Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
234 AM EST Wed Nov 20 2019

Valid 00Z Thu Nov 21 2019 - 00Z Thu Nov 28 2019

An upper-level low initially across Hawaii is forecast to fill as
it drifts eastward Wed-Thu, bringing a more stable air mass to the
state. This should serve to at least somewhat reduce the coverage
of showers and thunderstorms across the state from that seen over
the past few days. Strengthening trades by late this week are
expected as surface high pressure builds north of the state.
Strong trades in the 20-30 kt range (with higher gusts) are
possible by Thu night into the weekend. Sufficient moisture will
remain across the state to support a showery pattern. Models have
made some significant changes of the past 24 hours during the
extended period. An amplified flow regime farther north across the
Pacific will allow for a shortwave trough well northeast of Hawaii
to split off, with some of the energy drifting southwestward and
perhaps reaching Hawaii. The ECMWF has been more aggressive with
this idea, but the GFS also hints at the possibility. If this
occurs, a pool of enhanced instability would once again bring an
increase in coverage and intensity of shower and thunderstorm
activity across Hawaii by the early to middle portion of next
week. Additionally, models show an upper-level trough digging
southward west of Hawaii, which would potentially help to enhance
transport of tropical moisture northward across toward Hawaii. The
ECMWF is the most aggressive with pulling tropical moisture
northward, and shows the potential for very heavy/multi-inch rains
across Hawaii next week, while the GFS keeps the deepest moisture
south of the state. Given the significant change in guidance
(particularly the ECMWF) over the past day, confidence is low in
the deterministic solutions by that time, and would favor sticking
close to ensemble means at this time (which seem to favor keeping
the deepest moisture just south of Hawaii).

Ryan