000 WTPZ43 KNHC 232042 TCDEP3 Tropical Storm Marty Discussion Number 3 NWS National Hurricane Center Miami FL EP132021 300 PM MDT Mon Aug 23 2021 Marty remains a sheared tropical cyclone due to northeasterly vertical wind shear of about 10 kt as assessed by the GFS and ECMWF models, which seems underdone given the pronounced erosion of the deep convection since the previous advisory. Having said that, Marty has still managed to strengthen a little to 40 kt, which is based on a 1521Z ASCAT-A pass that contained one 39-kt surface wind vector embedded within a large field of 35-37-kt wind vectors located over most of the northwest quadrant of the cyclone's circulation. The initial motion estimate remains a little north of due westward, or 280/13 kt. There are no significant changes to the previous track forecast or synoptic reasoning. A strong ridge to the north of Marty is expected to remain dominant across the eastern Pacific basin, which should act to keep the cyclone moving on a general westward track for the next 48 hours. Thereafter, the ridge is forecast by the global and regional models to build slightly southward, which should force a weakening Marty in a west-southwestward direction. The new NHC track forecast is similar to but a little north, or right, of the previous advisory track, and lies along the southern edge of tightly packed consensus track models, which have once again shifted slightly northward. Although global model diagnostics indicate that the northeasterly shear has abated to about 10 kt, one would never guess that by looking at the visible and infrared satellite trends over the past few hours, which show a noticeable erosion of the overall convective pattern. Although the convection should re-develop some overnight during the convective maximum period, it is unlikely that Marty will strengthen due to the entrainment of nearby low-level stable stratocumulus clouds that will act to reduce the amount of available instability. The latest model runs continue to forecast the deep-layer shear to weaken a little more and remain low through the remainder of the forecast period. However, Marty will be moving over sub-26C SSTs and into a drier and more stable airmass by 24 hours, and those unfavorable environmental conditions should work to offset the favorable shear conditions, resulting in slow but steady weakening in the 24-96-h period. Marty is forecast to become a depression by Tuesday night or Wednesday, degenerate into a remnant low by Thursday, and dissipate on Friday. The official intensity forecast is essentially just an update of the previous NHC forecast, and closely follows the simple- and corrected consensus models IVCN, NOAA-HCCA, and FSSE. FORECAST POSITIONS AND MAX WINDS INIT 23/2100Z 20.9N 114.1W 40 KT 45 MPH 12H 24/0600Z 21.0N 116.2W 40 KT 45 MPH 24H 24/1800Z 21.0N 118.6W 35 KT 40 MPH 36H 25/0600Z 21.0N 120.6W 30 KT 35 MPH 48H 25/1800Z 20.9N 122.6W 30 KT 35 MPH...POST-TROP/REMNT LOW 60H 26/0600Z 20.9N 124.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 26/1800Z 20.8N 127.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 27/1800Z 19.4N 131.9W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 28/1800Z...DISSIPATED $$ Forecaster Stewart