Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
723 AM EST Tue Jan 13 2015

Valid 00Z Wed Jan 14 2015 - 00Z Wed Jan 21 2015

The 13/00Z GEFS/ECENS ensembles and their deterministic
guidance---are similar through 18/00Z. Both model solutions
project a north-to-south erosion of the broad-scale Subtropical
Ridge--between 17/00Z and 18/00z---then the 2 solutions begin to
generate differences across the north central Pacific, downstream
along the west coast of the Mainland and for Hawai'i.

In short, differences in the Bering Sea for day 3-5 (16/12Z and
18/12Z) cascade into nearly 'out-of-phase' type solutions on day 6
(19/12Z).

The GEFS/GFS solution becomes a faster/broader solution aloft
across the mid-latitude Pacific between 130W and 170W. By 18/12z,
it begins to generate an extra shortwave invof 40N 170W that
quickly ejects northeastward into the eastern Aleutians near 45N
160W (@ 19/12Z)---a shortwave the deterministic ECMWF has near 45N
170W (@ 19/12Z).

The 13/00Z GEFS/ECENS means are more stable---at the very
least---with a sharping trough at 500mb along the Dateline---from
the Bering Sea southward to 50N latitude---during this 19/12z time
frame. Both means do signal the formation of a significant long
wave trough along the Dateline---for days 7-8, but those details
were far from resolved in the 13/00z forecast cycle. 

A recommendation for the Hawai'i forecast---utilize the 13/00Z
ECENS means for the mass fields this forecast period. Its 13/00Z
deterministic ECMWF run deemed helpful for the smaller-scale
details until 19/12z---based on the comparison of its previous
12/00Z and the last two 00Z GEFS/GFS cycles (12/00Z and 13/00Z) .

This will allow---with some certainty---to track and forecast a
moderate 500mb mid-latitude trough passage from west to east---on
Sunday and Monday. Its arrival and departure providing the
sensible weather changes.

Vojtesak