Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
828 AM EDT Thu Mar 22 2018

Valid 00Z Fri Mar 23 2018 - 00Z Fri Mar 30 2018

An upper low initially near 35N/175W is shown by all
model/ensemble guidance to move southeastward over the next few
days, passing north of Hawai'i over the weekend, with an
associated surface front passing through the islands as well. A
surge high high precipitable water values (near 2.00 inches) ahead
of the front will support the development of widespread convection
and potentially heavy rainfall. The heaviest rainfall should focus
across the western islands Fri-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. The 06Z GFS shows
potential heavy rains across the western islands Fri-Sat night,
but then an extensive area of convection farther south seems to
serve to cut off more extensive convection across the eastern
Islands. Meanwhile, the ECMWF keeps the entire Hawaii'an chain
under a threat for heavy rainfall. ECENS probabilities suggest
there could be some credence to the GFS idea, with probabilities
of heavier amounts showing a slight reduction across the eastern
islands. At this time, confidence in the convective details is low
so either scenario seems possible.

After Monday, models/ensemble show general agreement that heights
will rise across Hawai'i with drier air advecting across the
state. 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