


351
wdpn31 pgtw 210300
msgid/genadmin/navmarfcstcen Pearl Harbor hi/jtwc//
subj/prognostic reasoning for tropical depression 04w
/warning nr 04//
rmks/
1. For meteorologists.
2. 12 hour summary and analysis.
A. Tropical depression (td) 03w, located approximately 45 nm
southeast of Yap, has tracked westward at 05 knots over the past
six hours. Animated multispectral satellite imagery (msi)
depicts a fully-exposed low-level circulation center (LLCC) with
an isolated burst of deep convection over the southeast quadrant.
The large area of intense deep convection displaced south of the
system has continued to shear southward under moderate northerly
vertical wind shear and has weakened diurnally over the past 08
hours. The system was downgraded to a TD based on a number of
factors: recent ascat data indicating 25-30 knot winds, surface
observations from Yap (45nm northwest) showing only 10-15 knot
sustained winds, Dvorak estimates decreasing to 25-30 knots, and
the lack of deep convective banding evident in the 2110z ssmis
37h image. There is good confidence in the current position
based on the exposed LLCC in msi. The upper-level environment
remains marginal with 10-20 knots of northerly vertical wind
shear hindering poleward outflow but offset slightly by good
equatorward outflow.
3. Forecast reasoning.
A. No change in forecast philosophy. The forecast track has
been shifted south slightly and the forecast intensification
rate through tau 24 was decreased.
B. TD 04w is tracking slowly westward under the steering
influence of the low- to mid- level subtropical ridge positioned
north of the system. TD 04w has tracked more westward than
expected under an east-west oriented low-level ridge over the
past 12 hours due to the weak organization. The system should
track west-northwestward within the next 24 hours as it
strengthens under an improving upper-level environment. In
particular, poleward outflow is forecast to improve ahead
of an approaching shortwave trough. Model guidance is in fair
agreement with JGSM and GFS on the northern edge of the aids
envelope and indicating a more poleward track through tau 36.
The ukmo model shows a more westward track into the central
Philippines. This forecast favors NOGAPS, GFDN, ECMWF and WBAR
and is positioned close to the consensus but slightly faster
at the extended Taus. TD 04w is forecast to intensify slowly
through tau 24 but should intensify at a faster rate through
tau 72 due to the aforementioned improving environment.
C. In the extended Taus, TD 04w is forecast to turn more
poleward along the southwest periphery of the mid-level
subtropical ridge. The system is expected to reach a peak
intensity of 105 knots by tau 120.//