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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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