msgid/genadmin/navpacmetoccen Pearl Harbor hi/jtwc//
subj/prognostic reasoning for typhoon 06w warning nr 25//
rmks/
1. For meteorologists.
2. 12 hour summary and analysis.
   A. Typhoon (TY) 06w (Nakri) has intensified slightly, against
previous expectations, from 70 knots to 80 knots over the past
12 hours. This intensification has likely resulted from passage
over a patch of higher ocean heat and enhancement of upper level
outflow toward a tropical upper tropospheric trough (tutt) cell
to the southeast and toward the midlatitude baroclinic zone to
the north. Forward track speed has increased to 16 knots under
the influence of enhanced flow along the western periphery of
the steering ridge.
   B. The current position is based on recent satellite fixes from
pgtw and rjtd. TY 06w exhibited a ragged, 32 nm diameter eye in
multispectral satellite imagery at 020000z. The presence of this
eye feature lends high confidence to the intial position. The
current intensity estimate is consistent with Dvorak fix values of
5.0 from pgtw and 4.5 from rjtd. Although deep convection surrounds
the eye, the majority of the convection associated with TY 06w is
now displaced to the east of the low level circulation center due
to the influence of increasing westerly vertical wind shear. TY 06w
is turning northeastward along the western periphery of a subtropi-
cal steering ridge to the east of the storm.
3. Forecast reasoning
   a. The tau 24 and tau 36 forecast positions have been adjusted
slightly northward in response to an anticipated poleward applifi-
cation of the steering flow. Forecast intensity values have also
been increased slightly based on recent intensification. Addition-
ally, the storm is expected to remain fully tropical through tau 12.
Therefore, the first phase of extratropical transition will be
delayed until tau 24.
   B. TY 06w will continue to track along the periphery of the sub-
tropical steering ridge to the east. This steering influence will
take the cyclone northeastward ahead of an approaching midlatitude
trough located to the northwest of the system. The exact track
of TY 06w will depend on the degree to which the steering ridge
amplifies ahead of the previously discussed midlatitude trough.
However, the numerical model vortex trackers are in good agreement
with the current forecast track. Additionally, the model consensus
has shifted very little over the past 24 hours, lending increased
confidence to the current forecast. TY 06w is expected to encounter
enhanced westerly flow ahead of the midlatitude trough by tau 12.
However, a well-developed mesoscale anticyclone aloft will buffer
the system somewhat, helping the storm to maintain typhoon intensity
through tau 12. Thereafter, increasing vertical wind shear, passage
over cooler water, the introduction of cooler, drier air to the
western portion of the circulation will result in more rapid weak-
ening. These influences will also bring about the first phase of
extratropical transition just after tau 12. The storm will become
fully extratropical by tau 36.
Forecast team: Alpha//