Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
716 AM EST Sun Nov 25 2018

Valid 00Z Mon Nov 26 2018 - 00Z Mon Dec 03 2018

Upper-level ridging will persist across Hawaii through the next
few days before things begin to change by mid to late this week. A
surface cold front is expected to weaken as it pushes south toward
Hawaii Mon-Tue, with little to no impacts in terms of sensible
weather for the islands. Trades will gradually strengthen slightly
into the 15-25 kt range through Wed. After that time, model
consensus continues to improve that a wave in the westerlies will
develop across the Central Pacific Mon-Tue, which will then
undergo significant amplification as it approaches Hawaii by late
in the week, with a surface low pressure system likely to track
several hundred miles northwest of the state. There remains some
uncertainty as to the exact degree of amplification that will
occur, with the UKMET and ECMWF the least amplified solutions and
the GFS/CMC the most amplified. In general, solutions seem to have
converged somewhat over the past day, however. Given the expected
evolution of this system, the potential will be there for
northward transport of moisture from the deep tropics, and the
potential for heavy rain for Hawaii late this week. Recent
models/ensembles have shown a bit of a trend away from a
significant heavy rain threat. The deterministic ECMWF has shown
the most significant change, now among the least amplified
solutions with the trough, it is much weaker with the northward
moisture flux into the system and much less with the heavy rain
threat for Hawaii. The ECENS seems to support the operational to
some degree, with 48-hour probabilities of exceeding 1 inch of
rainfall decreasing to less than 10 percent for the western
islands. The GFS on the other hand continues to suggest a bit more
potential, continuing to show 20-30 percent probabilities Thu-Sat.
There is also some question as to whether the trough will linger
north of Hawaii (as shown by the GFS/CMC) or more quickly move
east (as shown by the ECMWF). Ensembles are highly dispersed, but
perhaps show marginally more support for a slower progression.
Nonetheless, the forecast across Hawaii by late this week into
next weekend remains uncertain.


Ryan