Small Craft Advisory 59, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST Sat Dec 9 2017

.SYNOPSIS...
A weak cold front just northwest of the state will reach Kauai
this evening, then stall across the central islands by Sunday
before dissipating. Lighter winds will start to turn southerly on
Monday ahead of a stronger front, which is expected to cross the
state from Tuesday into Wednesday. Breezy north winds will fill in
behind the front through Thursday, with unsettled weather
possible from Friday into the following weekend.

&&

.DISCUSSION...
Water vapor imagery shows a longwave upper level trough northeast
of the state, with a shortwave impulse digging through the base
several hundred miles to our north. At the surface, a cold front
is located over the northwest offshore waters, moving
southeastward toward Kauai. High pressure is located behind the
front, with another cold front west of the date line that will
come into play later in the week. The 12z soundings from Lihue and
Hilo show inversions near 7kft and precipitable waters of 0.9-1.0
inches, but significant differences in the low-level moisture
profile. The Lihue sounding is pretty typical, with nearly
saturated conditions below the inversion, while the Hilo sounding
is particularly dry, with dewpoint depressions of 5-10C below the
inversion. MIMIC total precipitable water imagery shows values
increasing to 1.3-1.5 inches along the front.

A broad, ill-defined prefrontal convergence band was located near
Oahu/Maui County overnight. Offshore land breezes kept showers
offshore for a while, but as we transition to another onshore sea
breeze pattern during the day these showers are already making
inroads across some areas. The light background low-level flow is
out of the west to northwest. After cloud and showers initially
focus across western section, they will transition to interior and
southeastern sections by afternoon as more widespread sea breezes
develop. Kauai will be slower to cloud up this morning, but will
see more widespread clouds and showers by afternoon as the front
moves in.

With the bulk of the upper-level support passing well north of the
state, the front is expected to stall and weaken over the central
islands tonight/Sunday. Lingering low-level moisture will be left
behind as the front washes out, and a moderate north to northeast
gradient will focus showers back across windward sections of the
terrain. There is a significant amount of dry air behind the
front (MIMIC TPW values of 0.5-0.7 inches) but it won't make much
southward progress across the islands. The trailing high will
pass north of the state Sunday before dissipating, leaving a
surface ridge to shift southward over us Sunday night/Monday. The
light gradient will lead to another period of locally-driven land
and sea breezes. This time the background flow will be from the
south to southwest.

The GFS and ECMWF are generally similar in their handling of the
next front Tuesday-Wednesday, with the ECMWF a bit faster than the
GFS. Both show this as a stronger front with more upper-level
support to help propel it through the state by Wednesday. Stronger
north to northeast winds and more drying will also follow behind
this front. Beyond Wednesday, though, there are noticeable
differences between the models. There has also been a fair
amount of run-to-run variability, with the 12z ECMWF now backing
away from forming a kona low east of the state. As the upper level
trough amplifies and cuts off near/east of the state, the GFS
spins up a low along the old front and proceeds to retrograde it
westward across across the islands. The 00z ECMWF had a similar
solution, but the 12z run only has an inverted trough associated
with the upper low. While the GFS continues to develop and take a
broad low westward over the islands, the current run is faster
than earlier runs to bring moisture in from the northeast.
While specific details are difficult to pin down, we look to be
in store for a period of unsettled weather late next week.

&&

.AVIATION...
A weak surface ridge stalled near the Big Island is allowing a
west to northwest low level flow to prevail across the smaller
islands, while light and variable winds dominate around the Big
Island. A weak cold front currently located about 100 nm NW of
Kauai will reach the Garden Isle late this afternoon, bringing
MVFR ceilings and SHRA that could produce mountain obscuration.
Ahead of the front, shallow bands of clouds and isolated showers
will affect Oahu and Maui County, but mountain obscuration is not
expected. Meanwhile, afternoon sea breezes will produce stable
low clouds over Big Island slopes.

The weak front will continue to affect Kauai during the evening,
then stall over Oahu and Maui County tonight. AIRMET Sierra for
mountain obscuration may be needed along north facing terrain.

&&

.MARINE...
Winds will remain below Small Craft Advisory (SCA) levels for all
coastal waters today, as a weak ridge lingers over the area ahead
of an approaching cold front. This front is forecast to reach
Kauai late this afternoon, then move over the central islands
tonight, before stalling over Maui County Sunday. A brief period
of increased NNE winds is expected immediately behind the front,
with winds possibly reaching SCA levels in some areas around Kauai
and Oahu. Gentle to moderate trades will briefly prevail across
most areas Sunday into Sunday night as the front dissipates. Winds
will then swing around to S then SW Monday into Tuesday as the
next front approaches. This front is currently forecast to move
down the island chain Tuesday through Wednesday, with winds
shifting to north and increasing behind the front. N/NE winds will
likely reach SCA levels across most areas from Wednesday night or
Thursday through the end of next week.

Seas will remain below 10 feet today, then build rapidly tonight
into Sunday morning as the next NNW swell fills in and pushes
seas back above 10 feet for many areas. Seas should subside below
10 feet again on Monday, but build rapidly once again late
Tuesday into Wednesday, as another even larger NNW swell arrives.

Surf along north and west facing shores will continue to subside
today as the old NNW swell fades. The next NNW swell is forecast
to arrive this afternoon and build quickly tonight into Sunday
morning. This swell will likely produce warning-level surf for
north and some west facing shores as it peaks on Sunday, before
subsiding on Monday.

Another even larger NNW swell is forecast to arrive late Tuesday,
peaking Tuesday night into Wednesday. This swell is expected to
produce warning-level surf on north and west facing shores.

See the latest Oahu Surf Discussion (SRDHFO) for more details on
the swell sources.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

Discussion...Bravender
Aviation...Wroe
Marine...TS