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Tropical Storm RACHEL Forecast Discussion Number 11

2014-09-27 04:51:45| National Hurricane Center (East Pacific)

Issued at 800 PM PDT FRI SEP 26 2014 000 WTPZ43 KNHC 270251 TCDEP3 TROPICAL STORM RACHEL DISCUSSION NUMBER 11 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182014 800 PM PDT FRI SEP 26 2014 Geostationary and microwave satellite imagery show a much improved structure for Rachel this evening. A pronounced curved band has developed and wrapped around the center of the cyclone, while a mid-level eye is seen in 2002Z AMSR-E, 2130Z AMSU, and 0127Z SSMIS microwave passes. Correspondingly, the Dvorak classifications from TAFB and SAB have jumped up to a 3.5. This supports a 55 kt intensity, which may be conservative as the CIMSS AMSU method suggests maximum sustained winds of about 75 kt. It appears likely that Rachel is responding to a reduction in the vertical shear, as it has now dropped below 10 kt. Rachel has about 24 to 36 hr of conducive environmental conditions remaining before the atmosphere that it moves into becomes quite dry and stable. In about three days, the vertical shear increases which should hasten the demise of this system. The NHC intensity prediction is based upon the IVCN multi-model ensemble through 48 hr and upon the HWRF and GFDL hurricane models after that time. The peak intensity indicated here is above that from the previous advisory due to the jump in the initial intensity. The aforementioned AMSU pass also provided some guidance from the CIRA size technique for the initial tropical-storm-force wind radii, which have been expanded outward some as a result. The forecast wind radii, however, are anticipated to not grow much, based upon a blend of the dynamical and climatological models. Rachel's center apparently reformed southwestward closer to the deep convection earlier today, as suggested by a late arriving WindSat microwave pass. While the uncertainty in the initial position is low, the current motion is a bit ambiguous due to the reformation. It is estimated that the cyclone's center is moving toward the west-northwest at around 10 kt, primarily due to steering imparted by a mid-level subtropical ridge to its northeast. A vigorous shortwave trough near California is anticipated to induce a break in the ridge and allow Rachel to turn toward the north-northwest at a slower rate of forward speed by Sunday. In about three days, a decaying Rachel will start meandering in the weak low-level flow west of Baja California. The NHC track forecast is based upon the TVCE multi-model ensemble, which shows substantial divergence in solutions late in the forecast period, and is slightly east of the track shown in the previous advisory. FORECAST POSITIONS AND MAX WINDS INIT 27/0300Z 18.4N 115.3W 55 KT 65 MPH 12H 27/1200Z 19.1N 116.1W 60 KT 70 MPH 24H 28/0000Z 20.0N 116.4W 65 KT 75 MPH 36H 28/1200Z 20.9N 116.7W 65 KT 75 MPH 48H 29/0000Z 21.5N 116.9W 55 KT 65 MPH 72H 30/0000Z 22.1N 116.8W 40 KT 45 MPH 96H 01/0000Z 22.0N 117.0W 30 KT 35 MPH...POST-TROP/REMNT LOW 120H 02/0000Z 21.5N 117.5W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Landsea

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Tropical Storm RACHEL Forecast Discussion Number 10

2014-09-26 22:46:13| National Hurricane Center (East Pacific)

Issued at 200 PM PDT FRI SEP 26 2014 000 WTPZ43 KNHC 262046 TCDEP3 TROPICAL STORM RACHEL DISCUSSION NUMBER 10 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182014 200 PM PDT FRI SEP 26 2014 The center of Rachel appears to be reforming a little closer to the deep convection, and in fact looks somewhat elongated on the latest visible imagery. The SHIPS model and UW-CIMSS satellite analyses still show about 10 kt of shear affecting the cyclone, and overall there has been little net change to the cloud pattern today. The initial intensity remains 45 kt based on the latest Dvorak estimate from TAFB. The SHIPS model shows the shear decreasing over the next 24 hours, which should allow for some gradual strengthening, but the SHIPS intensity forecast has trended downward this cycle along with the GFDL. After 36 hours, gradual weakening is forecast as the cyclone moves into a drier and more stable environment over marginal SSTs and decay to a remnant low is expected in 4 to 5 days. The NHC forecast has been adjusted downward a little in the short range toward the latest IVCN intensity consensus and is close to IVCN after that time. Given the reformation of the center, the initial position is southwest of the previous advisory. Smoothing through this short- term motion yields a somewhat uncertain initial motion estimate of 295/10. The synoptic reasoning for the track forecast has not changed. Rachel is forecast to turn gradually northward into a break in the subtropical ridge with a decrease in forward speed during the next couple of days. After that time, the decaying cyclone is forecast to move little in a region of weak steering currents, with a slow southwestward motion possible by the end of the period. There continues to be a large spread in the track guidance after 36 hours, with the GFS and GFDL taking the cyclone farther north while the UKMET and ECMWF show a more westward to southwestward motion. The new NHC forecast track is generally between the two camps and is a little left of the TVCE multi-model consensus and not far from the HWRF. This track is west of the previous NHC advisory due to the initial position and motion. FORECAST POSITIONS AND MAX WINDS INIT 26/2100Z 18.1N 115.0W 45 KT 50 MPH 12H 27/0600Z 18.7N 115.9W 50 KT 60 MPH 24H 27/1800Z 19.7N 116.6W 55 KT 65 MPH 36H 28/0600Z 20.6N 117.1W 55 KT 65 MPH 48H 28/1800Z 21.3N 117.3W 50 KT 60 MPH 72H 29/1800Z 22.0N 117.3W 40 KT 45 MPH 96H 30/1800Z 22.0N 117.3W 30 KT 35 MPH 120H 01/1800Z 21.5N 117.5W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Brennan

