Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
818 AM EDT Sat Jul 15 2017

Valid 00Z Sun Jul 16 2017 - 00Z Sun Jul 23 2017

For the next several days, models agree on an elongated
positively-tilted trough slowly drifting westward across the
Hawai`ian island chain. Once it moves toward the International
Dateline, the system should become more closed off in nature as it
continues to push farther away from the state. Eventually
mid/upper-level ridging sets up in advance of what is currently
Hurricane Fernanda. The track forecast as depicted by the National
Hurricane Center seems to follow the 00Z ECMWF most readily while
the 06Z GFS is a hair quicker by mid-week. Weakening is indicated
toward the end of the period but what remains of the system bears
watching as it sits a few hundred miles east of Hawai`i by next
weekend.

At the surface, persistent ridging to the north will be a mainstay
feature throughout the forecast. Recent solutions favor nosing the
center of the anticyclone toward the west in time. Ultimately all
of this will favor a continued trade wind regime with speeds
generally between 15 and 20 knots. By next weekend, depending on
the remnant strength of Fernanda, some uptick in the local fields
may be possible. In terms of precipitation threats, the usual
windward showers are expected throughout the period amidst a bit
lower precipitable water environment. Models show values in the 1
to 1.25 inch range although a brief spike in such numbers is
possible by Tuesday/Wednesday. However, this does not seem to have
appreciable effects on the rainfall forecast. Eventually, a more
pronounced increase in the tropospheric moisture content is
evident in advance of Fernanda. The 00Z CMC and its ensemble
system increase rainfall amounts while other solutions remain more
conservative.


Rubin-Oster