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Tropical Storm ORLENE Forecast Discussion Number 5

Issued at 800 PM PDT SUN SEP 11 2016

ZCZC MIATCDEP1 ALL
TTAA00 KNHC DDHHMM

TROPICAL STORM ORLENE DISCUSSION NUMBER   5
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP162016
800 PM PDT SUN SEP 11 2016

Visible, infrared, and microwave satellite imagery indicate that the
cyclone has continued to become more organized, with a well-defined
low- to mid-level eye noted in a 2336Z SSMI/S overpass. Orlene now
has a large, circular CDO along with a symmetrical and expanding
upper-level outflow pattern. Dvorak intensity estimates from TAFB
and SAB are a consensus T3.5/55 kt while the objective UW-CIMSS ADT
is 60-65 kt. Given the rather large average eye diameter of about 35
nmi, the initial intensity is set below the ADT values and follows
the subjective estimate of 55 kt.

The initial motion is 305/08 kt based heavily on microwave eye
position estimates. Although there is some divergence in the models,
they are good agreement overall on Orlene gradually slowing down
and moving north-northwestward and northward into a break in the
subtropical ridge during the next 48 hours. After that, the break in
the ridge along 120W longitude is forecast to fill back in as a
shortwave trough lifts out quickly to the northeast. The
re-strengthening ridge to the north of Orlene is then expected to
accelerate the cyclone on a westward track on days 3-5. The new NHC
forecast track is essentially just an update and extension of the
previous advisory track, and lies close to a consensus of the
GFS-ECMWF model solutions.

Orlene is expected to remain in a very low-shear environment for the
next 72 hours, which typically favors rapid intensification.
However, the cyclone is also expected to remain embedded within a
fairly dry mid-level environment with humidity values of 50-55
percent. Given that Orlene has already developed a solid eyewall
structure, little if any dry air intrusions should be able to
penetrate into the eye in the short term.  Based on the favorable
low-shear and already impressive outflow pattern, a period of rapid
intensification is forecast to occur for the next 24 hours, followed
by a leveling off in the intensity due to cooler waters and possible
cold upwelling occurring beneath the cyclone. By 48 hours and
beyond, gradual weakening is expected due to Orlene moving into a
considerably drier and more stable environment characterized by
mid-level humidities in the 20- to 30-percent range. The intensity
forecast remains near the upper end of the guidance envelope close
to the LGEM and Florida State Superensemble models, and is just a
little higher than the previous advisory.

FORECAST POSITIONS AND MAX WINDS

INIT  12/0300Z 16.6N 118.3W   55 KT  65 MPH
 12H  12/1200Z 17.3N 119.1W   70 KT  80 MPH
 24H  13/0000Z 18.0N 119.7W   85 KT 100 MPH
 36H  13/1200Z 18.6N 119.8W   85 KT 100 MPH
 48H  14/0000Z 19.0N 120.0W   80 KT  90 MPH
 72H  15/0000Z 19.2N 121.7W   70 KT  80 MPH
 96H  16/0000Z 18.9N 125.7W   65 KT  75 MPH
120H  17/0000Z 18.9N 130.8W   60 KT  70 MPH

$$
Forecaster Stewart

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