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Tropical Storm RACHEL Forecast Discussion Number 9

2014-09-26 16:52:40| National Hurricane Center (East Pacific)

Issued at 800 AM PDT FRI SEP 26 2014 000 WTPZ43 KNHC 261452 TCDEP3 TROPICAL STORM RACHEL DISCUSSION NUMBER 9 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182014 800 AM PDT FRI SEP 26 2014 The organization of Rachel's cloud pattern has not changed this morning. The center of the cyclone remains on the northeast (upshear) side and outside of a large mass of very cold-topped convection, presumably a result of some northeasterly vertical wind shear. The deep convection continues to regularly burst, a trend that has been observed for a few days now. The initial intensity estimate is held at 45 kt in agreement with the 1200 UTC Dvorak fix from TAFB. Global models show Rachel moving underneath an upper- level ridge axis during the next 24 hours or so, which should result in a relaxation of the persistent shear and allow for some intensification since the storm will still be over warm enough waters. In 2-3 days, Rachel will have moved far enough to the north to experience an increase of southwesterly shear in response to a mid-latitude trough over the western United States. That shear and increasingly unfavorable thermodynamics factors, including somewhat cooler waters, are expected to hasten its weakening beyond 48 hours. The NHC intensity forecast is not much different than the previous one, and still shows remnant low status at the end of the forecast period. Rachel continues on a west-northwesterly track of 295/12. The cyclone is forecast to turn toward the northwest and north- northwest with a significant decrease in forward speed during the next couple of days when it encounters a weakness caused by the earlier-mentioned western United States trough. By 48 hours, there is a rather distinct split in the track guidance, with the GFS-based model solutions carrying the cyclone quickly northward and northeastward while the ECMWF/UKMET suggest a weaker cyclone steered toward the west-northwest and west. The NHC track forecast stays left of but closer to the GFS through 48 hours due to Rachel's forecast intensification and then shows slow motion for the remainder of the forecast period. The 48-120 hour portion of the track forecast is very near the multi-model consensus, TVCE, and it could trend westward in future forecasts if confidence in a weaker cyclone at that time frame increases. FORECAST POSITIONS AND MAX WINDS INIT 26/1500Z 18.2N 114.4W 45 KT 50 MPH 12H 27/0000Z 19.0N 115.2W 50 KT 60 MPH 24H 27/1200Z 20.0N 116.1W 55 KT 65 MPH 36H 28/0000Z 20.9N 116.8W 60 KT 70 MPH 48H 28/1200Z 21.8N 117.1W 50 KT 60 MPH 72H 29/1200Z 22.8N 117.2W 40 KT 45 MPH 96H 30/1200Z 23.0N 117.2W 30 KT 35 MPH 120H 01/1200Z 22.8N 117.4W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Kimberlain

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Tropical Storm RACHEL Forecast Discussion Number 8

2014-09-26 10:40:24| National Hurricane Center (East Pacific)

