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Hawaiian Islands Synoptic Discussion and Guidance

517
FXHW60 PHFO 181326
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
326 AM HST Sat Jan 18 2020

.SYNOPSIS...
Breezy and gusty trade winds will persist through the holiday
weekend as high pressure holds north of the state. Stable and
somewhat dry conditions will confine light showers to windward
slopes of most islands, though shallow moisture from a dissipated
front will produce modest rainfall on windward Big Island and
Maui. Trades will weaken early next week, and will likely be
disrupted Wednesday through Friday.

&&

.DISCUSSION...
Breezy, stable, and dry northeasterly trade winds remain in place
across much of the state, though moisture associated with a
dissipating front just east of the islands continues to produce
modest showers across windward slopes of the Big Island and Maui.
Broad surface high pressure centered nearly 900 miles northwest of
Kauai is moving eastward, maintaining a tight pressure gradient
just west of the dissipating front. This is driving breezy and
gusty northeast trade winds over all islands and is sending in a
rather dry air mass across the western half of the state.
Overnight soundings and recent aircraft data from Kauai to Maui
showed a strong inversion between 4,500 and 5,000 ft, and
satellite data show precipitable water values well below January
normal over the western half of the state. As a result, only light
showers have fallen over windward slopes from Kauai to Molokai
overnight, while windward slopes of Maui and the Big Island have
generally received about 1/4 to 1/2 of an inch of rainfall. Leeward
areas of all islands have been mostly dry.

A stable, rather dry, and breezy trade wind flow will dominate
over most areas this weekend. Surface high pressure will settle
north of the state, and mid level ridging will maintain stable
conditions will a strong low level inversion. Showers will remain
active across windward slopes of the Big Island and Maui this
morning and will taper off during the afternoon. For the rest of
the weekend, expect modest windward showers, at best, on those
islands, while the western half of the island chain sees minimal
windward rainfall and dry leeward weather. With dew points
hovering in the upper 50s to lower 60s in the breezy trades, it
will feel cool, even though temperatures will be close to January
normal.

Trades will gradually decline on Monday night and Tuesday as the
surface high moves off to the east. Stable trade wind weather will
persist, with light showers confined to windward slopes and
afternoon sea breezes possible by Tuesday.

Trades will likely be disrupted for the second half of the work
week as an approaching front pushes the surface ridge over the
islands. A stable land and sea breeze pattern is expected. The GFS
and ECMWF are now showing the front stalling near or just north of
Kauai on Friday night and Saturday as trades gradually rebuild.

&&

.AVIATION...
The front has dissipated over the Big Island. Post frontal
clouds/showers continue to affect mainly the Hamakua coast of the
Big Island (BI). A slow but gradual improvement is forthcoming
during the next few hours there. We hope to drop AIRMET Sierra for
windward BI by 16z. A patch of scattered showers that affected
mainly Oahu and Molokai is just about done. Now, we can proceed
with the building of the low level inversion eastward the rest of
today. Thus far, the inversion slopes from as low as 6k feet over
Kauai to 8k feet over Maui, to 10k feet over the Big Island. By
sunset this evening, the inversion is expected to lower to 8k feet
over the Big Island, while holding at 5-7k feet elsewhere.

Satellite infrared imagery shows a large field of stable
stratocumulus (sc) clouds affecting the northern section of Kauai.
This sc deck will be slow to clear Kauai, but is not expected to
spread eastward. Surface observation from LIH suggest a high cloud
base at 4k feet, and a top of 6k feet. Given this info, some patchy
MVFR ceilings with light drizzle or light showers can be expected
with these clouds, favoring the windward and mountain areas. Some
light showers embedded in the NNE wind flow are also expected to
affect the other islands, but remain VFR.

The brisk low level flow will be turning more NE by this evening.
AIRMET Tango for low level turb below 6k feet will continue as
the surface high, 750 nm NW of Kauai with a 1026 mb central
pressure, treks east-southeastward. The high is slated to pass
north of the islands Sunday night.

&&

.MARINE...
Surface high pressure centered far northwest of area is producing
strong northeasterly winds across the coastal waters. These
strong winds are causing rough, elevated seas. A Small Craft
Advisory (SCA) has been issued for all Hawaiian waters through
Sunday afternoon due to a combination of the strong winds and
elevated seas. The surface high will move slowly east to a
position north of the islands by Sunday. As a result, the winds
are forecast to gradually shift out of a more easterly direction
from later this weekend into Monday. The high will then move far
northeast of the area Monday night. A surface ridge trailing from
this high is forecast to then shift south over the state Tuesday
and Wednesday. This will cause the background flow to become
lighter, and shift out of the east-southeast by mid-week.

Rough surf is occurring along north facing shores of most of the
islands due to the strong northeast winds. However, the near
shore buoys appear to show some of the swell energy has subsided
overnight. Therefore, the High Surf Advisory (HSA) has been
cancelled for north facing shores of Kauai, Niihau, Oahu,
Molokai, and Maui. A longer period north-northwest swell is
expected to arrive early Sunday. This swell may cause surf to
reach the HSA thresholds along most north and west facing shores
of the smaller islands from Sunday afternoon into Monday. This
swell will gradually lower from late Monday through Tuesday.

A large low pressure system is forecast to race eastward from
near Japan to the International Date Line into early next week.
The captured fetch associated with this system is expected to
send a large northwest swell toward the islands late Tuesday
night and Wednesday. This may cause surf to approach the High
Surf Warning criteria along some north and west facing shores of
the smaller islands from late Wednesday into Thursday. In
addition, surf may also approach the HSA criteria along west
facing shores of the Big Island during this event.

Elsewhere, expect moderate, choppy surf along most east facing
shores into early next week. Small south-southwest swells will
maintain background surf along south facing shores through the
middle of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Sunday for all Hawaiian
waters.

&&

$$

DISCUSSION...Wroe
AVIATION...H Lau
MARINE...Houston

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

Current Conditions
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RH		10 %
Wind		NNE 8 mph
Road		Open (4x4)