Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
835 AM EDT Tue Sep 12 2017

Valid 00Z Wed Sep 13 2017 - 00Z Wed Sep 20 2017

To begin the period, a break in the subtropical ridge will allow a
closed upper low to sink southward from the mid-latitudes. It
remains quasi-stationary for a brief period before drifting
northeastward toward the eastern Pacific. Eventually a secondary
circulation forms just north and west of the Hawai`ian island
chain with the 00Z CMC being a slow eastern outlier. For much of
the weekend and into early next week, broad ridging extends over
much of the central Pacific although the 06Z GFS hints at another
disturbance swinging westward toward the region.

At the surface, a stalled boundary lingers just northwest of Kauai
through at least Thursday before the convergence zone weakens and
lifts toward the north. Wind fields across the state are quite
weak for the next couple of days before a surface ridge builds
over the mid-latitudes allowing for a return to the usual trade
wind regime by later in the work week and into the weekend. The
06Z GFS/00Z ECMWF show a marked increase in the low-level wind
fields by as early as Saturday as the ridge continues to
strengthen. As a result, sustained winds could reach the 20 to 25
knot range at times. In terms of precipitation, some frontal
showers may nose closer to Kauai during the next few days,
particularly noted in the 06Z GFS/GEFS mean which appears to be a
wetter solution. The boundary is accompanied by a sufficient spike
in precipitable water values as they rise into the 1.50 to 1.75
inch range. Thereafter, a return to the usual windward showers is
likely although the tropospheric moisture will be on the lower
side.


Rubin-Oster