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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
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