Hawaii Extended Forecast Discussion NWS Weather Prediction Center College Park MD 818 AM EDT Tue Apr 09 2019 Valid 00Z Wed Apr 10 2019 - 00Z Wed Apr 17 2019 ...Heavy rain possible later this week into the weekend... Guidance agrees that trades will be brisk to strong through the weekend into the start of next week, supported by strong eastern Pacific high pressure which by the late week/weekend time frame will retrograde and merge with a fast moving western Pacific high that moves in from the west-southwest. The high should linger north of the main islands into Monday and then progress eastward, finally leading to a weaker trend for trade flow. This pattern will promote a windward focus for rainfall through most of the period with some activity occasionally straying to other locations. Models and ensembles continue to show that an upper low will rapidly form to the east of the islands over the coming day as energy drops southward along 145-150W longitude. The upper low will then retrograde closer to the main islands into late week. Most solutions indicate that the upper low will open up during the weekend and leave an elongated trough whose axis may drift very slowly southward into early next week. Heaviest rainfall potential currently exists in the Friday-Saturday time frame over the Big Island and vicinity, corresponding to when guidance indicates the most favorable combination of high moisture and lowest heights aloft. There is greater uncertainty about Thursday and Sunday onward. ECMWF-based guidance takes longer to bring in highest precipitable water values versus the GFS/GEFS late in the week. Then the ECMWF keeps fairly high PWATs over the islands into next week in contrast to the GFS/GEFS which show an aggressive push of drier air from the northeast Sunday onward. A compromise looks reasonable for late this week given overall similarity of pattern evolution. Then from Sunday into early next week preference shifts to a compromise between the 00Z ECMWF mean and latest GFS/GEFS mean runs. Compared to the full ensemble spread the 00Z ECMWF keeps the upper feature somewhat northwest of the majority of members (though it should be noted, not an outlier). The ECMWF mean also shows somewhat less moisture over the main islands than the operational ECMWF Sunday-Tuesday, further supporting an average of the ECMWF mean and other non-ECMWF solutions. Rausch