Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
811 AM EDT Fri Oct 19 2018

Valid 00Z Sat Oct 20 2018 - 00Z Sat Oct 27 2018

Water vapor satellite loop early this morning shows a
mid/upper-level shortwave approaching Hawaii from the northwest.
Models show good consensus that this feature will move across
Hawaii through Saturday before upper ridging begins to build
across the state by late in the weekend. The shortwave will result
in a period of relatively weak trades, with scattered to numerous
showers over the eastern islands where the deepest moisture will
be in place. As upper-level heights rise across Hawaii early next
week and an expanding surface high north of the islands increases
trades, models/ensemble show general agreement than an easterly
wave will cross Hawaii by Tue-Wed. The wave will result in an
enhancement of trades, perhaps into the 20-30 kt range with higher
gusts, and an increase in moisture/shower potential. The ECMWF
solution has become much wetter with this system compared to 24
hours ago (and relative to the GFS), showing the potential for
heavy/multi-inch rainfall totals across the Big Island Tue
night/Wed as PWs approach 2.5 inches. At this time, confidence is
relatively low in that specific solution, as ECENS probabilities
for heavy rainfall during the same time period are relatively low
(10-15% chance of exceeding 1 inch of rainfall).

A rather active mid-latitude flow-regime and frontal zone should
remain north of Hawaii through most of next week, with some model
solutions suggesting the potential for amplification of a larger
trough across the north central Pacific by late next week, perhaps
pushing a cold front closer to Hawaii. Confidence in the amplitude
of this trough is low, however, with the GEFS mean much more
amplified relative to the ECENS/CMCE means.

Ryan