Hawaii Extended Forecast Discussion NWS Weather Prediction Center College Park MD 713 AM EST Sun Dec 20 2015 Valid 00Z Mon Dec 21 2015 - 00Z Mon Dec 28 2015 Strong trades over the next couple days should begin to weaken by Tue-Wed as high pressure to the northeast of the state is overtaken by an upstream high that will take a farther north track and progress rapidly into the far eastern Pacific. Precipitable water values should be at their lowest today (Sunday), then increase early in the week and stabilize for the rest of the week albeit with typical fluctuations. Thus expect continued shower activity of varying intensity focused over but not fully confined to windward terrain. Details will depend on specifics of mid level energy drifting northeastward from south of the islands around Tue and leading to a compact upper low to the east of the area by Thu. As was the case 24 hours ago, GFS forecasts are still weaker with the upper feature than the ECMWF/ECMWF mean and UKMET. By Fri and into the weekend there is reasonable agreement that a cold front trailing from a strong Aleutians system will approach the state. This front may disrupt trade flow somewhat and provide enhanced moisture focus. However an overwhelming majority of model/ensemble guidance suggests the 00z GFS may be too deep with energy aloft that pulls off the southern part of the upper trough, and as a result overdone with the extent of trade flow disruption and magnitude of increased PWATs. The type of evolution being depicted aloft by next weekend tends to have low predictability at extended time frames so confidence in the upper level forecast is low. At the very least the most likely forecast at the surface is represented by a more compatible cluster of 06z GFS, 00z GEFS mean, and 00z ECMWF/ECMWF mean. Note that even with this majority non-00z GFS scenario there will be potential for some rainfall enhancement with the arrival of this front. Rausch