Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
255 AM EST Sat Nov 18 2023

Valid 00Z Sun Nov 19 2023 - 00Z Sun Nov 26 2023

Latest guidance maintains the same general theme of winds quickly
turning more southerly on Saturday ahead of an approaching cold
front, but there has been some shifting over the past day in terms
of how far southeast the front and its associated band of enhanced
rainfall reach by Sunday-Monday.  While the ECMWF and its farther
northwest front had been in the minority due to a lower confidence
evolution that pulled off some energy from the southwest part of
the supporting upper trough, the 12Z ECens mean and 00Z UKMET/CMC
have joined the ECMWF with the idea of some energy in the southern
part of the upper trough separating just enough to hold the front
back.  This contrasts with the GFS/GEFS mean that continue to push
the front well southeastward, corresponding to a more intact and
thus amplified southern periphery of the upper trough.  The fine
scale nature of the crucial details aloft suggest predictability
may be below average for the time frame of interest, so would
favor a compromise approach for the time being.  Either way, the
front and its moisture band should dissipate quickly after Monday
with trades and windward/mountain shower focus becoming
established again from Tuesday onward.

Today's guidance is showing more of a hint that some combination
of either the leading shortwave energy (if it separates) or a weak
upstream shortwave may help to pull up an area of enhanced
moisture from the southeast, with some of this moisture then
progressing across the islands within the trade flow.  The 00Z GFS
is the most pronounced with the higher precipitable water values,
so preference would be toward an average of other solutions. 
Beyond this area of potential moisture enhancement, there are
other differences with moisture on a day-to-day basis with minimal
confidence among individual solutions.  There is relative
agreement that upper troughing should amplify and sharpen
northwest of the state toward Friday-Saturday, supporting an
approaching surface front.  Exact distribution of energy within
the trough will determine the position/orientation of the front
and how much the trades may weaken and/or shift to become more
southeasterly.  Associated moisture may extend into the western
islands by next Saturday.

Rausch