Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
803 AM EDT Sun Sep 16 2018

Valid 00Z Mon Sep 17 2018 - 00Z Mon Sep 24 2018

Surface easterlies varying in intensity from 10 to 25 kt will
prevail through much of the week as Hawaii will largely be under
the influence of upper ridging. By the middle to latter part of
this week models/ensembles continue to show general agreement that
an upper trough will begin to amplify across the north central
Pacific, an upper low likely to cut off northwest of Hawaii by Thu
or Fri. At the same time models show a weak easterly wave
approaching Hawaii, enhancing the trades with gusty conditions
likely. The degree of northern stream trough/upper low
amplification north of Hawaii will likely play a role in the
amount of deep tropical moisture that is able to stream northward
across Hawaii in association with the easterly wave, and thus the
coverage and intensity of any rainfall. Despite similar solutions
with respect to the upper low, the GFS continues to be more
aggressive with northward transport of deeper tropical moisture,
and shows somewhat more widespread/heavier precipitation across
Hawaii relative to the ECMWF. At this time due to the varying
deterministic solutions, an ensemble mean approach is preferred by
late week; ensemble probabilities suggest the greatest chance for
any heavy rains would be across the Big Island. By next weekend
the easterly wave should move west of Hawaii while the upper low
likely becomes absorbed in amplified northern stream flow. A drier
air mass is expected to once again overspread Hawaii with
relatively light trades next weekend.

Ryan