Hawaii Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EST Mon Dec 23 2019 Valid 00Z Tue Dec 24 2019 - 00Z Tue Dec 31 2019 ...Heavy rain possible across the western Hawaiian islands Tuesday night through Wednesday... An upper-level low initially well west of Hawaii is forecast to move northeastward by the middle of the week as it gets influenced and eventually picked up by stronger westerlies farther north. A cold front is forecast by all guidance to sweep eastward toward/across Hawaii by Wed 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 are expected to result in the potential for heavy rainfall, especially across the western Hawaiian islands, starting Tue night and continuing into Wed and possibly Thu. 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. Confidence is still moderate at best given the continued spread among the guidance on the specifics of the event, however the latest runs of the UKMET and CMC appeared to be more along the lines of the ECMWF in terms of a more significant heavy rain threat. ECENS probabilistic precipitation guidance shows the greatest chance for heavy rains across Kauai and Oahu between 12Z Wed and 12Z Thu, while similar GEFS probabilistic guidance resembles the GFS in keeping the highest chance for heavy rains north of Hawaii. Given the synoptic setup and anomalous PWs (+3 to +4 standard deviations), there seems to be sufficient synoptic support for heavy rainfall - and the potential for at least a couple inches of rain seems there across the western islands (with potentially much higher totals of the more bullish solutions verify). Strong/gusty winds may also accompany the front through the western islands on Wed. The cold front is forecast to sweep eastward across the remaining Hawaiian islands through late in the week before stalling and washing out. The ECMWF depicts a widespread heavy rain threat accompanying the front eastward across the state, as far east as Maui by Thu. This is a change from previous runs, however, and the ECENS continues to suggest the threat for heavy rainfall diminishing eastward with time. The upper-level support will also be quickly pulling away from the state by that time. 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 next weekend as the front dissipates and high pressure once again builds north of the state. Ryan