Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
810 AM EDT Mon Oct 02 2017

Valid 00Z Tue Oct 03 2017 - 00Z Tue Oct 10 2017

Latest satellite image shows the surface frontal boundary
discussed in recent days situated just west of the westernmost
Hawaiian Islands, with an upper low positioned generally along 160
deg west, about 300 miles north of the Islands. Models and
ensembles show good agreement that the upper low will drift
generally westward through the week, allowing the surface front to
also drift westward as it washes out. While the front is still in
close proximity, locally heavy rainfall will continue to be
possible across the western Islands through Tuesday, but models
continue to suggest the axis of heaviest rainfall amounts will
remain just west of the Islands, focused along the front where
precipitable water values will be maximized just shy of 2 inches.
In the wake of this system, upper ridging is forecast by all
guidance to once again expand across Hawaii, centered well north
of the state. This will allow rather brisk easterlies of 20-30 kt
to return by mid to late week. The ridge and easterly flow will
also advect much drier air across the state, with PWATs decreasing
to near or even just below 1 inch.

By late in the week, an amplifying upper trough across the
northeast Pacific is forecast by all deterministic/ensemble
guidance to cut off into an upper low near 140 deg west, which is
then forecast to briefly drift west-southwestward. While the upper
low is expected to remain well northeast of Hawaii, the resultant
falling heights across the central Pacific may push a cold front
southward to Hawaii by late Friday or Saturday. The GFS and ECMWF
are in general agreement on this scenario and the timing of the
frontal boundary. The front should once again disrupt the trades
by next weekend, with low-level flow turning more northeasterly
and gradually weakening through the weekend. A modest increase in
PWATs looks to accompany the front, with values rising generally
into the 1.3-1.5 inch range per the GFS/ECMWF. This should result
in an increase in shower activity by next weekend, although the
GFS and ECMWF both show shower activity remaining relatively
scattered. Behind the front, northeasterly flow should once again
bring drier air to Hawaii, with shower activity decreasing by
early next week.

Ryan