Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
818 AM EDT Sun Apr 29 2018

Valid 00Z Mon Apr 30 2018 - 00Z Mon May 07 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 600 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 early next
week, models show the surface front and the associated deep
moisture gradually lifting back northward across Hawai'i over the
next couple days, and lingering into next weekend as it gradually
washes out. Guidance shows low-level winds 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 suggests 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 another trough amplifies just northwest of Hawaii
(increasing upper-level support for precipitation). Both the ECMWF
and the GFS show the potential for more widespread heavy rains
across the western islands by Thu-Fri. The ECMWF would keep the
heaviest rain potential confined to Kaua'i, while the GFS would
also include O'ahu. Both models show the potential for multi-inch
rainfall totals. The ECENS supports the operational ECMWF in
showing the potential for heavy rains across Kaua'i later in the
week. The GEFS probabilities also mirror the operational GFS,
although the heavy rain probabilities are a bit lower than shown
in the ECENS. Thus, at this time there is slight to moderate
confidence in another round of potential heavy rains for the
western Hawaiian Islands by late in the week. The exact
timing/evolution of the late week upper trough, as well as the
strength and position of any lingering surface boundary by that
time will all play a role in the existence and placement of any
heavy rain potential.

Ryan