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Tropical Depression FOUR (Text)


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Tropical Depression Four Discussion Number   4
NWS National Hurricane Center Miami FL       AL042017
500 PM AST Thu Jul 06 2017

It's difficult to tell from visible and microwave satellite imagery
whether or not the depression still has a closed surface
circulation.  In addition, the forward speed is now about 20 kt,
which would make it difficult for a 25-kt depression to maintain
westerly winds to the south of the center.  For now, advisories
will be maintained until/if there is stronger evidence that the
circulation has opened up.

Convective cloud tops have warmed during the past few hours, and
Dvorak final-T numbers from TAFB and SAB have decreased.  Therefore,
the initial intensity will be held at 25 kt based on continuity.
Multispectral satellite imagery shows the Saharan Air Layer is
overtaking the depression, and it will be increasingly more
difficult for the system to maintain organized deep convection.  It
has also become more likely that the cyclone will not strengthen
beyond its current intensity.  The global models indicate that
southwesterly to westerly deep-layer shear will increase in about 36
hours, at which point the NHC official forecast now shows the
depression degenerating to a remnant low.  However, it's entirely
possible that the cyclone could open up into a tropical wave at any
time.

Based on the estimated center location, the depression has not
gained much latitude today, and the initial motion estimate is
275/20 kt.  The global models remain persistent that the cyclone
should turn west-northwestward in about 24 hours as it approaches a
break in the subtropical ridge, and they are generally in very close
agreement on the future track and forward speed through at least 48
hours.  The updated NHC track forecast is nudged southward to
account for the recent motion, but otherwise it is an update of the
previous forecast.

FORECAST POSITIONS AND MAX WINDS

INIT  06/2100Z 13.1N  44.9W   25 KT  30 MPH
 12H  07/0600Z 13.8N  47.8W   25 KT  30 MPH
 24H  07/1800Z 14.9N  51.5W   25 KT  30 MPH
 36H  08/0600Z 16.2N  55.1W   25 KT  30 MPH...POST-TROP/REMNT LOW
 48H  08/1800Z 17.8N  58.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  09/1800Z 20.9N  64.1W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  10/1800Z...DISSIPATED

$$
Forecaster Berg

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Page last modified: Wednesday, 18-Oct-2017 12:09:08 UTC