Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
819 AM EDT Mon Apr 23 2018

Valid 00Z Tue Apr 24 2018 - 00Z Tue May 1 2018

The forecast period begins with a surface high building north of
Hawai'i today and persisting through Wednesday as it tracks
towards the east near 35 degrees north, and an upper level ridge
situated over the western part of the Islands.  The high to the
north will ensure a steady east to northeast trade wind flow, and
becoming windy at times as the surface pressure gradient tightens.
 By Thursday, the guidance is indicating a change in this weather
pattern as a weakness forms within the ridge as an inverted
surface trough develops near the state followed by a new surface
low.  This is in response to an amplifying long wave trough over
the north-central Pacific by the end of the week.  Yesterday, the
EC mean was showing a more amplified solution than the GEFS mean. 
Today the opposite is true with the GEFS mean indicating a more
impactful system that is farther south for the weekend given the
more amplified upper trough.  Both the deterministic GFS and ECMWF
indicate the surface low dropping below 1000mb at times, and
guidance has been trending stronger over the past 48 hours.

The result in either case would be a decrease in the typical trade
winds along with southeasterly veering, with a frontal boundary
approaching the northwestern Islands by Saturday.  This will also
increase the coverage of showers and some thunderstorms as PWs
approach or even exceed 2 inches owing to the moist advection from
the Doldrums.  With the developing surface low expected between 30
and 35 degrees north, the strongest winds should remain north of
the state, however there will likely be an increase in swells on
north facing shores.

D. Hamrick