Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
722 AM EST Mon Dec 04 2017

Valid 00Z Tue Dec 05 2017 - 00Z Tue Dec 12 2017

With a mean closed upper ridge centered west of the International
Dateline, persistent northwesterly flow aloft will rule the
pattern. A series of shortwaves are forecast to track to the east
of the Hawai`ian island chain which should afford a couple of
frontal passages throughout the forecast period. One such system
will peel off from the prevailing westerlies with shortwave energy
swinging south of 30N, thus allowing weak height falls to graze
the region. The northern extent of the upstream ridge should
buckle somewhat which allows the mid/upper-level winds to shift to
a more west-northwesterly direction. Renewed upper troughing
returns to the picture early in the weekend which affords the next
cold frontal passage before the flow begins to flatten out into
early next week. Overall model agreement is above average given
only modest spread as noted in the ensemble spaghetti plot. The
biggest disagreement is the depth of the trough this coming
weekend with the 00Z GEFS members being more amplified than the
00Z ECMWF ensembles. A split between these two camps appears
appropriate here.

At the surface, an abrupt shift to north-northeasterly flow is
expected today with further intensification of these winds later
tonight and into Tuesday. Some solutions favor up to 30 to 35 knot
winds at times with even higher speeds over the mountainous
terrain. This is in response to the approaching baroclinic zone
which will force brisk moving showers through the island chain
through midday Tuesday. It should be fairly dry the following
couple of days as a sub-0.50 inch precipitable water air mass
infiltrates the region. Low-level winds should diminish
considerably into the middle of the week as pressure gradients
weaken with generally only 5 to 10 knot northerly winds remaining.
A return of a higher precipitable water environment returns by
Thursday through Saturday ahead of the next upper trough.
Eventually this will be shunted southward as north-northeasterly
winds return by Saturday in response to the next frontal passage.
This also affords the next rainfall chances although this threat
will be short-lived as well. Light southeasterly flow is expected
by next week as noted by the 00Z GFS/ECMWF. 

Rubin-Oster