subj/prognostic reasoning for tropical depression 20w warning nr 01//
rmks/wdpn31 pgtw 260300
1. For meteorologists.
2. 12 hour summary and analysis.
   A. Tropical depression (td) 20w has formed in a region of
strong surface troughing southeast of Okinawa. Deep convection
along the eastern periphery of the system, supported by strong
low level convergence, has waned slightly over the past few
hours under the influence of diurnal upper level warming and
increasing vertical wind shear. However, low level organization
has improved over the past 12 hours despite observed restruct-
uring in the convective pattern.
   B. The current intensity is based on recent Dvorak estimates
from pgtw, rjtd, knes, a 252101z Quikscat pass, and synoptic
observations. Although peripheral winds associated with the 
main bands of convection to the east and North-East of the low
level center exceed 35 knots, wind speeds at the center of
circulation are estimated to be near 25 knots. Analysis of recent
microwave, multispectral and water vapor satellite imagery
indicates a separation between the partially-exposed low level
circulation center and deep convection to the east. This separa-
tion between low level and upper level circulation centers
suggests a strong tilt in the vertical storm structure charac-
teristic of extratropical transition. Interaction with an
approaching mid-latitude shortwave and advection of dry air
associated with a northeasterly surge to the west of the storm
is further aiding this transition process.
3. Forecast reasoning.
   A. This is the first forecast for TD 20w.
   B. TD 20w is beginning to turn northward on the western peri-
phery of a subtropical ridge southeast of honshu. The system is
expected to crest the ridge axis during the next 12 hours and
steadily transition into an extratropical low pressure area by
tau 24. Core winds are not expected to increase significantly,
however stronger winds along the eastern and northern peripher-
ies of the transitioning system will likely persist through the
forecast period.
Forecast team: Alpha.//