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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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