Issued at 200 AM PDT FRI SEP 26 2014 000 WTPZ43 KNHC 260840 TCDEP3 TROPICAL STORM RACHEL DISCUSSION NUMBER 8 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182014 200 AM PDT FRI SEP 26 2014 Microwave data indicate that Rachel's structure has changed little during the past few hours, and the maximum winds are therefore held at 45 kt. Deep convection remains limited to the southwest of the low-level center, although a recent expansion of the cold cloud tops in infrared satellite imagery suggests that the northeasterly shear may be relaxing just a bit. The SHIPS guidance indicates that the shear should gradually diminish, reaching a minimum in about 36 hours, so gradual strengthening is anticipated up through that time. After 36 hours, the shear is expected to increase while Rachel moves over marginally cooler water and into a drier, more stable environment. Steady weakening is expected to begin by 48 hours, with the cyclone likely becoming a remnant low by day 5. The updated NHC intensity forecast is not too dissimilar from the previous one. However, it should be noted that the 60-kt forecast peak intensity is a little higher than all of the guidance except the HWRF. Rachel's initial motion is 300/12 kt. The track models are in good agreement during the first 36-48 hours, showing Rachel slowing down and turning north-northwestward toward a break in the subtropical ridge. There is significant divergence in the guidance after 48 hours, however. The notable outliers are the GFS and the GFDL ensemble mean, which have a stronger cyclone being pulled northeastward toward the Baja California peninsula by a deep-layer trough over the southwestern United States. The other models show a weaker cyclone stalling and then turning southward or southwestward within the prevailing low-level flow. Due to the model divergence, very slow motion is indicated between days 3-5, and Rachel is expected to become nearly stationary before drifting southwestward during its remnant low stage. The guidance shifted a bit to the west on this cycle, and although the NHC forecast was also nudged in that direction, it still lies on the eastern edge of the envelope. FORECAST POSITIONS AND MAX WINDS INIT 26/0900Z 18.0N 112.9W 45 KT 50 MPH 12H 26/1800Z 18.7N 114.3W 50 KT 60 MPH 24H 27/0600Z 19.7N 115.4W 55 KT 65 MPH 36H 27/1800Z 20.6N 116.2W 60 KT 70 MPH 48H 28/0600Z 21.4N 116.7W 55 KT 65 MPH 72H 29/0600Z 22.7N 116.8W 45 KT 50 MPH 96H 30/0600Z 23.0N 116.5W 35 KT 40 MPH 120H 01/0600Z 22.5N 117.0W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Berg

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Tropical Storm RACHEL Forecast Discussion Number 7

2014-09-26 04:36:40| National Hurricane Center (East Pacific)

Issued at 800 PM PDT THU SEP 25 2014 000 WTPZ43 KNHC 260236 TCDEP3 TROPICAL STORM RACHEL DISCUSSION NUMBER 7 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182014 800 PM PDT THU SEP 25 2014 While there is strong, deep convection associated with Rachel, microwave and last light visible imagery indicate that the tropical storm is still being sheared with the center at the edge of the convective overcast. Subjective Dvorak assessments from TAFB and SAB as well as the CIMSS AMSU method suggest peak sustained winds of about 45 kt, which is the initial intensity. While an upper-level anticyclone is contributing toward about 15 kt of northeasterly shear over Rachel currently, this should subside to quite low values from 24 through 72 hr. However, at the same time, Rachel's track should take it over gradually cooler waters and into much drier, stable air. The NHC intensity forecast calls for slow intensification through two days, followed by gradual weakening. This prediction is based upon a blend of the LGEM and SHIPS statistical and GFDL dynamical models and is about the same as in the previous advisory. No in situ observations were available for the tropical-storm-force wind radii, so no changes were made to the initial small size of Rachel. The global and regional hurricane models suggest that Rachel will remain relatively small for the next few days, which is the basis for the NHC wind radii forecast. Rachel is moving toward the west-northwest at 13 kt, primarily being steered by a mid-level subtropical ridge to its northeast. The tropical storm should round the western periphery of the ridge in about two to three days. After that time, Rachel will meander as a decaying vortex in the weak lower tropospheric flow. The NHC track forecast is based upon the tightly clustered TVCE multi-model ensemble and is about the same as that issued in the previous advisory. FORECAST POSITIONS AND MAX WINDS INIT 26/0300Z 17.7N 111.9W 45 KT 50 MPH 12H 26/1200Z 18.4N 113.4W 50 KT 60 MPH 24H 27/0000Z 19.2N 114.7W 55 KT 65 MPH 36H 27/1200Z 20.2N 115.6W 60 KT 70 MPH 48H 28/0000Z 20.9N 116.2W 60 KT 70 MPH 72H 29/0000Z 22.2N 116.3W 50 KT 60 MPH 96H 30/0000Z 23.0N 116.0W 40 KT 45 MPH 120H 01/0000Z 23.0N 116.0W 25 KT 30 MPH $$ Forecaster Landsea

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