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Hawaiian Islands Synoptic Discussion and Guidance

259
FXHW60 PHFO 180643
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
843 PM HST Fri Jan 17 2020

.SYNOPSIS...
North-northeast winds will shift out of typical trade wind
direction on Saturday and remain breezy and gusty through the
weekend as high pressure sets up 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 could produce modest rainfall on windward Big Island and
Maui. Trades will weaken early next week, and will likely be
disrupted Wednesday through next Friday.

&&

.DISCUSSION...
Dry, stable, and breezy north-northeast winds are in place across
much of the state, while moisture along a dissipated front
continues to produce modest showers across windward slopes of the
Big Island and Maui. Broad surface high pressure centered about
900 miles northwest of Kauai is moving eastward, creating a
rather tight pressure gradient just west of the dissipating front.
This is driving breezy and gusty north-northeast trade winds over
all islands and is ushering in a somewhat dry air mass across the
western half of the state. Afternoon soundings and recent
aircraft data from Kauai to Maui revealed a strong inversion
building between 4,000 and 5,000 ft, while satellite data show
precipitable water dropping well below January normal near Kauai
and Oahu. As a result, clouds will remain piled up along windward
slopes from Kauai to Molokai tonight with little or no rainfall.
Modest showers with accumulations below 1/2 inch are expected over
windward slopes of Maui and the Big Island as the front breaks
up. Leeward areas of all islands will be dry.

A stable, rather dry, and breezy trade wind flow will dominate
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. Precipitable water values are
expected to remain near or below normal, even though some moisture
from the dissipated front may linger near the Big Island and Maui.
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 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, and the
GFS and ECMWF stall the front north of the state.

&&

.AVIATION...
A dissipating front is over the Big Island this evening, moving
gradually east at about 15 kt. Post frontal low level moisture
extends westward to Molokai, which will be thinning out slowly but
surely as the night progresses. This air mass will be replaced by
a rather dry and stable air mass, which currently lies over Oahu
and Kauai.

AIRMET Sierra for mountain obscuration is still in effect for
windward slopes of Molokai, Maui, and the Big Island. We will be
watching closely how things progresses overnight, but most
certainly, Sierra is likely to persist across the Big Island
windward slopes to about sunrise Saturday.

The low level flow is from the north and north-northeast, and
brisk. Thus AIRMET Tango for low level turb is in effect below 6k
feet. The surface high generating this northerly wind flow is
currently 780 nm NW of Kauai with a 1026 mb central pressure. It
is forecast to move east-southeast, passing north of the islands
Sunday night at 1024 mb. This means the winds will be becoming
northeast on Saturday, then east-northeast Sunday night and
Monday. In short, the low level turb is likely to persist through
all day Saturday.

&&

.MARINE...
Surface high pressure building far northwest of area is producing
strong northeasterly winds across the coastal waters. These
strong winds are causing rough, elevated seas over most waters.
As a result, a Small Craft Advisory (SCA) has been issued for
most Hawaiian waters due to a combination of strong winds and
elevated seas. The winds are forecast to gradually shift out of a
more easterly direction later this weekend. The winds may become
lighter across most of the area from Tuesday through the second
half of next week based on the latest model guidance.

Elevated, choppy surf is occurring along north facing shores of
most of the smaller islands due to a combination of the strong
northeast winds and a long upstream fetch of north winds.
Therefore, a High Surf Advisory (HSA) has been issued for north
facing shores of Kauai, Niihau, Oahu, Molokai, and Maui. The HSA
is currently in effect through early Saturday morning. We will
monitor observations from the near shore buoys overnight in case
the HSA needs to be extended during the daytime hours Saturday
for some shorelines. In addition, surf along most east facing
shores, especially shorelines with a northerly exposure, will
trend up this weekend, before gradually lowering early next week.

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 through Sunday night. This swell will
gradually lower from Monday through Tuesday.

Two large gale- to storm-force low pressure systems are forecast
to race eastward from near Japan to the International Date Line
from this weekend into early next week. These systems are expected
to send larger northwest swells 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.

Surf along south facing shores will remain small through early
next week, with mainly small background southwest swells
expected.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Saturday for north facing
shores of Niihau, Kauai, Oahu, Molokai, and Maui.

Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Leeward Waters-Big Island Southeast Waters-Big Island Windward
Waters.

&&

$$

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

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



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