Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
301 AM EST Thu Jan 09 2020

Valid 00Z Fri Jan 10 2020 - 00Z Fri Jan 17 2020

Guidance continues to advertise a period of heavier rainfall over
the state late this week into the weekend as an upper low
initially to the east of the area elongates westward and crosses
the region.  During the weekend this upper low energy should
combine with a shortwave approaching from the west to yield a
better defined upper low west of the main islands, tracking
northwest/north with time.  The 00Z GFS/GEFS mean are still
stronger with this re-developing upper low to the west versus most
other guidance.  Likely due in part to this strength, and then
shortwave energy coming in from the west (keeping heights aloft
lower than other guidance), the GFS/GEFS hold an axis of high
precipitable water values (up to 3-4 standard deviations above
climatology) over the islands for multiple days.  Thus the
GFS/GEFS mean would extend the potential for periods of heavy rain
into the first half of next week.  On the other hand the 00Z ECMWF
keeps the best moisture axis somewhat westward though still at
times including the northwestern islands--leading to much lower
rainfall totals at most locations.  The 12Z ECMWF mean provides an
intermediate solution that may be the most reasonable starting
point given the current guidance envelope which as a whole
suggests less of a drying trend than consensus was showing 24
hours ago.  Meanwhile east-central Pacific high pressure will
support brisk trades during the next few days, with a gradual
trend from easterly to southeasterly in response to surface
troughing/low pressure to the west and northwest of the state. 
Weakening of the east-central Pacific high and a front coming into
the picture off to the northwest should allow for trades to
slacken late in the period.  Note that 00Z GFS progression of the
front by day 7 Thursday is overdone compared to most other
models/means.

Rausch