Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
704 AM EST Sat Jan 13 2018

Valid 00Z Sun Jan 14 2018 - 00Z Sun Jan 21 2018

A relatively weak flow pattern will be in place across Hawai'i
initially, with ridging centered east and west of the state, and a
more active mid-latitude flow pattern to the north of the state.
This setup will prevent low-level trades from taking hold at least
into early next week, with very light north-northeasterly surface
flow instead. This flow will keep dry air across the state through
the weekend, with Pwats near or just below 1 inch, and very little
in the way of shower activity. By early next week, models show
good agreement that upper ridging will migrate eastward across
Hawai'i, with the surface high centered north of the state. This
will allow brisk low-level east to east-northeast flow to resume
(reaching the 20-30 kt range by midweek), but will keep the
deepest moisture suppressed south of the state. Thus, expect a
modest increase early to mid next week in shower activity along
windward terrain due to the increasing low-level flow, but
widespread precipitation is not anticipated.

By late next week, a majority of the models/ensembles show a
weakness in the ridge developing west of Hawai'i as a piece of
energy separates from the higher latitude flow and drifts south.
The effects of this feature on Hawai'i appear rather limited at
this time, primarily just a gradual increase in moisture through
late in the week into the weekend (GFS and ECMWF both show Pwat
values rising to 1.0-1.3 inches). The result should be further
enhancement of windward showers, with perhaps some locally heavy
rain possible by late week into the weekend.

Ryan