Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 AM EST Fri Dec 17 2021

Valid 00Z Sat Dec 18 2021 - 00Z Sat Dec 25 2021

...Wet pattern to form over the islands...

Guidance continues to show a wetter period coming up as the state
will be located on the southern periphery of a blocky upper
pattern over the Pacific.  Initial shortwave energy may promote
some modest shower enhancement late this week, perhaps followed by
a brief drier trend this weekend as the energy departs and
moisture decreases slightly.  During the early to middle part of
next week the guidance consensus tracks an upper low west of the
state southeastward then eastward to the south of the islands
while strengthening central Pacific high pressure (which will help
to strengthen the trades) pushes a front into the area, leading to
a rebound in rainfall.  During the latter half of the week the
upper low shears out.  However predictability rapidly decreases
for specifics of upper flow to the south of the strong upper ridge
over the central Pacific, with considerable model and ensemble
member spread.  These important details will determine where the
old frontal boundary and enhanced moisture axis set up.  Latest
GFS runs would have this axis drift far enough northward to bring
a risk of heavy rainfall to the Big Island, as does the 12Z/16
ECMWF.  The new 00Z ECMWF is much lighter with its rainfall and
the 00Z CMC keeps the heavy rain axis to the south.  The ensemble
mean moisture forecasts are split as well, with the GEFS mean
keeping higher precipitable water values over the state versus the
more suppressed 12Z ECMWF mean.  At the moment the one
consideration that could tilt the probabilities a bit would be
teleconnections relative to the Pacific upper ridge's positive
height anomaly--suggesting the 00Z ECMWF/CMC/CMC mean could be a
little too amplified with their upper trough northeast of the
state.  In turn this means the suppressed cluster could be a
little too far south.  It remains to be seen whether moisture will
be as concentrated as seen in the GFS though. 

Rausch