Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
845 AM EDT Wed Aug 30 2017

Valid 00Z Thu Aug 31 2017 - 00Z Thu Sep 07 2017

It remains the case that models and ensembles are in good
agreement into Friday with the handling of the main features
affecting the Hawaiian islands. Solutions then diverge, primarily
with respect to the amplifying pattern in the polar stream. 

Trades return to near normal levels through Thursday as high
pressure builds to the northeast in response to a strengthening
upper ridge in the eastern Pacific. This suggests more limited
moisture in a pattern only favoring windward showers.

Recent GFS runs have dug a stronger short wave trough
southeastward to a position to the north of Hawaii from an
amplified west Pacific ridge Friday through the weekend compared
to most other guidance. This ridge is pumped up as Tropical Storm
Sanvu rides polewward around the western periphery of the ridge.
While the GFS based solution is certainly plausible considering
the amplitude of the ridge, GEFS and ECMWF ensembles support a
concensus amplitude of the ECMWF, UKMET, Canadian.

An associated surface front sets up well to the northwest of the
state late week and island trades begin to gradually weaken and
veer more southeasterly. This then leads to an increase in low
level moisture, but overall the models maintain modest
precipitable water values and limited showers this weekend. By
Monday-Wednesday, additional polar stream trough energy nudges the
front closer to the state. Increasing tropical moisture should
then support the chance for increasing showers and thunderstorms,
particularly across the western part of the state where a slow
moving plume of high precipitable water develops.

Schichtel