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Post-Tropical Cyclone Marty Forecast Discussion Number 7

2021-08-24 22:41:15| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Tue Aug 24 2021 738 WTPZ43 KNHC 242041 TCDEP3 Post-Tropical Cyclone Marty Discussion Number 7 NWS National Hurricane Center Miami FL EP132021 200 PM PDT Tue Aug 24 2021 There has been no significant deep convection within 100 nmi of Marty's center for more than 12 hours. As a result, Marty has degenerated into a remnant low. The initial intensity has been maintained at 30 kt based on ASCAT-B/-A passes between 1700-1800 UTC that still showed a significant fetch of 25-kt winds over much of the northern quadrant, including a few embedded 28-kt vectors. Since Marty is forecast to remain over sub-26C sea-surface temperatures and within a large field of cold-air stratocumulus clouds, gradual spin down of the vortex and weakening of the cyclone's peak winds are expected until dissipation occurs in about 72 hours. The new NHC intensity forecast is similar to the previous advisory, and closely follows a blend of the IVCN and NOAA-HCCA intensity consensus models. The initial motion estimate remains westward, or 270/10 kt. For the next 36 hours or so, the remnant low is forecast to move westward along the southern periphery of a sprawling deep-layer ridge located northwest through northeast of Marty. Thereafter, a motion toward the west-southwest is expected until the cyclone dissipates. The new NHC forecast track is essentially just an extension of the previous advisory track, and lies down the middle of the tightly packed consensus models, which have shifted a little to the south on this forecast cycle. This is the last advisory being issued on Marty. For additional information on the remnant low, please see High Seas Forecasts issued by the National Weather Service under AWIPS header NFDHSFEPI and WMO header FZPN02 KWBC. FORECAST POSITIONS AND MAX WINDS INIT 24/2100Z 20.5N 119.2W 30 KT 35 MPH...POST-TROP/REMNT LOW 12H 25/0600Z 20.5N 120.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 24H 25/1800Z 20.5N 122.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 36H 26/0600Z 20.4N 125.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 48H 26/1800Z 20.2N 127.2W 20 KT 25 MPH...POST-TROP/REMNT LOW 60H 27/0600Z 19.8N 129.6W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 27/1800Z...DISSIPATED $$ Forecaster Stewart

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Tropical Depression Marty Forecast Discussion Number 6

2021-08-24 16:33:07| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Tue Aug 24 2021 000 WTPZ43 KNHC 241433 TCDEP3 Tropical Depression Marty Discussion Number 6 NWS National Hurricane Center Miami FL EP132021 800 AM PDT Tue Aug 24 2021 There has been no significant deep convection within 100 nmi of Marty's center since about 0600 UTC. The cyclone's cloud pattern now consists of some mid-level clouds and mostly low-level cold-air stratocumulus clouds. The current intensity of 30 kt is based on a unanimous Dvorak satellite classification of T1.5/2.5 (25 kt/35 kt) from TAFB and SAB, which has resulted in Marty being downgraded to a tropical depression. Due to the cyclone's future track remaining over sub-26C sea-surface temperatures, coupled with additional entrainment of stable low clouds, Marty is now forecast to degenerate to a post-tropical remnant low later today. The new NHC intensity forecast closely follows the trends of the intensity consensus models IVCN and NOAA-HCCA. The initial motion remains westward, or 270/10 kt. The strong deep-layer ridge to the north of the weakening shallow cyclone is expected to keep Marty on a generally westward track over the next 36 hours or so, followed by a west-southwestward motion thereafter until the cyclone dissipates. The new official forecast track lies near the extreme southern edge of the model guidance envelope, owing to the models' slight poleward bias noted over the past 36 hours. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 20.6N 118.1W 30 KT 35 MPH 12H 25/0000Z 20.6N 119.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 24H 25/1200Z 20.6N 121.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 36H 26/0000Z 20.6N 124.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 48H 26/1200Z 20.5N 126.1W 20 KT 25 MPH...POST-TROP/REMNT LOW 60H 27/0000Z 20.2N 128.4W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 27/1200Z 19.6N 130.9W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 28/1200Z...POST-TROP/REMNT LOW $$ Forecaster Stewart

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Tropical Storm Marty Forecast Discussion Number 5

2021-08-24 10:39:03| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Tue Aug 24 2021 000 WTPZ43 KNHC 240838 TCDEP3 Tropical Storm Marty Discussion Number 5 NWS National Hurricane Center Miami FL EP132021 200 AM PDT Tue Aug 24 2021 Satellite imagery indicates that Marty is currently a swirl of low-level clouds with no associated convection, with this likely due to a combination of northeasterly vertical shear and marginal sea surface temperatures. Recent ASCAT data shows maximum winds of 30-35 kt over a small area in the northern semicircle, so the initial intensity is reduced to 35 kt. The initial motion is still westward, but slower than before, at 270/11 kt. The subtropical ridge to the north of Marty will continue to steer the cyclone generally westward over the next day or two. After that, the weakening cyclone is expected to turn west- southwestward during the latter part of the week within the low-level trade wind flow. The new forecast track is similar to, but a little slower than the previous forecast, and it lies near the various consensus models. Although the current shear is forecast to diminish, a combination of decreasing sea surface temperatures and a dryer air mass along the forecast track should cause continued gradual weakening. The new intensity forecast shows the system weakening to a depression in about 24 h and degenerating to a remnant low by 48 h. If convection does not re-develop near the center, both of these events could occur earlier than forecast. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 20.6N 117.0W 35 KT 40 MPH 12H 24/1800Z 20.6N 118.7W 35 KT 40 MPH 24H 25/0600Z 20.6N 120.9W 30 KT 35 MPH 36H 25/1800Z 20.7N 122.8W 25 KT 30 MPH 48H 26/0600Z 20.6N 124.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 26/1800Z 20.4N 127.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 27/0600Z 19.8N 129.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 28/0600Z 18.5N 134.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 29/0600Z...DISSIPATED $$ Forecaster Beven

