Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
829 AM EDT Wed Apr 22 2015

Valid 00Z Thu Apr 23 2015 - 00Z Thu Apr 30 2015

There is still good model and ensemble clustering with respect to
unusually deep/southward closed upper low development to the
northwest of the islands. Models continue to indicate that heights
with this feature may reach as low as 4-6 standard deviations
below normal for a time. This feature reflects at the surface as a
frontal zone around 165W longitude, helping to turn low level
winds over the islands more from the southeast allowing for deeper
moisture return. However, the heaviest associated band of heavy
rainfall settles just west of the northwestern islands late week
into the weekend. Models and ensembles are now in better agreement
with the subsequent evolution of the closed low, bolstering
forecast confidence. This solution would allow for slow weekend
ejection of the closed feature just to the north of the state and
entrainment into more mid-higher latitude flow by early next week.
This would bring a focus of heavier rainfall potential gradually
into the islands this weekend through early next week with
trailing frontal approach, passage and stalling into an airmass
with enhancing precipitable water values. Surface high reaching
the central Pacific in the wake of this system will meanwhile act
to strengthen trades through early-mid next week as high pressure
bridges to the north of the state to enhance terrain rainfall
focus.

Schichtel