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Tropical Storm Gert Discussion Number   6
NWS National Hurricane Center Miami FL       AL082017
500 AM EDT Mon Aug 14 2017

Although Gert's asymmetric convective cloud pattern has improved
slightly since the previous advisory, the system remains sheared
with the low-level center located near the far northwestern edge of
the deepest convection due to deep-layer and mid-level northerly
shear of 15 kt. Although the satellite intensity estimates are a
consensus T3.0/45 kt from TAFB, SAB, and UW-CIMSS ADT, the initial
intensity is being held at 40 kt due to the low-level center having
become more exposed since those 0600 UTC intensity estimates.

Gert has maintained a north-northwestward motion or 345/08 kt. There
are significant changes to the previous track forecast or reasoning.
The small cyclone is expected to turn northward shortly and then
turn toward the north-northeast by tonight, moving through a break
in the subtropical ridge roughly midway between the U.S. east coast
and Bermuda on Tuesday. By 48 h, Gert is forecast to steadily
accelerate northeastward over the north Atlantic through the
remainder of the forecast period ahead of a strong deep-layer
trough. The latest guidance has again shifted a little bit to the
west after recurvature, so the official forecast was nudged in that
direction slightly, but remains on the far eastern edge of the
guidance envelope.

Gert should continue to be affected by moderate northerly vertical
wind shear and dry mid-level air for the next 24-30 h, so only
slight strengthening is anticipated despite the very warm SSTs
beneath the cyclone. By 36-48h, a window of opportunity for more
significant strengthening is expected when the shear decreases to
less than 10 kt after the cyclone begins to recurve, with Gert
possibly becoming a hurricane by 48 h. However, the intensification
trend is expected to be short-lived due to the vertical wind shear
shifting to southwesterly and increasing to more than 30 kt by 60 h
and beyond. By 72 h, Gert will be moving over ocean temperatures of
20 deg C and colder, which should induce and sustain extratropical
transition. Gert's intensity is held to near-hurricane strength at
72 h due to the system getting a baroclinic kick when the cyclone
moves underneath a favorable region of the upper-level jetstream.
However, rapid weakening is expected by 96 h when the cyclone is
moving over 15 deg C SSTs, with the system being absorbed on day 5
by a larger extratropical low pressure system over the far north
Atlantic. The official intensity forecast is similar to the previous
advisory, and is lower than the consensus models IVCN and HCCA.
This is due to the HWRF model creating a high bias in those models
caused by an unrealistic forecast of Gert becoming a category 4
hurricane in 72 h when the cyclone will be moving over SSTs of 25
deg C or colder and in shear conditions greater than 35-40 kt.

FORECAST POSITIONS AND MAX WINDS

INIT  14/0900Z 29.7N  72.2W   40 KT  45 MPH
 12H  14/1800Z 30.9N  72.3W   45 KT  50 MPH
 24H  15/0600Z 32.7N  71.9W   50 KT  60 MPH
 36H  15/1800Z 34.9N  70.0W   55 KT  65 MPH
 48H  16/0600Z 37.3N  66.0W   65 KT  75 MPH
 72H  17/0600Z 42.7N  52.1W   60 KT  70 MPH...POST-TROP/EXTRATROP
 96H  18/0600Z 49.0N  38.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  19/0600Z...ABSORBED BY LARGER EXTRATROPICAL LOW

$$
Forecaster Stewart

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