Hawaii Extended Forecast Discussion NWS Weather Prediction Center College Park MD 707 AM EST Fri Dec 18 2015 Valid 00Z Sat Dec 19 2015 - 00Z Sat Dec 26 2015 Guidance agrees that trade winds will be strongest from now into early next week as strong surface high pressure initially well north of the islands drifts only slowly eastward. Although a reinforcing high will pass by to the north and continue into the eastern Pacific, its more northern track will allow the pressure gradient over the islands to relax somewhat during the mid-late week time frame. In addition by late next week a front trailing from a northern Pacific/Aleutians system may begin to approach from the northwest. Into Mon there is reasonable agreement from the models on precipitable water values varying between slightly below an inch and 1.25 inches or so. Around Sat sheared mid level energy and an associated weak surface front will likely drop southward across the region, around an upper level high. This may provide some enhancement of shower activity that should be most pronounced over/near the Big Island. Moisture details/shower intensity become more uncertain from Tue onward given model differences in what happens to the sheared energy that drops south of the state earlier in the period. Recent ECMWF runs with support of the EC mean aloft bring in higher PWATs due to northeastward progression of a compact upper circulation, in contrast to the more suppressed GFS. The UKMET is a slightly eastward-shifted version of the EC while the CMC at least has somewhat more of a weakness aloft than the GFS. So at this time would suggest somewhat higher moisture potential than 00z/06z GFS runs but with insufficient confidence to go beyond a compromise with the 00z EC. Rausch