Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
815 AM EDT Sat Mar 24 2018

Valid 00Z Sun Mar 25 2018 - 00Z Sun Apr 01 2018

An upper low initially near 26N/165W is shown by all
model/ensemble guidance to continue moving southeastward over the
next couple days, passing north of Hawai'i, with an associated
surface front passing through the islands as well before stalling
by Mon and gradually washing out through mid-week. A surge of high
precipitable water values (perhaps exceeding 2.00 inches) ahead of
the front will continue to support the development of widespread
precipitation and potentially heavy rainfall. The most widespread
rainfall should focus across the eastern islands on Sun-Mon. The
GFS and ECMWF now show general agreement that the absolute
rainfall maximum should remain just mostly south of the eastern
islands, with southern potions of the Big Island seeing the
greatest heavy rainfall threat. The idea seems to have some
support aloft, with somewhat better upper diffluence noted just
south of Hawai'i late Sat into Sun. ECENS probabilities support
this idea somewhat as well, with the highest probabilities of
heavy rainfall amounts exceeding 2 inches just south of Hawai'i,
and perhaps across the southern portion of the Big Island.

After Monday, models/ensemble show general agreement that heights
will gradually rise across Hawai'i with drier air advecting across
the state. As the front washes out, the Big Island could remain in
somewhat deeper moisture for a couple more days, with the
potential for showers persisting through Wed. Low-level flow
during this time period should be very light through mid-week,
with light easterlies resuming by late next week. A trough/upper
low is expected to amplify along 175W by late next week, and the
GFS/ECMWF show a front approaching by next weekend. The GFS is a
bit quicker with the timing of the front, bringing an increased
threat of showers into the western islands by next Sat, while the
ECMWF would hold the showers a couple hundred miles farther west.

Ryan