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Tropical Depression JOYCE


Tropical Depression Joyce Discussion Number  22
NWS National Hurricane Center Miami FL       AL102018
1100 PM AST Mon Sep 17 2018

Joyce remains a very resilient tropical cyclone. After an earlier
burst of deep convection just east of the low-level center that has
since dissipated, a new convective burst has developed just north of
the center with a curved convective band trying to develop in the
eastern semicircle. All of this convective activity has been
occurring despite westerly 850-200-mb vertical wind shear of near
35 kt and in the presence of very dry mid-level air. The initial
intensity is being maintained a 30 kt based on earlier 28-kt ASCAT
wind data, and the fact that no significant change has occurred to
Joyce's low-level appearance in satellite imagery.

The initial motion estimate is now 155/06 kt. Joyce is forecast to
move around the eastern periphery of a deep-layer ridge located
across the eastern Atlantic to the southwest and west of the
cyclone. This large-scale steering feature is expected to gradually
turn Joyce toward the south overnight and on Tuesday, and then force
the cyclone toward the southwest on Wednesday and Thursday. The new
NHC track forecast is to the west of the previous advisory track,
and lies near the middle of the guidance envelope close to a blend
of the consensus models TVCA and HCCA.

Convective development during the past 12 hours has been in large
part due to the strong atmospheric instability created by very cold
200-mb temperatures near -57 deg C overlaying relatively warm ocean
temperatures of 25.5 deg C. A contributing factor to the most recent
convective development appears to be enhanced uplift caused by an
approaching shortwave trough noted in water vapor imagery between
30W-35W longitude. The latter feature is forecast by most of the
global models to cut off south of the Azores and become entangled
with Joyce's circulation over the next few days, resulting in a
sharp decrease in the vertical wind shear and an increase in
atmospheric instability/CAPE by 48 h and beyond, which would aid
convective development and re-strengthening. For now, however, the
new intensity forecast continues the weakening trend of the previous
advisories due to an abundance of very dry mid-level air being
entrained into the circulation, which is expected to shut down
convective development, causing Joyce to dissipate shortly after
72 h. However, confidence in this forecast scenario is not very
high, and some adjustments may be required to subsequent forecasts
once the 18/0000Z model guidance becomes available, especially since
the GFS- and ECMWF-based SHIPS and LGEM intensity guidance maintains
Joyce as a 30- to 35-kt tropical cyclone through the next 120 hours.


INIT  18/0300Z 33.2N  27.3W   30 KT  35 MPH
 12H  18/1200Z 32.4N  27.1W   30 KT  35 MPH
 24H  19/0000Z 31.4N  27.6W   25 KT  30 MPH...POST-TROP/REMNT LOW
 36H  19/1200Z 30.5N  28.7W   25 KT  30 MPH...POST-TROP/REMNT LOW
 48H  20/0000Z 29.9N  29.8W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  21/0000Z 29.2N  33.0W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  22/0000Z...DISSIPATED

Forecaster Stewart