Hawaii Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 AM EST Sat Dec 21 2019 Valid 00Z Sun Dec 22 2019 - 00Z Sun Dec 29 2019 ...High winds possible though Saturday... ...Heavy rain possible across the western Hawaiian islands by the middle of next week... A cold front is forecast to cross Hawaii on Saturday, stalling south of the state as it washes out. Moisture along the front will bring some enhancement to rain shower activity before drier air moves in on Sunday. Strong surface high pressure north of Hawaii along with an area of low pressure well west of Hawaii are expected to result in an enhanced surface pressure gradient across the region through Saturday, resulting in the potential for strong trade winds, with high wind gusts possible (perhaps gusting as high as 60 mph for some areas). These winds should gradually relax somewhat by Sun-Mon as the surface high weakens and moves east. A second area of high pressure north of the state (albeit weaker than the first) could produce another breezy/gusty period by Tue/Wed. By the middle to end of next week, a cutoff low initially well west of Hawaii should get picked up by strong westerlies farther north, and begin heading eastward. Models/ensembles show general consensus on this idea. A plume of moisture should accompany the system eastward, crossing Hawaii as early as Tue and persisting into Thu. Models show somewhat better consensus surrounding this relative to yesterday, with the ECMWF and GFS now showing PW values exceeding 2 inches across the western Hawaiian islands by Tue night/Wed. With this moisture focused along a weak surface boundary, the potential for heavy rainfall appears to exist. The heavy rainfall threat will be greatest across the western islands starting late Tue and continuing into Wed, where both the GFS and ECMWF suggest that multi-inch rainfall totals are possible. The heavy rainfall threat should wane by Thu-Fri as the upper trough axis moves quickly eastward and the plume of strong moisture advection dissipates. Confidence in this heavy rain event for the western islands remains perhaps slightly below average (but has improved a bit over the past 24 hours) given evident spread among ECENS members on how far the moisture plume can progress eastward before it begins to diminish. Ryan