Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
704 AM EST Fri Jan 12 2018

Valid 00Z Sat Jan 13 2018 - 00Z Sat Jan 20 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 northerly surface low
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 (into the
20-30 kt range), but will keep the deepest moisture suppressed
south of the state. Thus, expect a modest increase early 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 (GFS and ECMWF both show Pwat values rising to
1.2-1.3 inches). The result should be further enhancement of
windward showers, with perhaps some locally heavy rain possible by
late next week.

Ryan