Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
829 AM EDT Fri Apr 27 2018

Valid 00Z Sat Apr 28 2018 - 00Z Sat May 05 2018

It remains the case that a rapidly amplifying central Pacific
trough aloft will close off an upper low to the north of the state
by Friday, reaching as far south as 30N latitude by late Friday
into the weekend. The upper low will begin to lift northeastward
during the first half of next week, but there are some differences
with specifics in guidance with the latest 06 UTC GFS now holding
lower heights at the base of the system longer than the 00 UTC
ECMWF/UKMET. The GFS has modest multi-model ensemble support and
the mid-scale trend at least may be reasonable considering
upstream flow amplitude. After early Monday heights over the
island will rise somewhat but a northeast-southwest trough should
persist to the north through midweek.

Developing low pressure associated with the upper low will anchor
a front that passes through the state from west to east during
Friday-Sunday. There will be potential for some bands of locally
heavy rainfall along the front as deep moisture increases. Trade
flow currently in place will become southeasterly ahead of the
front and northerly behind the front. After Sunday the front and
axis of highest precipitable water values will remain close enough
to the Big Island, or perhaps lift back to the northwest a bit
given the rise in heights aloft, to support a heavy rainfall
threat at times over the eastern part of the state. Differences in
location of the front/moisture axis as well as details aloft are
well within typical guidance error for 5-7 day forecasts and at
this time show no pronounced clustering. Thus confidence in any
specific solution is not particularly high, recommending an
intermediate solution for forecast specifics.

The western to central islands should see a drier pattern next
week with relatively light windward showers.  Most guidance
expects west-central Pacific high pressure gradually building
eastward to promote a strengthening of northeasterly winds by next
Wednesday-Friday.

Schichtel