Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
808 AM EDT Fri Mar 23 2018

Valid 00Z Sat Mar 24 2018 - 00Z Sat Mar 31 2018

An upper low initially near 34N/171W is shown by all
model/ensemble guidance to move southeastward over the next couple
days, passing north of Hawai'i over the weekend, with an
associated surface front passing through the islands as well. A
surge of high precipitable water values (perhaps exceeding 2.00
inches) ahead of the front will support the development of
widespread precipitation and potentially heavy rainfall. The
heaviest rainfall should focus across the western islands through
Sat and then across the eastern islands by Sun. While the GFS and
ECMWF both show agreement on the timing of the system,
convective/mesoscale differences affect the precise distribution
of heavy rainfall. There remains uncertainty as to whether an
extensive area of convection south of Hawai'i will cut off, to
some degree, more extensive convection farther north across
Hawai'i. Both the ECMWF and the GFS seem to show this occurring
although to different degrees. 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 across the
southwestern 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. There is some suggestion that the Big Island could
remain in deeper moisture with for another day as the surface
front potentially stalls and quickly washes out, with the
potential for showers persisting through Tuesday. Low-level flow
during this time period should be very light through mid-week,
with light easterlies resuming by late next week. A trough should
begin to amplify along 175W by late next week, but at this time
ensembles suggest that upper ridging should remain relatively
strong across Hawai'i at least through Fri, keeping the chances
for any widespread convection at bay.

Ryan