Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
803 AM EDT Fri Sep 29 2017

Valid 00Z Sat Sep 30 2017 - 00Z Sat Oct 07 2017

As an upper-level ridge weakens across Hawaii, a relatively benign
upper-level weakness will propagate westward across the eastern
Islands, perhaps causing a brief increase in shower activity
through Friday night. Deterministic models show generally good
agreement with respect to the large trough amplifying north of
Hawaii over the weekend, with most solutions showing a cutoff low
developing just northwest of Hawaii by early next week. The
ECMWF/GFS/UKMET/CMC all now show the upper low persisting into mid
week and then gradually drifting southwestward. As the trough
amplifies, falling heights across the north central Pacific are
expected to push a surface cold front toward the western Hawaiian
Islands by late in the weekend, with some potential for increased
precipitation activity. The ECMWF and GFS indicate relatively high
precipitable water values pooling along this front (1.7-2.0
inches). These forecast PWAT values along the front are 2 to 3
standard deviations above average. Thus, a fairly substantial
increase in precipitation would be possible across the western
Islands if the front can draw close enough before stalling early
next week. There remains a general consensus among model guidance
that the best axis of heavy rainfall should remain just northwest
of the western Islands. ECMWF ensemble probabilities show 48-hour
probabilities of exceeding 1 inch of rainfall near 10 percent
across Kauai and Ni'ihau from 00z Monday to 00z Wednesday.
Additionally, trades are expected weaken as ridging shifts more
toward the eastern Pacific and low-level flow gains a more
southerly to southeasterly component.

By late next week models and ensembles show better agreement that
whatever trough/upper low is left northwest of Hawaii will begin
to retrograde rather quickly as upper ridging once again builds
north of Hawaii. As a result, brisk low-level easterlies should
resume into the 20-30 kt range, with drier air advecting across
the Islands from the eastern Pacific through the end of next week.

Ryan