Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
826 AM EDT Thu Sep 28 2017

Valid 00Z Fri Sep 29 2017 - 00Z Fri Oct 06 2017

Upper-level ridging extending across Hawaii early in the period
will quickly break down by the weekend as a long wave upper trough
amplifies across the north central Pacific. Before the ridge
weakens, a relatively benign upper-level weakness will propagate
westward across the eastern Hawaiian Islands, perhaps causing a
brief increase in shower activity through Friday. Deterministic
models show generally good agreement with respect to the large
trough amplifying north of Hawaii over the weekend, with most
solutions now showing a cutoff low developing just northwest of
Hawaii by early next week. The ECMWF, GFS, and UKMET all show the
upper low persisting through Monday night, while the CMC shows
more interaction with another northern stream trough that
develops, which results in a more short-lived upper low near
Hawaii. While deterministic solutions seem to have trended toward
a more persistent upper low next week, ensemble means continue to
show a bit more spread than deterministic solutions would
indicate, with the NAEFS and the CMC ensembles providing a bit of
support for the operational CMC. Nonetheless, falling heights
across the north central Pacific look 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. At this time ECMWF
ensemble probabilities show the best axis of heavy rainfall
remaining just northwest of the western Islands, with 48-hour
probabilities of exceeding 1 inch of rainfall near 15 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 picks up 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, low-level easterlies should resume
to the 15-25 kt range, with drier air advecting across the Islands
from the eastern Pacific.

Ryan