Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
729 AM EST Fri Feb 09 2018

Valid 00Z Sat Feb 10 2018 - 00Z Sat Feb 17 2018

In the wake of the initial longwave trough exiting east of the
Hawai`ian island chain, a cold advection regime will ensue as
low-level northerly flow prevails into the weekend. This system is
forecast to close off and meander a bit along 150W as a rex block
sets up with the closed high building to the north. Local winds
weaken considerably throughout the weekend as the passage of a
surface wave encourages a shift to the more easterly direction. A
tongue of higher precipitable water air will peel back toward
Hawai`i raising the chance for scattered showers by as early as
Sunday.

Looking into next week, the circulation east of the state should
shear while all eyes move upstream with an impressive longwave
trough looming east of the International Dateline. While the
northern extension of these height falls should remain
progressive, operational and ensemble guidance suggest another
closed low will pinch off east of 170W by late Monday/early
Tuesday. This scenario is agreed upon by all guidance outside of
the 00Z UKMET. The change in the pattern will lead to a persistent
warm advection regime as south-southeasterly low-level flow takes
shape leading to more sufficient moisture transport out of the
equatorial central Pacific. By Wednesday morning, the
mid/upper-level circulation should spin just west of Kauai with
the strongest 564-dm clustering noted by the 00Z ECMWF ensemble
members. While the operational models agree on the development of
this system, they diverge with placement which would have
ramifications on where heavy rainfall likely would set up. Prefer
to lean in the slower direction given the tendency for models to
be too quick in ejecting closed circulations. This supports a
combination of the 00Z ECMWF/ECMWF ensemble mean later in the
period although confidence becomes rather low at this point. The
choice of models will of course also influence the wind fields as
the slower 00Z ECMWF/ECMWF ensemble mean combination would enforce
a lengthier period of southeasterly flow versus a frontal passage.
Where agreement does agree is with the heavy rainfall signal over
Hawai`i with precipitable water values likely reaching the 1.50 to
1.75 inch range. The wetter 00Z ECMWF would suggest such values
exceeding 2 inches with several inches of rainfall possible.
However, details are highly uncertain, but it is definitely
monitoring as future guidance arrives.


Rubin-Oster