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

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

...A Heavy Precipitation Threat early in the period...

Initially, a longwave trough will pass to the east of the
Hawai`ian island chain while the tail end of the height falls
should linger over the region this weekend. 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 southeast of the state by late next week.

At the surface, strong ridging to the north will encourage a
steady period of moderate trade winds, approaching the 20 to 25
knot range at times. This flow regime should continue into the
weekend with a gradual slackening of the wind fields. By Monday, 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 for the
remainder of the period. Regarding precipitation threats, locally
heavy rainfall will be possible during the next 24 hours as a high
precipitable water environment remains in place. By later in the
weekend, drier air will work its way into the region which should
decrease the coverage of shower activity. 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. 


Rubin-Oster