Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
824 AM EDT Fri Oct 27 2017

Valid 00Z Sat Oct 28 2017 - 00Z Sat Nov 04 2017

As a mid-latitude upper low slowly drifts eastward away from the
Hawai`ian island chain, shortwave ridging will set up for the
weekend. Looking toward the International Dateline, a broad band
of height falls begin to take shape within the active mid-latitude
flow regime. All signs point toward the system closing off,
generally along 170W which seems to be a slowing trend relative to
the spread noted yesterday. Eventually a northward jog is noted as
the circulation becomes absorbed into a broad negative height
anomaly across the northern Pacific. This will ultimately allow
heights to rise over Hawai'i with the 00Z CMC even showing a
588-dm closed high forming by Thursday. This overall shift in the
guidance is much more consistent among the ensemble members
although some spatial spread does persist later in the period.

In the wake of a shearing frontal zone, light northerly flow will
continue over the islands while a more easterly component is noted
toward the end of the weekend. Looking ahead to Monday and periods
onward, warm advection will ensue ahead of the next longwave
trough which ultimately favors general south-southeasterly flow.
By Tuesday, some intensification in the wind field is expected
given the strengthening of the surface gradients. As the upstream
trough lifts toward the north-central Pacific, a return to the
more climatological trade wind regime is in the forecast from
Wednesday onward. In terms of precipitation threats, scattered
showers are expected in the wake of the dissipating frontal zone
although overall moisture content is on the lower side. Warm
advection will eventually bring high precipitable water air from
the tropics toward Hawai`i by Monday/Tuesday which may enhance the
coverage of shower activity. Eventually the upstream frontal zone
may approach the island of Kauai by mid-week before the boundary
lifts northward by the following days. Depending on its ultimate
position, some heavy rainfall threat may affect Kauai but it could
easily stay off toward the northwest.


Rubin-Oster