Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
356 AM EDT Tue Oct 12 2021

Valid 00Z Wed Oct 13 2021 - 00Z Wed Oct 20 2021

The enhanced deep moisture supporting areas of moderate to heavy
rainfall on Tuesday will continue westward, yielding a drier trend
from Wednesday onward.  Guidance agrees that precipitable water
values will steadily decrease mid-late week, reaching at least one
standard deviation below climatology by the weekend.  Then
solutions diverge somewhat by Monday-Tuesday.  The 00Z GFS and
GEFS mean show a gradual increase of moisture from the south, with
the GFS showing an upper low/shear axis settling to the immediate
north and west of the main islands.  On the other hand the 00Z
ECMWF and 12Z ECMWF mean keep the moisture values low and even
suggest a further drier trend.  The ECMWF shows more of a
northerly push aloft that supports greater southward progression
of shortwave energy.  The 18Z GFS was actually closer to the 00Z
ECMWF in principle for flow aloft and moisture while the ECMWF has
better continuity from its 12Z run.  So while the GEFS mean hints
at an increase of moisture, ECMWF continuity and some progression
aloft seen in the 00Z CMC would seem to favor at least holding
moisture steady early next week if not decreasing slightly.  Still
confidence is not great since the state appears to be under an
area of relatively lower predictability for specifics aloft early
next week.

Models/ensembles continue to show a weakening of the trades over
multiple stages during the period.  Fairly brisk trades will
extend through midweek or so with some weakening noted late week
as a cold front pushes into high pressure to the north/northeast
of the state.  Trailing high pressure will steadily weaken during
the weekend and a stronger cold front north of the state will
further weaken the surface ridge north of the state early next
week.  Trade flow will support a windward/mountain focus for a
majority of shower activity during most of the period.  However
there is some uncertainty for how the front to the north could
affect trades by next Tuesday.

Rausch