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Hawaiian Islands Synoptic Discussion and Guidance

095
FXHW60 PHFO 181416
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
416 AM HST Mon Dec 18 2017

.SYNOPSIS...
Surface troughing will set up over the central and eastern
islands through mid week, while a strong upper level trough
approaches from the west. This will lead to increasing showers
across the state today, with locally heavy rainfall and
thunderstorms possible across the central and eastern islands
tonight through Wednesday night. A cooler and drier airmass will
overspread the state Thursday through Friday, with fewer showers.
A reinforcing cold front will then move down the island chain
Friday night and Saturday bringing a band of showers with it.
Trade winds are then expected to return on Sunday, with showers
favoring windward and mauka areas.

&&

.DISCUSSION...
Currently at the surface, a weak trough of low pressure is
located just northwest of Kauai. A weak pressure gradient remains
in place across the island chain, with land breezes dominant in
most areas early this morning. Aloft, a digging upper level
trough is evident in water vapor imagery around 700 miles to the
west-northwest of Kauai. Infrared satellite imagery shows clear
to partly cloudy skies over Kauai and Oahu, with partly to mostly
cloudy conditions across Maui County and the Big Island. Radar
imagery shows a few light showers over Maui County and the Big
Island, with rain free conditions over Kauai and Oahu. Main short
term concerns revolve around the chances for heavy rainfall and
thunderstorms through mid week.

Today through Wednesday night,
The digging upper level trough to the west of the state will
result in the development of surface troughing through the central
islands today and tonight. This surface trough will then linger
in the vicinity of Maui County and the Big Island through mid
week. Meanwhile, a cold front will approach the islands from the
northwest on Tuesday, reaching Kauai by late in the day or early
in the evening. The front will continue to slowly move
southeastward down the island chain Tuesday night through
Wednesday night, kicking the troughing over the central and
eastern islands to the east of the state by daybreak Thursday.

Deep layer moisture will increase over the eastern end of the
state on today, with precipitable water values climbing into the
1.5 to 1.8 inch range by late this afternoon. This deep moisture
will then linger over the central and eastern islands tonight and
Tuesday before shifting slowly eastward Tuesday night through
Wednesday night as the cold front progresses down the island
chain from northwest to southeast. Given the favorable jet
dynamics with entrance region forcing from a strengthening upper
level jet over the central and eastern islands, along with the
approaching upper level trough and plenty of deep layer moisture,
a fairly unsettled period is expected across the state,
particularly over the central and eastern islands. Showers are
expected to increase over the central and eastern islands today,
with some locally heavy rainfall and thunderstorms possible
tonight through Wednesday night, particularly over Maui County and
the Big Island. Across Kauai, the airmass won't be as moist, so
shower activity is not expected to be as widespread, with showers
expected to be most prevalent in association with the cold frontal
passage Tuesday night into Wednesday. Oahu will likely be on the
edge of the deeper moisture and any minor deviation of this
moisture axis west or east will have a significant impact on
coverage and intensity of the rainfall.

Given the potential for heavy rainfall and thunderstorms, a Flash
Flood Watch remains in effect for Oahu tonight through Tuesday
night, and for Maui County and the Big Island tonight through
Wednesday night.

Strong forcing for ascent along with plenty of deep layer shear
will overspread the central and eastern islands tonight through
Wednesday night, and this may result in a few waves of
thunderstorms across the islands, some of which may be strong.
The main limiting factor appears to be instability, with marginal
CAPE values generally remaining at or below 500 J/KG through the
period. This will need to be monitored closely over the next
several days, with the main threats from any stronger storms if
the most favorable kinematic and thermodynamic environments
align, being damaging straight line winds and/or hail.

Finally, with the increase in deep layer moisture over the eastern
end of the state, periods of accumulating snow are likely over
the Big Island summits tonight through Wednesday night. As a
result, the Winter Storm Watch has been upgraded to a Winter
Storm Warning, which is in effect from 6 PM this evening through
late Wednesday night. Upper level winds will also be on the
increase with the approach of the upper level trough from the
west, and this could lead to blizzard conditions at times Tuesday
through Wednesday night.

