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Tropical Depression Joyce Forecast Discussion


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960 
WTNT45 KNHC 181431
TCDAT5

Tropical Depression Joyce Discussion Number  24
NWS National Hurricane Center Miami FL       AL102018
Issued by the NWS Weather Prediction Center College Park MD
1100 AM AST Tue Sep 18 2018

Earlier in the morning, the low-level center of Joyce became
decoupled from a small area of deeper convection. However, another
convective burst has recently developed after 12Z closer to the
low-level center.  The initial intensity of Joyce was held at 30 kt
based on a recent ASCAT pass, which showed an area of 25-30 kt winds
on the north side of the circulation. The number of wind retrievals
close to 30 kt was rather limited, so Joyce's winds should decrease
below 30 kt when it eventually loses deep convection.  The
depression is in an environment with strong westerly vertical wind
shear (30-40 kt) and marginal sea surface temperatures (around 25C),
and thus the primary forecast question is when Joyce will succumb to
the relatively hostile environment, lose deep convection and become
a remnant low. The GFS and ECMWF models both show some deep
convection lingering into tomorrow before warming upper-level
temperatures and increasingly dry air aloft kill off any remaining
convection. Therefore, the 12-hour forecast point maintains tropical
depression status, and the 24-hour forecast point indicates a
post-tropical system.

The initial motion of Joyce is now more southerly (185 degrees at 7
kt), and it will begin to turn increasingly to the southwest and
west as the central Atlantic ridge builds to the north. The new
forecast track is essentially an update of the previous one, which
was close to the TVCA and HCCA consensus models. Minor adjustments
were made in the first 24 hours to account for a slightly faster
observed motion to the south in the past 6-12 hours.


FORECAST POSITIONS AND MAX WINDS

INIT  18/1500Z 31.6N  27.4W   30 KT  35 MPH
 12H  19/0000Z 30.6N  27.7W   25 KT  30 MPH
 24H  19/1200Z 29.9N  28.7W   25 KT  30 MPH...POST-TROP/REMNT LOW
 36H  20/0000Z 29.4N  29.9W   20 KT  25 MPH...POST-TROP/REMNT LOW
 48H  20/1200Z 29.0N  31.5W   20 KT  25 MPH...POST-TROP/REMNT LOW
 72H  21/1200Z...DISSIPATED

$$
Forecaster Lamers/Blake