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Tropical Storm Douglas Discussion Number 35
NWS Central Pacific Hurricane Center Honolulu HI EP082020
500 PM HST Tue Jul 28 2020
With no associated deep convection, Douglas continues to drive
westward across the Papahanaumokuakea Marine National Monument far
northwest of the main Hawaiian Islands. While the clearly defined
low level circulation center (LLCC) has passed just west of Maro
Reef, waters from Maro Reef to French Frigate Shoals continue to
lie within the tropical storm force radius. The Tropical Storm
Warning for those waters therefore remains in effect. The Tropical
Storm Warning for waters from Maro Reef to Lisianski also
remains in effect. However, the Tropical Storm Watch for waters
from Lisianski to Pearl and Hermes Atoll has been cancelled.
Subjective Dvorak satellite intensity estimates for Douglas range
from 2.0/30 kts, from HFO and JTWC, to too weak to classify from
SAB. UW-CIMSS ADT was 43 kt. A late morning ASCAT pass showed a
broad swath of 35 to 40 kt wind barbs within the north semicircle.
Based on all of this, intensity remains set at 40 kt for this
forecast package.
Initial motion is 285/18 kt, representing a gradual shift to the
west compared to the 290/17 kt 12 hour motion. Douglas has sped up
a bit as steering is now completely low level. This system
continues to track generally westward to the south of a low to mid
level ridge. Once again, the forecast track is quite close to the
last one, closely matching UEMI along the northern portion of the
tightly packed guidance envelope through 24 hours, then gradually
shifting to the center of the envelope afterwards through system
dissipation at 72 hours. As Douglas remains deep convection-free
after 18 hours and will not recover as a tropical system, the only
question of interest is when will transition to a post-tropical
remnant low begin. Global models keep a weak remnant circulation
alive at the surface, with little or no reflection above 700 mb, all
the way through 72 hours as it begins to merge with a frontal system
west of the dateline. On the other hand, DSHP unrealistically keeps
Douglas at 45 to 50 kt through 120 hours. Given the deteriorating
satellite presentation and the increasing time with no deep
convection, we will tilt toward the global models and make Douglas a
post-tropical remnant low in 12 hours but delay system dissipation
to 72 hours.
FORECAST POSITIONS AND MAX WINDS
INIT 29/0300Z 24.6N 170.9W 40 KT 45 MPH
12H 29/1200Z 24.9N 173.7W 30 KT 35 MPH...POST-TROP/REMNT LOW
24H 30/0000Z 25.5N 177.7W 30 KT 35 MPH...POST-TROP/REMNT LOW
36H 30/1200Z 26.3N 178.4E 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 31/0000Z 27.3N 174.5E 25 KT 30 MPH...POST-TROP/REMNT LOW
60H 31/1200Z 28.4N 170.6E 20 KT 25 MPH...POST-TROP/REMNT LOW
72H 01/0000Z...DISSIPATED
$$
Forecaster Powell
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