Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
819 AM EDT Sat Oct 20 2018

Valid 00Z Sun Oct 21 2018 - 00Z Sun Oct 28 2018

Water vapor satellite loop early this morning shows a
mid/upper-level shortwave skirting just north of Hawaii with a
trailing plume of moisture across the eastern islands. Trades are
relatively light in the vicinity of this shortwave, averaging
around 10 kt. This shortwave is forecast by all model guidance to
move east of Hawaii over the next couple days as upper-level
ridging builds east across the Central Pacific - with somewhat
drier air gradually filtering into the state and an increase in
trades to around 20 kt. Until the shortwave moves east, locally
heavy rain will remain possible across the Big Island. Guidance
continues to show an easterly wave approaching the state by
Tue-Wed. Compared to yesterday, latest model solutions seem to be
keeping the deepest moisture associated with this feature a bit
farther south, so resultant precipitation amounts look lighter.
ECENS probabilities of heavy rainfall associated with this system
have also decreased significantly from yesterday. 

A rather active mid-latitude flow-regime and frontal zone should
remain north of Hawaii through much of next week, with some model
solutions still suggesting the potential for amplification of a
larger trough across the north central Pacific by late next week,
perhaps pushing a cold front closer to Hawaii. Confidence in the
amplitude of this trough is low, however, with the GFS and GEFS
mean continuing to show a much more amplified trough relative to
the ECMWF/CMC or ECENS/CMCE means. At this time, a compromise
solution seems like a prudent approach, staying away from the
significant amplification shown by the GFS (until forecast
confidence increases).

Ryan