Tropical Storm Marty

Post Tropical Marty Track 1400 Hours August 24 2021
Post Tropical Marty Track 1400 Hours August 24 2021

Tropical Storm Marty Wind Speed FieldTropical Storm MartyNWS National Hurricane Center Miami FL 200 PM PDT Tue Aug 24, 2021 (see video below)

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.

FORECAST POSITIONS AND MAX WINDS

INIT  24/2100Z 20.5N 119.2W   30 KT  35 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 12H  25/0600Z 20.5N 120.8W   25 KT  30 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 24H  25/1800Z 20.5N 122.9W   25 KT  30 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 36H  26/0600Z 20.4N 125.0W   20 KT  25 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 48H  26/1800Z 20.2N 127.2W   20 KT  25 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 60H  27/0600Z 19.8N 129.6W   20 KT  25 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 72H  27/1800Z...DISSIPATED

NWS National Hurricane Center Miami FL  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 - Depression (WSW Cabo San Lucas,MX)
 12H  25/0000Z 20.6N 119.9W   25 KT  30 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 24H  25/1200Z 20.6N 121.9W   25 KT  30 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 36H  26/0000Z 20.6N 124.0W   20 KT  25 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 48H  26/1200Z 20.5N 126.1W   20 KT  25 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 60H  27/0000Z 20.2N 128.4W   20 KT  25 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 72H  27/1200Z 19.6N 130.9W   20 KT  25 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 96H  28/1200Z...POST-TROP/REMNT LOW

NWS National Hurricane Center Miami FL 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 - Tropical Storm (WSW Cabo San Lucas,MX)
 12H  24/0600Z 21.0N 116.2W   40 KT  45 MPH - Tropical Storm (WSW Cabo San Lucas,MX)
 24H  24/1800Z 21.0N 118.6W   35 KT  40 MPH - Tropical Storm (WSW Cabo San Lucas,MX)
 36H  25/0600Z 21.0N 120.6W   30 KT  35 MPH - Depression (WSW Cabo San Lucas,MX)
 48H  25/1800Z 20.9N 122.6W   30 KT  35 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 60H  26/0600Z 20.9N 124.8W   25 KT  30 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 72H  26/1800Z 20.8N 127.0W   20 KT  25 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 96H  27/1800Z 19.4N 131.9W   20 KT  25 MPH - Post Tropical (WSW Cabo San Lucas,MX)
120H  28/1800Z...DISSIPATED

NWS National Hurricane Center Miami FL 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 - Tropical Storm (WSW Cabo San Lucas,MX)
 12H  24/0000Z 20.6N 114.5W   40 KT  45 MPH - Tropical Storm (WSW Cabo San Lucas,MX)
 24H  24/1200Z 20.6N 117.2W   40 KT  45 MPH - Tropical Storm (WSW Cabo San Lucas,MX)
 36H  25/0000Z 20.5N 119.3W   35 KT  40 MPH - Tropical Storm (WSW Cabo San Lucas,MX)
 48H  25/1200Z 20.4N 121.3W   30 KT  35 MPH - Depression (WSW Cabo San Lucas,MX)
 60H  26/0000Z 20.3N 123.3W   25 KT  30 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 72H  26/1200Z 20.2N 125.5W   20 KT  25 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 96H  27/1200Z 19.3N 130.4W   20 KT  25 MPH - Post Tropical (WSW Cabo San Lucas,MX)
120H  28/1200Z...DISSIPATED

NWS National Hurricane Center Miami FL 300 AM MDT Mon Aug 23, 2021

Recent scatterometer wind data indicate that the circulation associated with the low pressure area located a couple of hundred miles south-southwest of the southern tip of the Baja California peninsula has become better defined. In addition, convective activity has persisted and expanded over the western portion of the circulation overnight, resulting in Dvorak T-numbers of 1.5 and 2.0 from SAB and TAFB, respectively. Both ASCAT-A and B instruments revealed 30 to 35 kt winds over the western portion of the circulation and based on these data, advisories are being initiated on Tropical Storm Marty, the thirteenth named-storm of the 2021 eastern Pacific hurricane season.

Marty is moving westward at about 13 kt. A strong mid-tropospheric ridge that extends from the south-central United States westward across northern Mexico and the eastern Pacific is forecast to steer Marty westward over the next couple of days. After that time, the cyclone is forecast to weaken and become vertically shallow. This should cause it to turn toward the west-southwest as it steered by the low-level trade wind flow. The dynamical model guidance is in excellent agreement, and the NHC track is near the middle of the tightly clustered model envelope.

Although the tropical storm is currently located over warm waters, it is within an area of moderate northeasterly shear. In addition, it is likely to ingest drier and more stable air located to the northwest of the system over the next couple of days. As a result, only slight strengthening is indicated in the official forecast. By 36 hours, Marty is forecast to cross the 26C isotherm and gradual weakening is anticipated after that time. Less favorable thermodynamic conditions should result in the system becoming a remnant low in 60 to 72 hours, if not sooner. The NHC intensity forecast is in good agreement with the HCCA and IVCN consensus aids.

FORECAST POSITIONS AND MAX WINDS
 
INIT  23/0900Z 20.0N 111.0W   35 KT  40 MPH - Tropical Storm (WSW Cabo San Lucas,MX)
 12H  23/1800Z 20.3N 113.2W   40 KT  45 MPH - Tropical Storm (WSW Cabo San Lucas,MX)
 24H  24/0600Z 20.4N 116.0W   40 KT  45 MPH - Tropical Storm (WSW Cabo San Lucas,MX)
 36H  24/1800Z 20.2N 118.4W   35 KT  40 MPH - Tropical Storm (WSW Cabo San Lucas,MX)
 48H  25/0600Z 20.1N 120.4W   30 KT  35 MPH - Depression (WSW Cabo San Lucas,MX)
 60H  25/1800Z 20.0N 122.3W   25 KT  30 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 72H  26/0600Z 19.8N 124.3W   20 KT  25 MPH - Post Tropical (WSW Cabo San Lucas,MX)
 96H  27/0600Z 19.0N 129.2W   20 KT  25 MPH - Post Tropical (WSW Cabo San Lucas,MX)
120H  28/0600Z...DISSIPATED

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