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Temp1.6 C
RH9 %
WindENE 8 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 230200

Area Forecast Discussion
National Weather Service Honolulu HI
400 PM HST Sat Apr 22 2017

Clouds and showers from an old front will track in with moderate
trade to east southeast winds. Showers will focus over windward
slopes, particularly Kauai and Oahu. Monday and Tuesday will be
drier statewide with an increase in trade wind showers expected by
midweek. Unsettled weather may return around Friday into next


A surface high far north northeast of the state is driving
moderate trade winds across Hawaii. The high is expected to move
east tonight and forcing the winds to weaken slightly and veer
from the east southeast.

Moisture from an old front continues to track over the island
chain within the trade wind flow. Most of the associated clouds
and showers have been focused over windward sections of Kauai and
Oahu today, but these showers have become more diffuse and spread
out this afternoon. Models, along with radar and satellite
extrapolation, indicate windward slopes of most islands will be
the focus of rainfall through Sunday morning, but particularly
for Kauai and Oahu. A few showers may track onto leeward sides of
the smaller islands.

Meanwhile, a nearly stationary upper level trough remains west of
the state is producing some high clouds over the state. The afternoon
sounding at Lihue showed an above average precipitable water (PW)
value of 1.82 inches with an elevated inversion at 17000 feet.
This is in sharp contrast to the Hilo sounding which showed 0.94
inches with a 8000 foot inversion. Thunderstorms are observed on
satellite about 200 miles to the west, but is not expected to impact
the main islands.

By Sunday night, the clouds and showers will lift north of Kauai
with a drier airmass reaching the entire state. Some clouds and
isolated showers will continue to filter in with the trades, but
we are not expecting any significant weather Monday or Tuesday.

Trade winds are likely to strengthen again by midweek as the same
high pressure system builds back towards the islands. More
moisture will push back in with the trades resulting a few more
windward and mauka showers.

Long range models are beginning to hint that a new front with
good upper level support will approach the island chain from the
northwest around Friday and into next weekend. Timing and details
are still a bit different between the GFS and ECMWF model runs,
so will not be too specific with the forecast quite yet. However,
it is becoming more likely that we will be under a wetter and more
unsettled weather pattern during this period.


AIRMET SIERRA continues for Kauai and Oahu for mountain
obscuration. The old frontal boundary continues to bring clouds
and showers to those islands which is also bringing MVFR and
occasionally IFR conditions. Expect conditions to improve
overnight. Some of the clouds and showers are moving over the
islands of Maui County, so expect some pockets of MVFR over those
islands. VFR conditions continue over the Big Island, but expect
more clouds and showers to form over the island overnight.


The mid-morning scatterometer passes showed some Small Craft
Advisory (SCA) winds around Ka Lae on the Big Island. In addition,
sustained winds reached SCA criteria at PHOG and have been gusting
over 30 kt there for the last few hours. However, as the
background flow veers a little, this will likely put Maalaea Bay
in the wind shadow of Haleakala. At the same time, winds will
likely accelerate around the Hamakua Coast and around N and E
parts of Maui and Molokai. We have added some windward marine
zones to the SCA for tonight, and will likely need to extend the
SCA into Sunday. At the same time, areas like Maalaea Bay may not
need an SCA extension. Have alluded to these finer details in the
MWW product.

Otherwise, relatively quiet for the next few days with a number of
mainly small swells affecting NW, S, and E shores. A small NW
swell will arrive Sun night into Mon, and gradually diminish by
Tue. Another will arrive later Tue and spread down the chain Tue
night, then diminish Wed. A couple of very small S-SSW swells will
bring minor bumps to S facing beaches for much of this week as
well. A surface high will stall out roughly halfway between HI
and CA for a few days will keep a long fetch of moderate trades
aimed at us, but the high will not be particularly strong. Thus,
while sloppy E shore surf will be a little elevated, it should
stay below advisory criteria.

Lingering moisture associated with the old frontal band
interacting with an upper level low about 300 nm west of Kauai
could result in a few thunderstorms over the far W and NW offshore
waters through about Sunday. Will be adding these to the Offshore
Forecast, but deep convection is not anticipated over the Coastal
Waters within 40 nm.


Small Craft Advisory until 6 AM HST Sunday for Maui County
Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-
Big Island Windward Waters-Big Island Leeward Waters-Big Island
Southeast Waters.



AVIATION...M Ballard
MARINE...R Ballard

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