Hawaii Extended Forecast Discussion NWS Weather Prediction Center College Park MD 830 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 through 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 that rides polewward on the western periphery of the ridge. While the GFS based solution is certainly plausible considering the amplitude of the ridge, ensemble member spreads including the GEFS and ECMWF ensembles instead support a more concensus solution amplitude of the ECMWF, UKMET, Canadian. An associated surface front will set up well northwest of the islands by later this week and should result in trades weakening a bit as well as veering around to a more southeasterly direction. This will likely result in an increase in low level moisture, but overall the models maintain slightly below average PWs, so not expecting a large increase in shower activity through Saturday. Into Sunday-Wednesday, a trough in the polar stream and have a pretty well-defined surface front approaching the northwest part of the island chain. As a result, trades should weaken considerably, with winds turning more southerly. Increasing tropical moisture should support the chance for increasing heavy showers and thunderstorms, particularly across the western part of the state where a slow moving plume of high precipitable water develops. Schichtel