ZCZC MIATCDEP3 ALL
TTAA00 KNHC DDHHMM
 
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
 
NNNN