Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
817 AM EDT Sun Oct 21 2018

Valid 00Z Mon Oct 22 2018 - 00Z Mon Oct 29 2018

Water vapor satellite loop early this morning shows drier air
beginning to overspread Hawaii at the mid-levels as the
upper-level shortwave begins to move east of the islands.
Upper-level ridging is expected to build eastward across Hawaii
over the next day or two in the wake of the shortwave. Models
continue to show an easterly wave tracking south of Hawaii during
the Tue-Wed time period, with some continued disagreement on
exactly how far north the associated deep moisture will spread.
The ECMWF keeps the deepest moisture associated with the system
south of Hawaii, while the GFS brings it farther north, with the
potential for heavy rainfall. Ensemble probabilities can perhaps
shed a little light on the heavy rainfall potential, with the
ECENS probabilities of exceeding 1 inch in 24 hours only reaching
5-10 percent across the Big Island during the Tue-Thu time frame,
and similar GEFS probabilities approaching 30 percent. Given
oscillating solutions among the guidance, taking a compromise
approach is probably best at this point, which would suggest the
potential for at least locally heavy rainfall by the middle of the
week with the passage of the wave. Additionally, trades will
gradually increase as the wave approaches, reaching the 20-30 kt
range by mid-week.

Models consensus has improved a bit by late in the week with
respect to amplifying flow at the mid-latitudes with some
potential to affect Hawaii. The GFS has backed off on its previous
solution which brought strong height falls all the way south to
Hawaii, and is now more in line with other guidance showing a
trough of moderate amplitude developing across the north central
Pacific by Fri-Sat, with the associated cold front likely
remaining north of Hawaii (keeping Hawaii under the influence of
moderate trades). As the mid-latitude flow becomes more
progressive, ensembles show general agreement that a second,
similar trough will amplify across the north central Pacific by
next Sun.

Ryan