Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
816 AM EST Sat Dec 02 2017

Valid 00Z Sun Dec 03 2017 - 00Z Sun Dec 10 2017

Initially, a closed upper low will pass well east of the Hawai`ian
island chain while the tail end of the height falls should linger
over the region into early next week. The elongated band of
vorticity is forecast to drift southward in time toward the
equatorial Pacific by early next week. Eventually another potent
system will swing southward out of the mid-latitudes with
additional shortwave energy following in its wings. While the
strongest forcing should stay well northeast of Hawai`i, a
lingering batch of height falls should swing through the region by
mid-week. While the global guidance is in agreement with the
overall forecast evolution the next week, they vary with the
details, particularly the 00Z ECMWF which shows a cut-off upper
low looming just east of the state by the middle of next week.
This closed low would linger just east of Hawai`i through much of
next week as the system edges slowly toward the east. However,
compared to the 00Z ECMWF ensemble members, its solution does
appear to be outlying to some degree so would not put significant
stock in its forecast just yet.

At the surface, strong ridging to the north will encourage a
steady period of east-northeasterly winds, approaching the 20 to
25 knot range at times. This flow regime should persist through
the weekend with a gradual slackening of the wind fields by late
Sunday/early Monday. By early next week, a deepening cyclone to
the north and east should support a strengthening cold advection
pattern as moderate to occasionally strong northerly flow ensues.
This will ultimately shunt a great deal of the tropical moisture
equatorward. By the middle of next week, a weakening pressure
gradient will eventually decrease the strength of the northerlies
with lighter winds expected through at least the end of the work
week. The next system taking shape should lead to an additional
frontal passage but details remain uncertain as suggested by what
the 00Z ECMWF had depicted. Regarding precipitation threats, the
initial high precipitable water environment will be in the process
of dropping south which should decrease the coverage of showers in
time. By later in the weekend, drier air will work its way into
the region which should further shut off any appreciable
precipitation chances. The next frontal surge should bring another
threat for showers by Tuesday/Wednesday although this should be
brief given the quick progression of the frontal boundary. An
additional threat is possibly by the Day 6/7, December 8/9,
timeframe with the next frontal passage expected.


Rubin-Oster