Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
808 AM EDT Sun Oct 01 2017

Valid 00Z Mon Oct 02 2017 - 00Z Mon Oct 09 2017

Satellite imagery this morning show a mid/upper-level trough
across the north central Pacific, with a cold front moving
southward toward the Hawaiian archipelago. Infrared satellite
shows a band of relatively cold cloud tops, indicating widespread
convection along the front. Models continue to show general
agreement that this cold front will move southeastward toward the
western Hawaiian Islands through tonight before briefly becoming
stationary as a closed upper low develops northwest of the front.
Flow surrounding the front will disrupt low-level easterlies
across Hawaii, with weak south to southeasterly flow expected
across the state into Monday. The upper low is then forecast to
drift westward by mid week as ridging builds north of Hawaii once
again, allowing the surface front to drift westward and wash out.
Models continue to show some spread with respect to how far east
the front will get before stalling, with the GFS and ECMWF keeping
the front just west of the westernmost islands, while the
CMC/UKMET bring the front to Ni'ihau and Kauai. With a plume of
high precipitable water values focused along the front (1.7-2.0
inches), as well as the impressive height falls in the vicinity,
the boundary will serve to focus potentially heavy rainfall.
GEFS/ECENS/NAEFS means continue to support the GFS/ECMWF in
keeping the frontal boundary (and the heaviest rainfall) just west
of Kauai and Ni'ihau. Given some remaining uncertainty in the
exact frontal position, and its close proximity to the western
Islands, some areas of heavy rainfall across the western islands
cannot be ruled out, with Ni'ihau seeing the highest probabilities
for locally heavy rains. ECMWF ensemble probabilities have come up
a bit compared to this time yesterday, with 48-hour probabilities
for exceeding 1 inch of rainfall (ending 00Z Wed) of 15 to 20
percent across Ni'ihau, and less across Kauai, 5 to 10 percent.

By late next week as the upper low retrogrades west and ridging
becomes reestablished north of Hawaii, brisk low-level easterlies
should resume into the 20-30 kt range. Additionally, much drier
air is expected to advect across the Islands from the eastern
Pacific into next weekend. By late next weekend deterministic and
ensemble solutions all generally show a cutoff upper low
developing over the northeastern Pacific, which retrogrades toward
Hawaii by late next weekend before becoming absorbed in stronger
westerlies associated with another amplifying trough south of
Alaska. This feature will lower heights across Hawaii through next
weekend and will once again disrupt easterly trades, with weak
south to southeasterly flow across the Islands into Sunday. A
relatively weak frontal boundary west of the upper low may also
approach Hawaii next weekend, with moisture along the boundary
resulting in an increase in shower activity, especially across the
eastern Islands where height falls will be the strongest.

Ryan