Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
723 AM EST Tue Jan 30 2018

Valid 00Z Wed Jan 31 2018 - 00Z Wed Feb 07 2018

A Rex Block pattern will dominate the north central Pacific into
the weekend, before beginning to break down to some degree by
early next week. While this pattern remains in place, an upper low
will persist south of the Aleutians, with a broad trough axis
extending into the central Pacific, and greatest negative 500 hPa
height anomalies centered roughly 1000 miles west of Hawai'i.
Models and ensembles show general agreement on this large scale
pattern, which should keep a series of shortwaves traversing the
southwesterly upper flow near or just west of Hawai'i. An
initially dry air mass in place across Hawai'i (Pwats under 1
inch) should moisten quickly by the weekend owing to deep
southerly flow east of the trough, and Pwats should rise into the
1.50-1.80 inch range by Saturday. Thus, expect a gradual increase
in shower activity over the next several days, particularly for
the western islands which will be in closest proximity to the
stream of upper shortwave energy and the best upper diffluence.

As the blocking pattern begins to weaken by Sat night into Sun,
there is general agreement among the guidance that the upper
trough should begin to propagate eastward toward Hawai'i, perhaps
bringing a surface cold front east to the Islands as well. The
ECMWF is quicker to bring the surface front east to Hawai'i
relative to the GFS/CMC, and the ECENS mean is also slower,
suggesting the deterministic ECMWF may be a bit too quick with the
front. These differences manifest in the model QPF fields as well,
with the ECMWF a bit quicker to bring heavier QPF amounts east
across the Islands on Sun, but most guidance does spread rainfall
across much of Hawai'i during the Sun-Mon time frame. Given deep
southerly flow, high Pwat values, and increasingly diffluent upper
flow ahead of the approaching trough by late in the weekend, the
environment does appear supportive of potentially heavy rainfall.

Model consensus suggests that the upper trough may further amplify
as it crosses Hawai'i Mon-Tue, with the GFS a bit stronger in this
respect relative to the ECMWF. By this time the best moisture axis
should shift either to the Big Island or perhaps even east of
Hawai'i if the GFS solution were to verify. Thus, there is some
increased forecast uncertainty by the Mon-Tue time frame,
centering primarily around how much upper trough amplification can
occur over Hawai'i and its resultant impacts on pushing the
surface front/moisture axis eastward.

Ryan