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Tropical Storm MADELINE Forecast Discussion


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000
WTPZ44 KNHC 271437
TCDEP4

TROPICAL STORM MADELINE DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP142016
800 AM PDT SAT AUG 27 2016

A GPM overpass from 1146 UTC indicated that the low-level center of
Madeline was located northeast of previous estimates, near the
northeastern edge of the convection.  This is consistent with the
moderate northeasterly shear analyzed over the cyclone by the SHIPS
model output based on the GFS and ECMWF fields.  The initial
intensity is held at 45 kt for this advisory in agreement with the
latest Dvorak estimates from TAFB and SAB, and this value is also
close to the latest UW-CIMSS SATCON analysis.

Given the relocation of the center, the best estimate of the initial
motion is an uncertain 310/08.  Madeline is expected to continue
moving generally northwestward for the next 72 hours around the
southwesterly periphery of a mid-level ridge centered near 125W.
After that time, a ridge centered north of the Hawaiian Islands
becomes the dominant steering mechanism, and should impart a
westward motion through the remainder of the forecast period.  While
the overall track forecast philosophy has not changed, the more
northerly initial position has resulted in a northward shift of the
guidance envelope and the NHC forecast by 30 to 50 n mi through the
forecast period.  The new NHC track is near the middle of the
guidance envelope and close to the latest multi-model consensus.

The SHIPS model shows the shear decreasing over Madeline during the
next 12 hours or so, which should allow for some intensification
through 72 hours while the cyclone is moving over SSTs above 27C.
The NHC forecast continues to show Madeline reaching hurricane
intensity in about 36 hours.  At days 4 and 5, the SSTs along the
track drop below 27C and the westerly shear increases, which should
result in some weakening.  The new NHC intensity forecast is a
little lower than the previous one after 48 hours due to the more
northerly track and is near or a little above the latest intensity
consensus.

It is too early to determine what impacts Madeline could have on
the Hawaiian Islands late in the forecast period.  It is important
to remind users that the average day 4 and 5 track forecast errors
for eastern Pacific tropical cyclones are around 145 and 170 miles,
respectively.

FORECAST POSITIONS AND MAX WINDS

INIT  27/1500Z 15.2N 138.5W   45 KT  50 MPH
 12H  28/0000Z 15.9N 139.4W   50 KT  60 MPH
 24H  28/1200Z 16.7N 140.7W   60 KT  70 MPH
 36H  29/0000Z 17.7N 142.0W   70 KT  80 MPH
 48H  29/1200Z 18.8N 143.6W   70 KT  80 MPH
 72H  30/1200Z 20.5N 147.0W   70 KT  80 MPH
 96H  31/1200Z 21.0N 151.2W   60 KT  70 MPH
120H  01/1200Z 21.0N 155.5W   50 KT  60 MPH

$$
Forecaster Brennan