Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
718 AM EST Mon Feb 12 2018

Valid 00Z Tue Feb 13 2018 - 00Z Tue Feb 20 2018

Highly amplified mid-latitude flow will prevail over the central
Pacific while the southern extension of height falls should close
off northwest of the Hawai`ian islands by later tonight. As this
circulation edges closer to 165W longitude, the low-level wind
fields should pick up a bit as south-southeasterly warm advection
ensues. This will also be accompanied by a slight increase in
shower activity as higher precipitable water air lifts out of the
equatorial Pacific. Over the following days, the parent
low/mid-level circulation should remain slow moving with a
position just to the north of Kauai and Ni'ihau on Thursday
morning. This will ultimately afford a frontal passage by mid-week
while the boundary should eventually stall and begin to shear out
by later in the week. While a bulk of the rainfall with this
system should occur in closer proximity to the upper low where
better dynamics will reside, models do afford a fair amount of
precipitation along this mentioned boundary. Unsettled conditions
could linger into early Friday over the Big Island as shown by the
06Z GFS and 00Z CMC/UKMET solutions.

Looking out to Friday onward, the guidance gradually allow the
upper low to weaken and become absorbed by a broader-scale
longwave trough intercepting from the north. Within this same
feature, there is a growing consensus that an additional
circulation may peel off and loom north of the Hawai`ian islands
over the weekend. By Day 6/7, February 18/19, the 06Z GFS and 00Z
CMC/ECMWF all indicate a broad upper low may sit right over
Hawai`i which is accompanied by a fair share of ensemble support.
While this will afford a threat for increased shower activity,
outside of the wetter 00Z CMC, it appears the better moisture
fluxes would reside well to the east. There should also be decent
low-level support for showers as a north-south oriented
convergence zone sets up. This solution is particularly evident in
the 06Z GFS although overall wind fields are on the lighter side.


Rubin-Oster