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RH96 %
WindSW 28 mph
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Hawaiian Islands Synoptic Discussion and Guidance

524
FXHW60 PHFO 101948
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
948 AM HST Sat Dec 10 2016

.SYNOPSIS...
An upper level trough near the western end of the state will bring
overcast skies and rain from Molokai eastward today. This band of
clouds and showers will slowly shift eastward tonight through
Monday night, resulting in slowly improving conditions from west
to east across the island chain. Drier weather is then expected
Tuesday through Wednesday as weak high pressure builds north of
the state. This will be short lived however, as unsettled and
potentially very rainy weather may return for the latter part of
the upcoming work week.

&&

.DISCUSSION...
Water vapor satellite shows an upper level trough skirting along
the western end of the island chain and lifting towards the
northeast. Infrared satellite shows a layered cloud band extending
north from the deep tropics covering from Oahu to about 250 miles
east of the Big Island. Deepest clouds within the band extend up
to an estimated 32 kft. Radar reflectivity shows mostly stratiform
precipitation concentrated across Maui County and the northern
side of the Big Island. Isolated thunderstorms were observed
earlier this morning within the shower band, but the overall
intensity of the band has weakened a little bit since then.

At the surface, a couple of weak closed lows are located north of
the islands linked together by a front that extends towards
California. A 1026 mb high sits about 1800 miles east northeast of
the Big Island. This is resulting in a weak wind regime across
most of the state this morning, with gentle winds out of the
south southeast near the Big Island, and variable elsewhere.
Soundings from early this morning show precipitable water values
of 1.38 inches at Lihue and 1.81 inches at Hilo, with the latter
showing a saturated vertical profile. The Lihue sounding has a
weak inversion about 10 kft and pockets of drier air through the
midlevels, though lapse rates are fairly steep with the upper
trough passing through. Temperatures on the Big Island summits remain
just below freezing this morning with deep enough layered clouds
to support snowfall into this evening. A Winter Storm Warning is
in effect until midnight tonight.

Expect little change to current conditions today with overcast
skies and rain for Molokai eastward. Oahu will see more sun
through the day and Kauai is already mostly sunny this morning.
However, the light wind flow will allow for sea breezes to bring
afternoon clouds and showers for interior sections this afternoon.
The passing upper trough is firing thunderstorms about 300 miles
north of Kauai this morning, but thinking it will stay north of
Kauai and Oahu today. The limiting factor should be the drier
midlevel air and weak inversion (indicated by the Lihue sounding).

The upper trough will lift towards the northeast and push the layered
cloud band eastward through Sunday. Expect to see a slow eastward
progression of the layered clouds and showers shifting east. The
surface lows will also lift northeast, which will allow a weak
ridge to build in from the north and bringing gentle north winds
across the western portion of the state Sunday. Cooler and drier
air will accompany this flow regime. The Big Island, and to a
lesser extent, Maui, will still deal with some lingering clouds
and showers Sunday and Monday, and push east of the Big Island by
Tuesday.

Model solutions are in good agreement showing improving conditions
Tuesday through Wednesday as weak high pressure builds north of
the island chain, bringing gentle trade winds for all islands. It
appears that this improvement will be short lived however, as the
12Z GFS and 00Z ECMWF, along with support from the GFS/ECMWF
ensemble members, show a deep stacked low developing just to the
west of the island chain Wednesday night and moving little through
the end of the upcoming work week. Although model solutions differ
on the exact details at this time, they both indicate that we may
move back into a very unsettled rainy pattern for the latter half
of next week.

&&

.AVIATION...
A thick layer of stratiform layered clouds lies over Maui County
and the Big Island...ahead of a slow moving upr lvl trough. Thus
expect little if any improvements during at least the daylight
hours. Judging from the most recent sfc obs...significant low
cloud coverage appears to be confined to mainly Maui County. Thus
expect AIRMET SIERRA for mtn obscuration acrs Maui County to
continue well into the afternoon hours. Will update should the
weather improves. Northern Big Island is the next area to watch
out for regarding AIRMET SIERRA and mtn obscuration.

As for Kauai and Oahu...we will be monitoring closely for some
sct showers to develop over and in the surrounding waters as the
day progresses. No AIRMETS expected at this time.

AIRMET TANGO for upper level turbulence will be kept for the area
from oahu ewd starting with the noon package for fl280-fl400.

AIRMET ZULU will also stay in effect beyond at least 22z in the
thick layered clouds for psbl light to moderate icing in clouds
between 140-FL300.


&&

.MARINE...
A compact gale is located several hundred miles north of the
state. The overnight ASCAT pass showed a fetch of 30-40 knot
winds about 150 miles long aimed at the state. Models show the
low weakening this weekend, but moving southward as it does. This
will contribute to a short-period north to north-northeast swell
through early next week. This swell will larger over the western
half of the state given the location of the low and direction of
the fetch.

A longer-period northwest swell associated with a gale currently
near the date line is expected to fill in Monday, peak Monday
night, then fade Tuesday through Wednesday. Models remain in
good agreement showing another gale developing northeast of Japan
Sunday and tracking toward the Aleutian Islands early next week. A
new northwest swell will likely fill in from this source Thursday
and hold into the weekend before trending down. Although
confidence remains low this far out in the forecast, advisory-
level surf along north and west facing shores is possible from
Friday into Saturday.

There remains a fair degree of uncertainty with a strong low
developing near or to the northwest of the area late in the week,
which could lead to a period of moderate to strong southerly
winds. A combination of local winds and the building northwest
swell will likely translate to Small Craft Advisory conditions
beginning as early as Friday.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until midnight HST tonight for Big Island
Summits.


&&

$$

DISCUSSION...Foster
AVIATION...Lau
MARINE...Bravender

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