Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
727 AM EST Tue Feb 05 2019

Valid 00Z Wed Feb 06 2019 - 00Z Wed Feb 13 2019

Between now and Friday guidance agrees that initially diffuse
upper level troughing just west of the state will gradually
amplify and sharpen as it approaches/crosses the region.  This
evolution aloft will promote development of a surface trough over
the main islands by Wednesday, significantly interrupting trade
flow.  Embedded low pressure that develops within the surface
trough will ultimately track northeast of the area with a trailing
front ushering in brisk northerly winds aided by strong central
Pacific high pressure.  Before frontal passage there should be
some sea/land breeze focus for rainfall given the lack of trade
flow with dynamics aloft offering some potential for locally
moderate-heavy activity though most guidance does not expect
precipitable water values to reach much above one standard
deviation above climatology.

From late Friday into early next week a majority of guidance is
showing an unusual pattern evolution toward a Rex block
configuration featuring strongly positive height anomalies/ridging
south of the Alaska Peninsula and a deep trough with likely
embedded closed low just north of Hawai'i.  However within this
generally agreed-upon evolution there are significant differences
for low position at the surface and aloft and thus for sensible
weather effects across the islands.  At the very least the full
array of model/ensemble guidance suggests that the 00Z ECMWF may
track the low at the surface and aloft too far east/northeast away
from the islands due to a separate northern stream upper low that
drops toward the mean trough and which is not really depicted in
other solutions.  The old 12Z/04 ECMWF runs is closer to an
intermediate track.  Meanwhile the 00Z CMC/CMC mean keep the deep
layer low north or even northwest of the main islands, while the
00Z/06Z GFS and 00Z GEFS mean show a low track closest to the
islands in the Sunday-Tuesday time frame.  The 00Z FV3 GFS loops
the low around on a track a little farther away from the islands
and faster than operational GFS runs.  Based on the current
guidance envelope would initially recommend a compromise of the
00Z GEFS/ECMWF ensemble means for low track at the surface and
aloft, with some input from GFS/FV3 GFS runs to represent strength
including the potential for strong winds to the west/southwest of
the surface low.

Rausch