Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
730 AM EST Fri Feb 24 2017

Valid 00Z Sat Feb 25 2017 - 00Z Sat Mar 04 2017

The mid-latitude pattern will generally consist of a rex block
with a pair of closed lows meandering underneath a mean closed
ridge south of the Aleutians. The more western negative height
anomaly should drift toward the International Dateline which will
help establish mid-level southwesterly flow toward the Hawai`ian
island chain. This is well agreed upon by all forecast solutions
with the upper trough passing through the state by the middle of
next week. Where the guidance diverges is with the next large band
of height falls moving out of the western Pacific with general
timing/amplitude differences evident. The 06Z GFS/GEFS mean were
on the slower side as noted in deterministic/ensemble comparisons
as well as ensemble spaghetti plots.

The surface wind regime across Hawai`i will be changing quite a
bit throughout the period with the initial modest
east-northeasterly trades weakening in response to a diminishing
pressure gradient. Gradually the flow will pick up again by late
in the weekend into early next week before a more pronounced
change takes place by the middle of next week. An emerging area of
low pressure which begins to further strengthen as it departs the
islands should allow for low-level southwesterly flow accompanied
by a significant increase in low/mid-level moisture content.
Consequently, there should be a marked rise in the threat for
precipitation, particularly across the Big Island where many
solutions maintain modest rainfall in the forecast. However, the
06Z GFS/GEFS mean do keep the heaviest activity to the south. Much
of this depend on the strength of the surface reflection as well
as the quality of the moisture lifting out of the tropical
Pacific. Regardless, a wet pattern does appear to be in store
during parts of next week given the change in the overall pattern.


Rubin-Oster/Ryan