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Tropical Depression TWELVE-E (Text)

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TROPICAL DEPRESSION TWELVE-E DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP122014
800 AM PDT MON AUG 18 2014

The unusually large depression, at least 800 nmi in diameter, has
changed little since the previous advisory. A 1047 UTC SSMI
microwave overpass indicated that the depression has a well-defined
but fully exposed low-level circulation center, with a smaller
secondary circulation located about 120 nmi east of the main center.
Deep convection has been unable to develop and persist near the main
low-level center, possibly due to the aforementioned secondary swirl
and modest east-northeasterly mid- to upper-level shear undercutting
the otherwise favorable outflow pattern. A blend of the TAFB, SAB,
and ADT satellite intensity estimates supports maintaining the
initial intensity at 30 kt.

Recent passive microwave fix data suggest that the depression has
turned toward the west-northwest and is now moving 290/07 kt. The
cyclone is expected to turn northwestward later today as the
depression moves around the southwestern periphery of a deep-layer
ridge located over the southwestern U.S. and northwestern Mexico. A
mid-latitude trough/low currently digging south-southeastward along
the U.S. west coast is expected to erode the ridge on Tuesday and
Wednesday, allowing the cyclone to move more northward. However, the
trough is forecast to shift eastward by late Wednesday, which will
allow the ridge to build back in to the north of the cyclone and
forcing it back on a northwesterly track toward colder water through
Days 4 and 5. The new forecast track is similar to the previous
advisory track, and lies in the middle of the NHC guidance envelope
and close to the consensus model TVCE.

The large circulation and associated radius of maximum winds, plus
modest easterly shear, are expected to hinder the intensification
process for the next 72 hours or so despite the favorable outflow
regime. After that, the system is forecast to move over sub-26C and
progressively cooler waters, which should induce slow weakening.
The intensity forecast closely follows the ICON consensus model
through 72 hours, but is a little above ICON after that due to the
large cyclone requiring more time than normal to spin down.

Although the cyclone is forecast to remain well offshore of the
southwestern United States, a northward surge of moisture partly
associated with this system could move into that region by late
Wednesday and Thursday.  Please monitor products issued by your
local National Weather Service office for more details on a possible
heavy rainfall threat.

FORECAST POSITIONS AND MAX WINDS

INIT  18/1500Z 16.7N 117.7W   30 KT  35 MPH
 12H  19/0000Z 17.1N 118.7W   35 KT  40 MPH
 24H  19/1200Z 17.7N 119.8W   40 KT  45 MPH
 36H  20/0000Z 18.3N 120.3W   45 KT  50 MPH
 48H  20/1200Z 18.9N 120.5W   50 KT  60 MPH
 72H  21/1200Z 20.4N 121.0W   50 KT  60 MPH
 96H  22/1200Z 22.8N 122.6W   45 KT  50 MPH
120H  23/1200Z 25.5N 125.0W   35 KT  40 MPH...POST-TROPICAL

$$
Forecaster Stewart

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