Hawaii Extended Forecast Discussion NWS Weather Prediction Center College Park MD 233 AM EST Wed Dec 25 2019 Valid 00Z Thu Dec 26 2019 - 00Z Thu Jan 02 2020 ...Heavy rain possible across the western Hawaiian islands Tuesday night through Wednesday... An upper-level low initially northwest of Hawaii is forecast to move northeastward through the middle of the week as it eventually gets picked up by stronger westerlies farther north. A cold front is forecast to sweep eastward across Hawaii over the next couple days south of the upper-level/surface lows. A plume of deep tropical moisture along/ahead of the front along with enhanced instability associated with the upper-level shortwave will continue to result in the potential for heavy rainfall, especially across the western Hawaiian islands into Wed. The GFS and ECMWF continue to show some differences that impact the degree of heavy rain threat, with the ECMWF continuing to be the more bullish of the two solutions, with PWs (near 2.25 inches) and resultant rainfall amounts. The GFS continues to weaken the moisture plume more quickly as it moves eastward, and keeps the heaviest rainfall amounts north of Hawaii, likely driven by closer proximity to the best mid/upper-level forcing. Given ongoing flash flooding tonight across portions of Kauai, would expect this threat to continue until the front and deepest moisture move east of the island during the day on Wed, with the potential for at least 2-3 inches of additional rainfall. The cold front is forecast to sweep eastward across the Hawaiian islands through late in the week before stalling and washing out. The threat for heavy rainfall should gradually diminish eastward with time as the upper-level support quickly pulls away from the state. Thus, expect scattered (locally heavy) showers to persist across Hawaii into next weekend. As the front passes it will disrupt the trades, with a period of light and variable winds possible by Thu night-Fri. More typical trades should resume by the weekend and continuing into early next week as the front dissipates and high pressure builds north of the state. Ryan