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Hawaiian Islands Synoptic Discussion and Guidance

882
FXHW60 PHFO 181335
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
335 AM HST Tue Dec 18 2018

.SYNOPSIS...
A cold front is forecast to quickly move down the island chain
later today through Wednesday, bringing increasing rain chances.
Gusty northerly winds along with drier and cooler conditions will
follow tonight through Thursday. Rain chances may linger over the
eastern end of the state through Thursday as the front stalls and
diminishes. A return of a more typical trade wind pattern will be
possible Friday into the upcoming weekend.

&&

.DISCUSSION...
Short-term (through Thursday) guidance remains in good agreement
and shows a cold front, currently northwest of Kauai, advancing
down the island chain beginning later today for then northwest
islands, then to Maui County and the Big Island tonight through
Wednesday. A slight increase in moisture (albeit shallow)
accompanying this boundary and upper trough will support a band
of clouds and increasing rain chances as the boundary moves down
the island chain. The bulk of the showers will focus over the
northern and windward sides of the islands. Accumulations should
remain on the light side due to the limited source of moisture
(precipitable water values only extending into the 1-1.2" range).
Rain chances will likely remain up over the Big Island through
Thursday as the boundary hangs up and gradually diminishes.

Gusty northerly winds along with much drier and cooler conditions
will follow the frontal passage tonight through Thursday. Wind
advisory criteria being exceeded for the typically windier areas
exposed to northerly winds can't be ruled out, mainly through
the day Wednesday. Dewpoints are forecast to dip into the upper
50s for portions of the state. High temperatures may struggle to
reach the 80 deg mark Wednesday afternoon.

For the extended (Friday through the weekend), guidance supports a
return of a more typical trade wind pattern by the weekend with
clouds/showers focusing over windward areas.

&&

.AVIATION...
A weak surface front will advance to Kauai late this morning,
then through the rest of the island chain by tonight. Clouds and
showers will spread from the north and will focus on windward and
mountain areas. Mainly VFR conditions are expected this morning,
though MVFR ceiling will develop later today, mainly over windward
and mountain areas at times as the moisture from the front edges
south. Brisk north winds are expected to spread from the north.

No AIRMETs are in effect or expected early this morning, though
AIRMET Tango for TURB may be needed later this morning for the
northern islands due to the increasing northerly winds. AIRMET
MTN OBSC is also a possibility along the windward mountains.

&&

.MARINE...
Light to moderate trades will hold in place through the morning
hours today, as a cold front approaches the state from the
northwest. The cold front will sweep through Kauai late this
afternoon, through Oahu and Maui County tonight, then hang up in
the vicinity of the Big Island on Wednesday. Strong northerly
winds will fill in along and behind this front, bringing solid
Small Craft Advisory (SCA) level winds to all marine zones, and
potentially a brief period of near Gales to the Alenuihaha
channel. High pressure will build eastward well to the north of
the islands Wednesday through early next week, with strong winds
returning to a more typical east-northeasterly trade wind
direction late Wednesday and Wednesday night. Moderate to locally
strong trades will then continue Thursday through early next week.

The incoming large long-period northwest swell has overspread the
NOAA Buoys 51001 and 51101, and is arriving at the Hanalei Buoy
51208 and beginning to register at the Waimea Bay Buoy 51201 early
this morning. This swell will continue to spread down the island
chain today. Surf is expected to rapidly build over exposed north
and west facing shores this morning, reaching warning levels by
late this afternoon. Surf is expected to peak tonight through the
first half of the day on Wednesday, then gradually lower through
the rest of the work week. Additionally, the peak of the surf is
expected to occur during the pre-dawn hours on Wednesday near the
peak of the spring high tides. This is expected to result in above
average coastal wave run-up, increasing the risk for significant
beach erosion and possibly leading to some minor overwash onto
coastal roadways.

The High Surf Warning for exposed north and west facing shores has
been extended through 6 PM Wednesday, and a High Surf Advisory is
now in effect for north facing shores of the Big Island through 6
PM Wednesday. Advisory level surf will likely continue for
exposed north and west facing shores of the smaller islands
Wednesday night and into the day Thursday. A SCA is now in effect
for all Hawaiian waters for winds, seas or a combination of the
two through Wednesday night. The SCA will likely need to be
extended for much of the marine area through Thursday, and may
continue for the typically windy waters around Maui and the Big
island right through early next week.

Surf is expected to drop below the winter time average along
north and west facing shores Friday through the weekend. A new
small to moderate northwest swell will be possible early next
week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST Wednesday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

High Surf Advisory until 6 PM HST Wednesday for Big Island North
and East.

Small Craft Advisory until 6 AM HST Thursday for all Hawaiian
waters-

&&

$$

DISCUSSION...Gibbs
AVIATION...Hui
MARINE...Jelsema

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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