Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EST Sat Feb 11 2023

Valid 00Z Sun Feb 12 2023 - 00Z Sun Feb 19 2023

Latest models and ensembles continue to agree on the general
themes of brisk to strong trades supported by a series of
mid-latitude Pacific surface highs and the trend toward a wetter
windward-focused rainfall pattern.  There are some embedded
uncertainties though.

The first step toward higher rainfall totals will be from this
weekend through the first half of next week as flow around an
upper low over/east of the main islands brings increased moisture
into the area.  Consensus shows the best axis of moisture
extending across the central/northwestern islands by the middle of
next week.  Then some combination of this upper low's energy and
shortwave(s) to the west should consolidate into an upper low
west/southwest of the state Thursday through Saturday.  Currently
there is better than average agreement among the models and
ensemble means that southeasterly flow supported by this latter
upper low will further increase precipitable water values over the
state.  By next Saturday the 00Z GFS/ECMWF both show PWATs
reaching 3 to locally 4 standard deviations above normal while the
00Z GEFS mean reaches plus 2.5-3 sdevs and the 12Z ECens mean
exceeds plus 3 sdevs over the southeastern half of the main
islands.

What appears to be the greatest uncertainty in the forecast
involves a possible surface trough/wave which operational runs are
suggesting could cross the region from east to west during or a
little after midweek when the upper low emphasis switches
location.  The 00Z ECMWF is currently on the fast side of the
spread with this feature while the 00Z CMC is slowest along with
having the northernmost surface wave.  The ensemble means imply a
surface trough timing in the middle to faster half of the spread. 
If this feature ends up being sufficiently well defined, it may
serve to disrupt the trades and alter the rainfall pattern for a
brief time.  By the end of the period on Saturday the latest GFS
runs are somewhat on the extreme side with additional surface low
pressure just south of the islands.  These surface features will
be very sensitive to exact details of individual upper level
impulses whose relatively small scale leads to low predictability.
 

Rausch