Hawaii Extended Forecast Discussion NWS Weather Prediction Center College Park MD 705 AM EST Sat Dec 19 2015 Valid 00Z Sun Dec 20 2015 - 00Z Sun Dec 27 2015 Expect the strongest trades over the next couple days or so as strong high pressure to the north-northeast of the state drifts eastward by only a few degrees of longitude. Then upstream high pressure, taking a more northern track, will merge with the initial high and move more quickly into the eastern Pacific. By late next week a cold front extending from a strong Aleutians storm will approach the islands. Trailing high pressure may support some strengthening in the trades though the expected surface pressure gradient appears weaker than at the start of the forecast. Southward push of upper level energy and associated surface front will bring somewhat drier air into the region this weekend, especially over the central/northwestern islands. Around the middle of the week models are still at odds over the details of energy lifting back to the northeast from south of the islands. A fair cluster of solutions including the ECMWF/EC mean/UKMET/CMC suggest a stronger reflection aloft which based on ECMWF precipitable water forecasts would bring more moisture across the islands than GFS runs. Later in the week there is somewhat better agreement toward an upper low lingering to the east of the islands. Farther west by next Sat the 06z GFS differs from other guidance in dropping an upper low west of the state, so its surface gradient may be underdone at that time. Expect showers to focus over favored terrain and to a greater extent over the Big Island, though other locations may see some periods of rainfall as well given strength of the trades. Especially around the middle of next week intensity will be sensitive to still uncertain details aloft/PWAT values. Rausch