Thursday through next Sunday,
The upper trough axis is expected to shift east of the Big Island
Thursday morning, and this will allow a cooler and drier post
frontal airmass to overspread the entire island chain. Locally
breezy northerly winds on Thursday will begin to diminish Thursday
night and Friday as a ridge of high pressure settles southward
over the state. Yet another cold front is then expected to swing
southward through the island chain Friday night and Saturday, with
trades returning by next Sunday.

As for sensible weather details, scattered showers are expected
mainly across north facing slopes and coasts Thursday and
Thursday night as low level lapse rates remain steep in the cool
airmass following the frontal passage. Shower coverage will
decrease on Friday as the surface ridge builds southward over the
islands, with a band of showers then expected to accompany the
next front Friday night through Saturday as it progresses down the
island chain. Fairly dry conditions are then expected Saturday
night and Sunday, but given steep low level lapse rates in the
post frontal airmass, scattered showers will likely continue
across windward and mauka areas.

&&

.AVIATION...
A low pressure area north of the Hawaiian Islands and an
approaching cold front from the northwest will keep the state in a
light south to southwest flow pattern. Wind speeds will be light
enough for afternoon sea breezes to develop with onshore wind
direction shifts from late morning to afternoon. Overnight winds
will become offshore. The large scale southerly flow will
continue to pull up tropical moisture across all islands except
Kauai today with increasing clouds and isolated to scattered
showers.

TEMPO MVFR will linger eastern half of the Big Island today with
MVFR conditions expanding to the southern slopes by tonight.
Prevailing VFR conditions this morning over Molokai, Lanai and
Maui will transition to TEMPO MVFR ceilings by afternoon. Expect
VFR conditions to prevail for Kauai and Oahu through tonight.

AIRMET SIERRA for mountain obscuration remains in effect for
eastern sections of the Big Island. AIRMETs for mountain
obscuration will likely be expanded to Maui, Lanai and Molokai by
this afternoon.

&&

.MARINE...
Light winds will likely hold into Tuesday as a trough of low
pressure continues to impact the local weather. Moderate to fresh
northerly winds are forecast to fill in Tuesday through midweek
as a cold front moves down the island chain. Heavy showers and
storms associated with this front are expected tonight through
midweek, especially over the southeast waters. There will be a
brief lull in the wind speeds Thursday night into Friday, but wind
direction/flow is forecast to remain out of the north and
northwest into Saturday before shifting out of the east through
the second half of the weekend. Winds should remain below
advisory levels through the week.

Surf associated with the long-period northwest (300-320 deg)
swell that filled in overnight will hold below advisory
thresholds along north and west facing shores through the day
today.

A reinforcing shorter-period (11-12 sec) north-northwest (330-000
deg) swell associated with a gale developing north of the state
is expected to fill in tonight through midweek. This new swell
could result in surf reaching advisory levels along north facing
shores Tuesday night into Wednesday.

A long-period northwest (320-330 deg) swell associated with a large
area of gales forecast to setup Wednesday through Thursday over
the northwest Pacific between 40N and 50N (near the western end
of the Aleutians) will fill in Friday. Surf heights should remain
well below advisory levels from this source.

For the extended, guidance remains in decent agreement and shows a
gale developing just east of the date line around 40N Wednesday
through Thursday. A large area of strong- to gale-force winds
associated with this feature is forecast to setup within the
330-350 directional band relative to the islands. Surf associated
with this source will come up, likely exceeding advisory levels
over the weekend along north facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from this evening through late Tuesday night
for Oahu.

Winter Storm Warning from 6 PM this evening to 6 AM HST Thursday
for Big Island Summits.

Flash Flood Watch from this evening through late Wednesday night
for Molokai-Lanai-Kahoolawe-Maui-Kona-South Big Island-Big
Island North and East-Kohala-Big Island Interior.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Gibbs

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