Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
711 AM EST Fri Jan 11 2019

Valid 00Z Sat Jan 12 2019 - 00Z Sat Jan 19 2019

In most respects guidance clusters well and maintains good
continuity from yesterday.  Consensus continues to show an upper
ridge initially to the southwest of the state expanding over the
islands by the weekend and early next week before giving way to a
progressive shortwave later in the week.  Through midweek expect
the upper ridge to keep frontal boundaries to the north of the
state while precipitable water values remain below climatology. 
This combination will likely keep shower activity fairly light
over the next few days.  Meanwhile through Wednesday the center of
high pressure to the east and/or north of the islands will meander
from day to day.  The surface ridge will promote some background
trade flow and a general windward focus for any shower activity,
but trades may be light enough at times to allow for sea breezes
and associated shower focus in the afternoon.

By Thursday-Friday the models and ensemble means still display
better than average agreement for the progressive shortwave that
will bring a cold front and a brief period of somewhat more
organized rainfall into and through the region.  This front should
reach the northwestern islands Thursday and perhaps even clear the
Big Island by Friday.  There will be a narrow band of relatively
higher moisture along the front but precipitable water may not
reach too far above normal, and fast movement of the front should
tend to keep rainfall amounts down somewhat as well.  By late
Friday the one difference in the guidance is that the operational
models are stronger with trailing high pressure than the
GEFS/ECMWF means.  At this time a model/mean compromise for
strength of high pressure and associated trades should provide a
reasonable balance considering forecast uncertainty versus
operational models sometimes having a better lead on details of
features.

Rausch