Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
730 AM EST Sun Feb 11 2018

Valid 00Z Mon Feb 12 2018 - 00Z Mon Feb 19 2018

One upper low will continue to spin just east of the Hawai`ian
island chain with some scattered showers swinging southwestward
back toward Oahu and Moloka`i. This is in response to a weak area
of low pressure, 1009 mb, being analyzed just to the northeast of
the state. Models gradually shear out the system while a transient
shortwave ridge settles in through early Monday. The focus then
shifts to a strong upper trough lurking upstream with an upper low
likely peeling off from the mid-latitude feature. However, the big
shift in the guidance from yesterday has persisted which favors an
eastward drifting upper low tracking north of Kauai and Ni`ihau. A
small fraction of the 00Z GEFS, ECMWF, CMC ensemble members show
the 570-dm closed circulation grazing these northern most islands.
Amongst the operational models, the 00Z CMC is on the southern
extent of the spread which leads to a much wetter solution. The
general consensus is for this weakening circulation to be absorbed
by a broader scale trough dropping down from the north by the end
of the work week. This would ensure maintaining an axis of lower
heights into next weekend as perhaps another closed low may spin
off over the Hawai`ian islands next weekend. However, given how
much the current forecast has changed, will likely see a multitude
of changes during the next few days of model runs. Ensemble
spaghetti plots show quite a bit of spread which verifies this
notion.

Considering the surface pattern next week, general
east-southeasterly flow will take shape as a warm advection regime
ensues ahead of the approaching baroclinic zone. Depending on the
strength and position of the parent surface cyclone to the
northwest of Hawai`i, the wind fields vary among the guidance. A
frontal passage would appear likely by mid-week with the surface
wave possibly lingering to the north into Thursday. It is
difficult to say with confidence what the surface wind fields will
look like later in the week given increasing model spread. Like
yesterday, GFS solutions remain quicker and favor a cold advection
pattern by Thursday. It does seem likely that an inverted surface
trough will likely take a north/south orientation into next
weekend with positions quite variable. One thing that seems
certain is daily rainfall chances are in the picture, particularly
later in the period as that mentioned inverted trough sets up.
Models show precipitable water values exceeding 1.50 inches with a
broad axis of precipitation spread over the region.


Rubin-Oster