Hawaii Extended Forecast Discussion NWS Weather Prediction Center College Park MD 719 AM EST Fri Feb 16 2018 Valid 00Z Sat Feb 17 2018 - 00Z Sat Feb 24 2018 Models/ensemble suggest a blocky pattern developing across the north/central Pacific over the next several days, with an ridge amplifying south of Alaska and persistent negative height anomalies in the vicinity of Hawai'i in the form of a couple upper-level lows. The first upper low closes off directly over Hawai'i by Sun, with the low weakening and a second upper low closing off west of the Islands by Wed. Low-level southerly flow will transport copious amounts of moisture northward across Hawai'i, with Pwats of 1.50-2.0 inches by early next week, especially across the eastern Islands. Models show some disagreement on what happens by mid-week as the second upper low forms, with the ECMWF developing a surface wave of low pressure that moves east of Hawai'i, allowing drier air to move in in its wake, while the GFS keeps the moist air mass in place for longer. Thus, the ECMWF shows a period of potential for very heavy rainfall amounts from today through Wed (especially across the eastern Islands), while the GFS keeps precipitation around for longer (although amounts are not as heavy as the ECMWF). ECENS data suggest the potential for 24-hour rainfall totals exceeding 2 inches becoming more likely across the Big Island by Mon-Tue (probabilities reaching 40 percent), with multi-day totals of several inches possible. These heavy rains will support the potential for some areas of flash flooding across the Big Island. Ensemble data show some potential for an ECMWF-like solution with a surface wave developing, but there is a range of spread as to the exact position/track of the system. This, while a solution resembling the ECMWF is possible, confidence in the specific details is relatively low at this time. In terms of surface winds, flow will generally be east-southeasterly through the weekend, gradually increasing into early next week. Depending on whether a surface low develops by Mon night/Tue (as the ECMWF) shows, this will play a role in the evolution of low-level winds by that time. If the ECMWF solution verifies, would expect increasing east-southeasterly flow (20-30 kt) by Mon night, with higher gusts possible as the system passes on Tue, before a wind shift to westerly or northwesterly in the wake of the system. If a surface wave does not develop, expect broad east-southeasterly flow to persist through much of the period. Ryan