Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
350 AM EDT Mon Oct 11 2021

Valid 00Z Tue Oct 12 2021 - 00Z Tue Oct 19 2021

A couple fairly strong mid-latitude surface highs will support
brisk to strong trades into about midweek.  Then trades should
weaken as one cold front pushes into the overall mean surface
ridge late in the week.  A trailing surface high during the
weekend will be short-lived as another front (extending from a
strong North Pacific system) will further weaken the surface ridge
north of the islands by next Monday.  At that time the 00Z
ECMWF/12Z ECMWF mean keep the front farther north and the surface
ridge a little stronger while latest GFS/GEFS mean runs and the
00Z CMC mean are on the opposite side of the envelope.  The 00Z
CMC is in-between.

The GFS and ECMWF have finally converged for the area of enhanced
moisture that will cross the area approximately late Monday into
early Wednesday, bringing a period of heavier rainfall that will
focus over windward/mountain locations but occasionally stray
elsewhere.  Model/ensemble consensus agrees on the trend toward
lighter rainfall after midweek but differences persist for how
much and where precipitable water values will decrease. 
GEFS/ECMWF means have been consistently dropping PWATs to 1.25
inches or lower from around late week onward.  On the other hand
the ECMWF is still keeping some higher values around the Big
Island while the GFS has trended somewhat lower than yesterday but
is still a little above the means over the Big Island.  Some of
these differences may be related to elongated shortwave energy
that may approach/reach the islands from the north per the
GFS/ECMWF.  Relatively small scale of this feature limits
predictability that far out in time. 

Rausch