Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
730 AM EST Sat Feb 09 2019

Valid 00Z Sun Feb 10 2019 - 00Z Sun Feb 17 2019

...Unusually deep and southward low pressure brings a threat of
strong/damaging winds, high waves and precipitation to the state
Sunday into next week...

The recent passage of a lead shortwave over the region is
supporting an area of low pressure to the north-northeast of the
main islands. A trailing cold front passed through the state and
ushered in brisk northerly winds and drier air, with an axis of
lowest precipitable water values (1-2 standard deviations below
climatology) passing through from northwest to southeast today.

Models and ensembles match well with water vapor loops and have
been extremely consistent in showing a dramatic amplification of
central Pacific energy leading to closed low formation now just
north of 30N that digs southward to near 25N by Monday, bolstering
forecast confidence. At the low's most extreme combination of
depth and southward extent, recent GFS/ECMWF runs and their
ensemble means still indicate that 500mb/850mb heights could reach
as low as 4-6 standard deviations below normal. In response to
this upper low there is good consensus the surface low north of
the state will deepen and loop back to the south, getting as close
to the islands as 24N or so Sunday/Monday. The upper low will be
slow to open up next week in a pattern within a persistent upper
trough that gradually weakens. Expect surface low pressure to
meander/loop just northeast of the state through mid-late week,
maintaining unsettled weather over the islands as reinforcing
shortwave energy/height falls dig back over the state. Confidence
in the specific scenario surface/aloft (along with associated
sensible weather effects) decreases into mid-later week.

Despite smaller scale differences that increase over time, the
latest GFS/ECMWF and GEFS/ECMWF ensemble means forecasts are well
clustered in this threatening pattern with above normal
predictability.

Accordingly, there remains a high threat for strong
northerly/northwesterly winds and high waves around this system,
with multiple periods of heavier rains embedded within the flow
starting Sunday and lingering well into next week. There is also
enough cold air aloft in this pattern to also support wintry
weather at the higher summits. The Honolulu WFO has already issued
a Special Weather Statement (SPSHFO) to discuss all these upcoming
weather threats and please refer to the office for all watches and
warnings.

Schichtel