Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
826 AM EDT Sat Apr 21 2018

Valid 00Z Sun Apr 22 2018 - 00Z Sun Apr 29 2018

After several days of unsettled weather across the region, a
return to a more typical trade wind pattern will ensue for this
weekend and into early next week.  The upper low will continue
departing towards the east away from the Islands and allow an
upper level ridge to build in across the northwest part of the
state.  This will allow much drier air (PWs in the 0.9 to 1.2 inch
range) to spread across the state, thus reducing the coverage of
windward showers that develop.  Low-level easterlies should also
increase into the 20-30 kt range by mid-week as the surface
pressure gradient tightens owing to a building surface high to the
north, centered near 35-40N. 

Models and ensemble means show general agreement that this drier
pattern should persist into late next week, before a northern
stream Pacific trough amplifies northwest of the state.  Models
show the amplifying trough pushing a cold front and pre-frontal
surface trough toward Hawai'i by Friday.  Solutions vary on timing
but at this time an ensemble mean approach is preferred.  The 6Z
GEFS mean would have a greater impact on Hawai'i given the closer
proximity of the surface low by next Saturday morning, whilst the
EC mean is farther northeast with the low.  The system should
begin to disrupt the low-level easterlies by Friday, and advecting
more of a tropical airmass from the equatorial region towards the
state and increasing the PWs to above 1.75 inches and thus greater
coverage of shower activity.

D. Hamrick