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TROPICAL STORM GRACE DISCUSSION NUMBER   7
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072015
1100 PM AST SUN SEP 06 2015

A partial ASCAT-B overpass at 2346 UTC, along with several passive
microwave passes, indicates that Grace has a well-developed, compact
low-level circulation that extends upward into the mid-levels of
the cyclone. However, both conventional and microwave imagery also
indicate that the inner-core convection has been significantly
disrupted by the entrainment of dry mid-level air into the western
and southern quadrants of the circulation, and has penetrated into
the cyclone's center. The initial intensity of 45 kt, which could be
generous, is based on Dvorak current intensity estimates of T3.0/45
kt from both TAFB and SAB, which is supported by a 40-kt ASCAT-B
wind vector.

Grace's motion continues to be 280/13 kt. The small tropical cyclone
is expected to move a little north of due west throughout the
forecast period due to moderate low- to mid-level easterly flow on
the south side of a large subtropical ridge located to the north
of Grace. The NHC model guidance is tightly packed around the
previous advisory track, so the new forecast is just an update and
extension of the previous advisory, and lies close to the consensus
model, TVCN.

Grace's fairly robust circulation should be able to mix out any dry
air intrusions for the next 24 hours or so while the vertical wind
shear remains low and, therefore, a return of inner-core deep
convection is expected later tonight. Shortly after that time,
however, the vertical shear is forecast by both the GFS and ECMWF
models to increase to 25-30 kt from a westerly direction and for the
mid-level humidity to decrease to near 40 percent, a combination
that will halt any intensification and induce a steady weakening
trend despite warm SSTs of near 28 deg C beneath the cyclone. The
official intensity forecast is a little below the previous advisory
forecast, and is similar to the intensity consensus model, IVCN. It
is worth noting that the GFS and ECMWF models show significant
weakening of the cyclone in 4 to 5 days, with possible dissipation
occurring by 120 h.

FORECAST POSITIONS AND MAX WINDS

INIT  07/0300Z 13.3N  32.5W   45 KT  50 MPH
 12H  07/1200Z 13.5N  34.7W   45 KT  50 MPH
 24H  08/0000Z 13.8N  37.7W   50 KT  60 MPH
 36H  08/1200Z 14.0N  40.7W   45 KT  50 MPH
 48H  09/0000Z 14.5N  44.0W   40 KT  45 MPH
 72H  10/0000Z 15.3N  50.3W   40 KT  45 MPH
 96H  11/0000Z 16.1N  56.5W   35 KT  40 MPH
120H  12/0000Z 17.2N  62.0W   30 KT  35 MPH

$$
Forecaster Stewart

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