000 WTPZ42 KNHC 280238 TCDEP2 TROPICAL STORM MARTY DISCUSSION NUMBER 6 NWS NATIONAL HURRICANE CENTER MIAMI FL EP172015 1000 PM CDT SUN SEP 27 2015 Recent infrared and microwave satellite imagery indicate that a strong burst of deep convection has developed in the northwestern quadrant of the circulation, with very cold cloud tops having spread southwestward over the previously exposed low-level circulation center. This increase in Marty's cloud pattern suggests that the cyclone has strengthened despite moderate to strong westerly vertical wind shear. The initial intensity has been increased to 55 kt based on a Dvorak satellite intensity estimate of T3.5/55 kt from both TAFB and SAB. The initial motion is an uncertain 360/02 kt. Overall, Marty doesn't appear to have moved much, if at all, since the previous advisory. Steering currents are weak since the cyclone is embedded in the base of a broad deep-layer trough that extends from the Texas coast southward across central Mexico and into the eastern North Pacific. Only a slow north or northeastward drift if expected for the next two days. The high terrain of southern Mexico is likely helping to induce a narrow ridge just offshore the Pacific coast, which should prevent the cyclone from gaining much latitude. The global and regional models, excluding the GFS model, maintain some degree of low- to mid-level ridging throughout the forecast period, which gradually forces Marty on a westward track over water after 48 hours. In contrast, the GFS model marches the cyclone inland over Mexico in about 48 hours, but this is considered to be an outlier scenario since the GFS-Ensemble Mean keeps Marty well offshore similar to the ECMWF and the other model solutions. The official forecast track is essentially just an update of the previous track through 72 hours, and then is a little slower than the previous advisory after that as per the latest TVCE consensus model solution. The overall atmospheric and oceanic environments surrounding Marty are expected to essentially remain unchanged for the next 24 hours or so. As a result, little change in intensity is expected during that time. In the 36-48 hour time period, however, the global models are forecasting the vertical wind shear to back around to the southwest and increase to around 25 kt, which should induce steady weakening through at least 72 hours. By 96 hours, the shear is forecast to sharply decrease to around 10 kt or less, possibly giving Marty a chance to re-strengthen on days 4 and 5 if a decent low-level circulation still remains at that time. The new NHC intensity forecast follows the trend of the previous advisory in calling for Marty to become a depression by around 96 hours and a remnant low by 120 hours. However, the remnant low forecast is still somewhat uncertain. Given that Marty has slowed down considerably and is expected to move somewhat little slower than indicated in the previous advisory, a tropical storm warning is not necessary at this time. However, a tropical storm warning could be required for portions of the coast of Mexico by Monday morning. FORECAST POSITIONS AND MAX WINDS INIT 28/0300Z 15.3N 103.0W 55 KT 65 MPH 12H 28/1200Z 15.7N 102.6W 55 KT 65 MPH 24H 29/0000Z 16.2N 102.2W 55 KT 65 MPH 36H 29/1200Z 16.6N 102.0W 55 KT 65 MPH 48H 30/0000Z 16.8N 102.3W 45 KT 50 MPH 72H 01/0000Z 16.9N 103.5W 35 KT 40 MPH 96H 02/0000Z 17.0N 105.0W 30 KT 35 MPH 120H 03/0000Z 17.2N 106.0W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Stewart