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Tropical Storm Greg Forecast Discussion


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000
WTPZ42 KNHC 212050
TCDEP2

Tropical Storm Greg Discussion Number  18
NWS National Hurricane Center Miami FL       EP072017
200 PM PDT Fri Jul 21 2017

Greg has thrown me a curveball today with the center unexpectedly
becoming exposed southwest of the main ball of convection.  While
there is clearly an impressive low-level circulation, as seen by
both conventional and microwave images, Greg has become less
organized overall.  Satellite intensity estimates, consequently,
have dropped, and the initial wind speed is reduced to 45 kt.

While Greg is forecast to remain over warm waters for the next
couple of days, dry air in the mid-levels and some westerly shear
is likely to impede the strengthening process, despite an intact
low-level core present on microwave data.  Thus the intensity
forecast is held steady for about a day.  Thereafter, the regional
hurricane models suggest Greg will have a chance to reintensify,
although this is far from a slam dunk with an uncertain
environment.  I have elected to show some intensification on day 2
of the forecast, but reduced by only about 10 kt from the previous
prediction due to continuity constraints.  The guidance continues to
indicate a weaker storm, so it would not be surprising if later
shifts had to lower the forecast winds more.

The initial motion has shifted to 275/11.  The subtropical ridge
should hold to the north of Greg for the next few days, steering
the storm generally westward.  Thereafter, Greg should turn to the
west-northwest due to a small low- to mid-level trough dropping out
of the mid-latitudes, weakening the periphery of the aforementioned
ridge. Model guidance has trended southward, following the lead of
the 00Z ECMWF.  With a weaker storm expected, it makes sense that
Greg would feel less of the trough, so the official forecast is
shifted to the southwest, close to a blend of the 12Z ECMWF and
model consensus forecasts.


FORECAST POSITIONS AND MAX WINDS

INIT  21/2100Z 14.9N 120.7W   45 KT  50 MPH
 12H  22/0600Z 15.1N 122.4W   45 KT  50 MPH
 24H  22/1800Z 15.3N 124.6W   50 KT  60 MPH
 36H  23/0600Z 15.5N 126.8W   55 KT  65 MPH
 48H  23/1800Z 15.6N 129.0W   55 KT  65 MPH
 72H  24/1800Z 15.8N 132.4W   50 KT  60 MPH
 96H  25/1800Z 16.8N 135.0W   45 KT  50 MPH
120H  26/1800Z 18.0N 138.0W   35 KT  40 MPH

$$
Forecaster Blake