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Tropical Storm Marty Forecast Discussion Number 4

2021-08-24 04:36:26| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Mon Aug 23 2021 000 WTPZ43 KNHC 240236 TCDEP3 Tropical Storm Marty Discussion Number 4 NWS National Hurricane Center Miami FL EP132021 800 PM PDT Mon Aug 23 2021 Northeasterly vertical wind shear continues to disrupt the organization of Marty. The limited shower activity associated with the cyclone remains displaced to the southwest of its exposed low-level center. Despite its poor satellite appearance for much of the day, earlier scatterometer data showed 35 to 40-kt winds in the sheared cyclone. The latest objective satellite estimates and subjective Dvorak classifications support an intensity of 35 kt. However, the initial intensity for this advisory is conservatively held at 40 kt, with hopes that new scatterometer data become available overnight to better reassess Marty's intensity. Marty is moving westward, or 270/15 kt. The subtropical ridge to the north of Marty will continue to steer the cyclone generally westward over the next couple days. Then, the weakening cyclone is forecast to turn west-southwestward during the latter part of the week within the low-level trade wind flow. The latest NHC track forecast is adjusted just a bit south of the previous one, near the middle of the guidance envelope and close to the multi-model consensus aids. Despite the modest wind shear values diagnosed from the SHIPS guidance, Marty is clearly struggling to maintain organized convection. Furthermore, Marty only has a brief window over sufficiently warm SSTs before it crosses the 26 deg C isotherm on Tuesday and moves into a drier, more stable airmass. Therefore, gradual weakening is expected to begin on Tuesday and continue through the rest of the week. In fact, the latest global model simulated satellite imagery suggests Marty could struggle to generate any organized convection beyond tonight's convective maximum period. The latest NHC intensity forecast is essentially unchanged. Marty is forecast to weaken to a depression by early Wednesday, degenerate into a remnant low by Wednesday night, and dissipate late this week. But, it is possible these changes could occur even sooner than forecast if Marty's convective structure does not improve soon. FORECAST POSITIONS AND MAX WINDS INIT 24/0300Z 20.7N 115.8W 40 KT 45 MPH 12H 24/1200Z 20.7N 117.8W 35 KT 40 MPH 24H 25/0000Z 20.7N 119.9W 35 KT 40 MPH 36H 25/1200Z 20.7N 121.8W 30 KT 35 MPH 48H 26/0000Z 20.7N 123.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 26/1200Z 20.5N 126.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 27/0000Z 20.1N 128.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 28/0000Z 18.7N 133.1W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 29/0000Z...DISSIPATED $$ Forecaster Reinhart/Pasch

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Tropical Storm Marty Forecast Discussion Number 2

2021-08-23 16:32:46| National Hurricane Center (East Pacific)

Issued at 900 AM MDT Mon Aug 23 2021 000 WTPZ43 KNHC 231432 TCDEP3 Tropical Storm Marty Discussion Number 2 NWS National Hurricane Center Miami FL EP132021 900 AM MDT Mon Aug 23 2021 Marty is now a sheared tropical cyclone owing to northeasterly vertical wind shear of about 15 kt, which has displaced the bulk of the deep convection, with cloud tops colder than -80C, into the western semicircle. The low-level center is estimated be located near the northeastern edge of the sharp convective cloud shield. The initial intensity remains 35 kt based on a subjective Dvorak satellite intensity estimate of T2.5/35 kt from TAFB and an objective estimate of T2.6/37 kt from UW-CIMSS ADT. The initial motion remains westward or 280/13 kt. A strong deep-layer subtropical ridge to the north of Marty is forecast by the global and regional models to remain entrenched across the eastern Pacific basin, keeping Marty moving in a general westward direction for the next couple of days. On days 3 and 4, the ridge is forecast build southward slightly, nudging the cyclone in a west-southwestward direction. The new NHC track forecast is similar to but slightly north, or right, of the previous advisory track, and is also a little south of the tightly packed consensus track models which have shifted northward on this cycle. The moderate northeasterly shear currently affecting Marty is forecast to gradually decrease to 10 kt or less and also become easterly during the next 24-36 hours while the cyclone remains over sea-surface temperatures (SST) of 26 deg C or greater. These favorable conditions should allow for at least slight strengthening during that time. Although the shear is expected to remain low through the remainder of the forecast period, Marty will be moving over sub-26C SSTs and into a drier and more stable airmass. Those unfavorable environmental conditions should combine to erode the central deep convection and weaken the cyclone on days 3 and 4. Marty is forecast to become a depression on Wednesday, degenerate into a remnant low on Thursday, and dissipate on Friday. The official intensity forecast is essentially just an update of the previous NHC forecast, and is about 5 kt above all of the available intensity guidance through 60 hours. FORECAST POSITIONS AND MAX WINDS INIT 23/1500Z 20.5N 112.4W 35 KT 40 MPH 12H 24/0000Z 20.6N 114.5W 40 KT 45 MPH 24H 24/1200Z 20.6N 117.2W 40 KT 45 MPH 36H 25/0000Z 20.5N 119.3W 35 KT 40 MPH 48H 25/1200Z 20.4N 121.3W 30 KT 35 MPH 60H 26/0000Z 20.3N 123.3W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 26/1200Z 20.2N 125.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 27/1200Z 19.3N 130.4W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 28/1200Z...DISSIPATED $$ Forecaster Stewart

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