Hawaii Extended Forecast Discussion NWS Weather Prediction Center College Park MD 830 AM EDT Mon Sep 10 2018 Valid 00Z Tue Sep 11 2018 - 00Z Tue Sep 18 2018 All eyes will initially be on Hurricane Olivia as it is expected to reach the Hawai`ian island chain late Tuesday night. While forecast to weaken to a tropical storm, it will still be responsible for heavy rainfall, gusty winds, and rough seas. There still remains some north-south spread with the cyclone track but the model differences have diminished markedly from a couple of days ago. Based on the latest Tropical-Storm-Force Wind Speed probability graphic from the Central Pacific Hurricane Center, there is a decent chance such winds will be observed across the central/southern islands. The fact the system should be on the move could somewhat limit a more extensive precipitation event. Based on the latest model guidance, the 00Z ECMWF would support 4 to 6 inches of rain over Oahu while the 06Z GFS favors similar amounts down over Maui. Interestingly, the latter solution suggests additional heavy rainfall lingering in the wake over the more northern islands. However, this appears to be an isolated solution as the 06Z GFS depicts additional positive vorticity advection accompanying a westward propagating upper low. Other solutions show this synoptic-scale feature but will more limited precipitation potential. Once Olivia clears the region, a building dome of surface high pressure across the mid-latitudes will lead to a brisk period of trades, potentially in the 20 to 30 knot range. This is primarily just after Olivia exits Hawai`i given the stronger large-scale pressure gradient. Trades continue throughout the remainder of the period although with a more reduced strength. Further, daily rounds of showers will be likely which would fall on top of anything that occurs with the tropical cyclone. Thus, some hydrologic issues may take place during the period. Rubin-Oster