Hawaii Extended Forecast Discussion NWS Weather Prediction Center College Park MD 725 AM EST Thu Nov 20 2014 Valid 00Z Fri Nov 21 2014 - 00Z Fri Nov 28 2014 The 20/00Z and 20/06Z forecast cycles did not stray from the current short term Hawai'i forecasts---with a trend towards diminishing trades and increasing cloudiness. The 20/00Z guidance continues to identify the migration of a mid-latitude upper-level trough and attendant cold front across Hawai'i---in the 24/12Z to 26/12Z time frame---but the deterministic runs offered more spread in those details versus 24 hours ago. Of note though---satellite confirms that this next system approaching Hawai'i is currently dropping southward from the southern Bering Sea and spreading out across the Aleutians. Rather good global model agreement in principle---with the origins of this system. The 20/00Z Canadian would be the very slow outlier at 25/12z---with its cutoff 500mb low west of Kauai (near 24N 164W). On the faster side---the 20/00Z & 20/06Z GFS (invof 33N 150W)---both are consistent with a quicker northeastward migration of the feature. Either solution is plausible in the Subtropics at the moment---though the differences at mid-latitude support a GFS-type solution downstream with emphasis on an 'atmospheric-river' like connection between the feature---in whatever form---and the downstream ridge along the mainland's west coast. The Canadian solution has far more 'stream separation'. Then there is the deterministic 20/00Z ECMWF---which appears to be the middle-ground solution---but an aggressive one. It's 25/12Z position at 33N 156W is closed off and deep---which splits the difference versus the Canadian/GFS with respect to lat/long positioning at 25/12z---and is 80 meters deeper than its own mean (20/00Z ECENS). The presence of a 500mb cutoff---or lack thereof---will continue making the Monday/Tuesday precipitation forecast a challenge. At this point, would recommend continuing with a mass-field blend of the 20/00Z GEFS/ECENS means---both of which support the presence of a disturbance aloft---an extension of the mid-latitude trough migration across the northeast Pacific. The reason for recommending and continuing with a mean solution is how both handle the pattern downstream from the central Subtropical Pacific northeastward into the west coast of North America this medium range period---i.e. the longwave trough-ridge pattern---looks to be reshuffling. And those details are still being sorted out---most evident over British Columbia with direct impacts from the Subtropics---north of Hawai'i still in the mix. Vojtesak