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Hawaiian Islands Synoptic Discussion and Guidance

258
FXHW60 PHFO 170203 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
403 PM HST Sat Feb 16 2019

.SYNOPSIS...
A weak area of low pressure northeast of the islands will lift
northwest tonight, then shift westward well to the north of the
state Sunday and Sunday night. Shower coverage and winds will
slowly diminish from east to west across the island chain through
the remainder of the weekend. Unsettled weather appears to make a
return during the early to middle part of next week, as deep
tropical moisture lifts northward over portions of the island
chain. This could bring heavy rainfall, thunderstorms and the
potential for flash flooding particularly to the central and
eastern islands. Improving conditions are expected across most of
the state late next week, but a lingering trough of low pressure
could keep conditions a bit unsettled over the eastern end of the
island chain.

&&

.DISCUSSION...
Currently at the surface, a weak 1010 mb area of low pressure is
located around 350 miles north-northeast of Hilo, while a strong
1040 mb high is centered around 1550 miles north-northeast of
Honolulu. The gradient has relaxed from Maui eastward, with light
and variable winds here, while moderate to breezy northerly winds
continue to the west of Maui. Visible satellite imagery shows
partly to mostly cloudy skies across the state this afternoon, a
mixture of clouds moving through the western islands on the
northerly winds and daytime convective cloud cover over the
eastern end of the state. Radar imagery shows scattered showers
moving into north facing slopes and coasts from Kauai eastward to
Molokai, with showers also reaching leeward areas here. Some
scattered convective shower development is also noted over Maui,
Lanai and the Big Island. Main short term concerns revolve around
rainfall and wind trends through the weekend as the area of low
pressure passes by to the north of the state. Longer term focus
remains on the potential for heavy rainfall, thunderstorms and
flooding during the early to middle portion of next week.

Tonight through Sunday night,
The latest model guidance remains in general agreement showing
the area of low pressure lifting northwest tonight, then tracking
westward well to the north of the islands Sunday and Sunday
night. There remain some differences however in both the track and
intensity of the low, but these differences shouldn't have too
much of an impact on the weather here locally. Winds will continue
to slowly diminish from east to west across the islands over the
next couple of days as the gradient weakens in response to the low
passing by to the north of the state. Light and variable winds
are expected through the period across Maui and the Big Island.
Meanwhile, moderate to breezy northerly winds are expected to
continue across Kauai through Sunday and possibly into Sunday
night. Winds on Oahu will remain at light to moderate levels out
of the north tonight and early Sunday, before diminishing to
light and variable levels on Sunday afternoon into Sunday night.
In the areas where winds become light and variable, daytime sea
breezes and overnight land breezes can be expected.

Shower coverage is expected to remain the highest in areas where
onshore flow is the strongest through the remainder of the weekend,
with showers favoring north facing slopes and coasts. Where winds
become light and variable, daytime convective shower development
is expected over interior and mauka areas, with very little if any
showers at night.

Monday through next Saturday,
Model solutions are in fairly good agreement through the long
term periods, with both the GFS and ECMWF indicating that more
unsettled weather is in store for portions of the island chain.
Although the track and intensity of the low to the north of the
islands continue to differ between the models, both the GFS and
ECWMF do show the area of low pressure shifting westward and
further away from the state early next week, while a strong 1045
mb high builds well to the north-northeast of the island chain.
As a result of the convergent southeasterly flow to the east of
the Big Island and an upper level trough west of the state,
surface troughing is expected to develop over the eastern end of
the island chain early next week. The models indicate the surface
trough will shift westward into the eastern portion of the state
and possibly as far west as the central islands during the Monday
night through Tuesday night time frame. Both models show a tap
into deep tropical moisture originating from the intertropical
convergence zone, with precipitable water values climbing well
above normal into the 1.5 to 2.2 inch range across the central and
eastern islands, with the ECMWF the most aggressive in the deep
moisture transport into the area. This ECMWF solution is favored
by the majority of the global models as well as the ensemble
guidance, and will bring the potential for widespread heavy
rainfall, flash flooding, and a slight chance of thunderstorms to
portions of the state, with the central and eastern islands
appearing to be the favored targets. During the middle to latter
portion of next week, drier air will begin to work back into the
region from west to east, reducing the shower coverage and
intensity across the island chain. It could remain a bit unsettled
over the eastern end of the state however, as the latest model
solutions indicate the potential for a surface trough to linger
in the vicinity through late next week.

&&

.AVIATION...
A stacked low pressure system lies just northeast of the island
chain this afternoon. Stronger northeast winds on the western end
of the system will be felt across Kauai tonight, while lighter and
variable winds persist over the rest of the state. AIRMET Tango
is in effect for moderate low level turbulence along leeward sides
of the mountains on Kauai due to the strong northeast winds.
These winds will gradually taper off late tonight as the low
pressure system moves towards the northwest.

No other AIRMETs are in effect. However, scattered passing showers
will continue to pass over the western and central islands. Mountain
obscurations are isolated along windward sides this afternoon for
now, but may become more extensive later on tonight.

&&

.MARINE...
Locally strong and gusty winds are expected to continue over the
western waters tonight with strong high pressure to the north and
a low to the northeast. For rest of the weekend, the low is
expected to begin drifting northwest tonight and westward on
Sunday. As the low moves westward, the strong northerly winds will
also shift westward. By Sunday, the locally strong north winds
are expected to be over the far western coastal waters near Kauai
with light and variable winds for waters east of Oahu. Across the
offshore waters, gales will be possible on Sunday if the low
tracks near the northern offshore waters. The models still appear
to be having a difficult time handling the position and strength
of the low on Sunday. Beyond the weekend, the low is expected to
be west of the area by Tuesday and will lead to light to moderate
east to southeast flow over the coastal waters and locally strong
southeast flow over the eastern offshore waters.

NOAA Buoy 51000 to the north of the islands jumped to 24 ft, 11
seconds early this morning and as a result, surf is rapidly
building this afternoon across exposed east facing shores. Latest
observations at buoy 51000 are holding steady at 14 to 16 feet
at 11 to 12 seconds. This should translate to surf lowering
overnight but holding at warning levels across east facing
shores. The current High Surf Warning for east facing shores and
advisory for north facing shores has been extended through
tonight. A new long period west-northwest swell is starting to
fill in across north and west facing shores this afternoon and
should continue to build through tonight and peak on Sunday. This
swell is expected to peak at heights just below advisory levels
for north and west facing shores.

The Small Craft Advisory (SCA) has been extended through Sunday
night for most Hawaiian Waters, primarily for the seas generated
by the above mentioned swell, however some areas are expected to
have some SCA level winds through Sunday as well. With the new
west-northwest swell building tonight and tomorrow, expect seas
to remain above 10 ft in many locations at least through Sunday
night.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Sunday for Niihau-Oahu North
Shore-Molokai Leeward-Maui Central Valley.

High Surf Warning until 6 AM HST Sunday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-Big Island North and East.

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Foster
MARINE...Kino

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