Hawaii Extended Forecast Discussion NWS Weather Prediction Center College Park MD 829 AM EDT Fri Oct 26 2018 Valid 00Z Sat Oct 27 2018 - 00Z Sat Nov 03 2018 ...Heavy rain threat for Hawai'i next week... Longwave pattern across the Pacific indicates a 500 mb ridge developing and moving west to east over the islands this weekend, and then east of the islands Mon 29 Oct. The models continue to show a significant upper trough and surface cold front drifting east towards the islands early next week. This will support an increasingly wet pattern for first the western islands and then eastward. The model majority now shows surface low development along the front Monday night and Tue, with the low pressure possibly crossing the islands on Tue before departing to the northeast on Wed. The 00-06z GFS are on the eastern edge of the guidance envelope and 00z UKMET the westernmost solution, placing the 00z ECMWF/ECMWF ensemble mean and Canadian global as the middle of the cluster of solutions and preferred. Heavy rainfall threat potential is high with this pattern given the surge of tropical moisture from the ITCZ with both the GFS and ECMWF showing a 2.25 inch precipitable water maxima in advance of the wave crossing the islands, plus the synoptic lift provided by the approaching low pressure center. Several inches of rain are possible over especially the western islands and windward locations. Mesoscale processes/convection will also play a role in focusing rainfall. The wave departure on 31 Oct allows the trailing front lingering in the islands to drift east. This provides potential for cooler and drier air to filter in to the western islands Wed and Thu. This would be aided as the models generally allow the upper trough over the islands to drift east and the upper ridge west of Hawaii to drift east. Petersen