Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
816 AM EDT Mon Apr 30 2018

Valid 00Z Tue May 01 2018 - 00Z Tue May 08 2018

The large scale pattern across the central and north Pacific will
remain relatively unchanged through the week. Models/ensemble show
a strong signal for a broad area of negative height anomalies
persisting in the vicinity of 35-40N/150-170W. The persistent
large scale trough will be maintained by a series of
mid/upper-level shortwaves that traverse the northern stream flow
across the Pacific and then amplify as they encounter the existing
large scale cyclonic flow in place north of Hawaii.

Initially, satellite imagery this morning show a relatively deep
upper-level low in place approximately 700 miles north of Hawai'i.
South of the upper low, a weakening surface front was located just
east of the Big Island, with deep tropical moisture (PWs > 2.00
inches) pooling along the front. As the initial upper low weakens
and becomes absorbed in the higher latitude westerlies, and a
second shortwave enters the large scale mean trough over the next
couple days, models show the surface front and the associated deep
moisture gradually lifting back northward across Hawaii, and
lingering into the weekend as it gradually washes out. Guidance
shows low-level winds initially remaining relatively light and
variable in the vicinity of the surface front as it washes out
through mid-week, thus the potential for sustained orographic
rains appears fairly limited, despite the deeper moisture.

By later in the week (Wed night onward), guidance continues to
suggest that low-level easterlies may begin to pick up into the
10-20 kt range (increasing flow perpendicular to any remaining
weak surface boundary), while the next trough amplifies just
northwest of Hawaii (increasing upper-level support for
precipitation). Both the ECMWF and GFS continue to indicate some
potential for heavy rains across the western islands, especially
Kaua'i, by Thu-Fri. Both deterministic model solutions show
somewhat lighter QPF amounts during that time period compared to
yesterday, however ensemble probabilities from both the ECENS and
the GEFS continue to indicate the potential for multi-inch
rainfall totals.  Thus, the potential remains, with slight to
perhaps moderate confidence in at least locally heavy rains across
the western islands by late in the week. The threat for heavy
rainfall should begin to wane by Sunday night as models are in
general agreement on another cold front passing through Hawaii as
yet another shortwave amplifies north of the islands.


Ryan