECAST POSITIONS AND MAX WINDS INIT 28/2100Z 21.7N 117.4W 65 KT 75 MPH 12H 29/0600Z 22.3N 117.5W 55 KT 65 MPH 24H 29/1800Z 22.6N 117.6W 45 KT 50 MPH 36H 30/0600Z 22.6N 117.7W 35 KT 40 MPH 48H 30/1800Z 22.5N 117.8W 30 KT 35 MPH 72H 01/1800Z 22.0N 118.3W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 02/1800Z 22.0N 119.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 03/1800Z...DISSIPATED $$ Forecaster Brown

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