Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
814 AM EDT Sat Sep 30 2017

Valid 00Z Sun Oct 01 2017 - 00Z Sun Oct 08 2017

Satellite imagery this morning show an amplifying mid/upper-level
trough across the north central Pacific, with a cold front moving
southward toward the Hawaiian archipelago. Models continue to show
general agreement that this cold front will move southeastward
toward the western Hawaiian Islands through Sunday night before
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 through early next week. The upper low
is then forecast to drift westward early to mid next week as
ridging builds north of Hawaii once again, allowing the surface
front to drift westward as it washes out. Models 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 brings 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, some chance
of heavy rainfall by early next week 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) approaching 20
percent across Ni'ihau, and 10 percent across Kauai. While
currently the less probable scenario, if the front were to move
farther east as shown by the 00Z CMC, heavier rainfall amounts
would become more likely.

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 through into next weekend.

Ryan