Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
823 AM EDT Tue Aug 13 2019

Valid 00Z Wed Aug 14 2019 - 00Z Wed Aug 21 2019

Latest models and ensemble means offer similar ideas through the
rest of the week into the weekend.  Light to moderate trades
should trend a bit lighter into early Wednesday, with wind speeds
light enough to promote sea/land breezes at some locations.  After
early Wednesday expect trades to strengthen as eastern Pacific
high pressure builds westward.  Consensus shows the strongest
surface gradient from early Friday into early Saturday.  Then
trades will trend lighter in response to weakening of the surface
ridge to the north.  The stronger trades after midweek will lead
to greater windward focus for any showers.  The combination of
near to below normal precipitable water values and an upper ridge
over/north-northeast of the region should lead to mostly light
rainfall.  There is some clustering toward a modest pocket of
greater moisture passing through around Saturday but without much
enhancement of rainfall.

From the weekend into the first half of next week the GFS/ECMWF
and their means have the upper ridge continuing westward while an
upper weakness moves over the main islands, with some differences
in timing.  The 00Z CMC and a fair number of CMC ensembles provide
a less probable scenario of an east-central Pacific upper
low/trough dropping to a position north of the state near 30-35N
latitude by next Tuesday.  No GEFS/ECMWF ensembles are as extreme
as the 00Z CMC but a small number of members do show trough
amplification.  Differences aloft lead to a range of possibilities
for the surface pattern with the CMC/CMC mean showing a surface
trough over the islands versus the 06Z GFS that holds onto a
strong enough surface ridge to maintain the trades.  The 06Z GEFS
mean is most comparable to the 00Z GFS.  The 00Z ECMWF mean shows
somewhat less disruption of trade flow than its operational
counterpart so would ultimately recommend a compromise among the
00Z GFS, ECMWF mean, and 06Z GEFS mean as the best way to depict
the general trend toward weakening trades without full disruption.
 Meanwhile solutions vary with individual pockets of moisture with
minimal confidence in any specific solution.  Lower heights aloft
should lead to at least a modest increase in rainfall versus
earlier in the forecast period.

Rausch