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Hawaiian Islands Synoptic Discussion and Guidance

138
FXHW60 PHFO 022019 CCA
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1004 AM HST Fri Dec 2 2016

.SYNOPSIS...
A series of upper level disturbances passing over or near the
state combined with a deep moist southerly flow, will result in
periods of unsettled weather across the island chain through the
weekend. Locally heavy rainfall and thunderstorms will be possible
at times. The unsettled weather is expected to briefly exit the
state Monday and Monday night, with more unsettled weather then
moving back in Tuesday through the middle of next week.

&&

.DISCUSSION...
Currently at the surface, a 1034 mb high is located around 1700
miles northeast of the state, with an inverted trough developing
around 400 miles west of Kauai. These features place Hawaii in a
moderate to fresh east southeast flow. Aloft, a deep upper level low
is evident in water vapor imagery just north of the state embedded
within an upper trough with an axis running perpendicular over the
island chain. Infrared satellite shows deep layered clouds across
the eastern half of the island chain along the eastern flank of the
upper trough. Forecast highlights include a Flash Flood Watch that
is in effect through Sunday and a Winter Storm Warning through
Saturday. The presence of the upper low / trough will allow for
greater vertical development of clouds to bring potentially heavy
rain (snow at high elevations on the Big Island) and a slight chance
of thunderstorms through the weekend.

The high to the northeast of the state will remain nearly stationary
today, while an inverted trough west of the island chain becomes
better developed. The resulting southeast flow will keep stronger
winds blocked downstream by the island terrain allowing sea breezes
to set up interior clouds and showers this afternoon. With the
atmospheric dynamics and moisture as it is, these clouds and showers
also have the potential to be heavy.

Another upper level trough will dig to the west of the island
chain tonight, amplify west of the state Saturday and Saturday
night, then weaken slowly as it lifts northeastward across the
islands Sunday through early next week. Models show the inverted
trough to the west of the islands keeping us in southeast flow
through Saturday night, then bring it over the state Sunday and
Sunday night.
As for sensible weather details this weekend, south to southeasterly
flow at the surface and aloft, will allow deep tropical moisture
to funnel northward across the island chain. In combination with
an unstable airmass remaining in place, will keep conditions
unsettled, with periods of heavy showers and isolated thunderstorms
moving through the area. As these features will cover most of the
state this weekend, low confidence exists for exact timing and
location of these heavy showers or thunderstorms.

Monday through next Thursday,
Model solutions are in general agreement keeping a weak pressure
pattern and troughing in place across the state through the
period. They also show another upper level trough developing west
of the islands Monday through mid week, but disagree on the
amplitude and progressiveness of this feature. Overall, we may see
a brief break in the unsettled weather Monday and Monday night as
the deepest moisture and best forcing ascent lift northeast of the
area. This will likely be short lived however, as the models show
deep southerly and southeastly flow lifting deep tropical moisture
back into the area Tuesday through next Thursday, keeping
conditions unsettled and rain chances above normal.

&&

.AVIATION...
Unsettled weather over the state will continue today. An upper
level trough over the state with ample moisture will lead to
periods of heavy rain and isolated thunderstorms throughout the
state. The Big Island will continue to hold the highest chance for
thunderstorms and widespread showers, expect PHTO to see
prevailing MVFR with periods of IFR not unlikely throughout much
of the day. Reduced visibilities, turbulence, icing, and
lightning can be expected with any thunderstorms that develop.
Expect CB tops associated with thunderstorms to reach upwards of
FL400

Winds will continue to weaken and veer to the southeast today as a
surface trough develops west of Kauai. AIRMET TANGO for low level
turbulence has been cancelled this morning. As the winds trend
down, some areas may pockets of isolated turbulence.

AIRMET SIERRA for mountain obscuration is posted over most north
thru east facing slopes on all islands. AIRMET ZULU for icing
within thick layered clouds is posted over the Big Island and
adjacent waters. AIRMET TANGO is posted for tempo light to
moderate high level turb between FL240/400 from Oahu- Big Island
from an upper level jet on the east side of the trough.


&&

.MARINE...
An overnight ASCAT pass showed winds speeds ranging from 20 to 30 kt
across the Hawaiian waters, highest winds over the channels and
the waters south of the Big Island. A Small Craft Advisory (SCA) is
in effect for most coastal waters through Saturday due to a
combination of the winds as well as a building northwest swell.
Buoy observations reflect this pattern and continue to record seas
around 10 to 11 ft at exposed buoys.

The latest model guidance lines up well with the current conditions
and generally depicts the winds trending down and shifting toward
the south over the weekend in response to a surface trough developing
over the western waters. Winds and seas should drop below SCA
levels by Sunday and remain below this level through the upcoming
week. Isolated thunderstorms will remain in forecast through the
weekend due to a broad upper low centered over the state. Gusty
winds, locally higher waves and poor visibility can be expected in
the vicinity of thunderstorms.

East facing shores will continue to experience advisory-level surf
through the day today and possibly into tonight due to the
persistent strong trade winds over and upstream of the islands.
Although the winds will become light over the weekend through early
next week, a moderate easterly swell will linger and continue to
generate moderate surf along east facing shores into early next
week. Along north and west facing shores, advisory-level surf is
expected today (due to the northwest swell) and trend down this
weekend. A similar source (310 deg) is forecast to fill in
Tuesday night and peak Wednesday and Wednesday night before
trending down Thursday.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Sunday afternoon for all Hawaii
islands.

High Surf Advisory until 6 PM HST this evening for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Windward Haleakala-
South Big Island-Big Island North and East.

Winter Storm Warning until 6 PM HST Saturday for Big Island
Summits.

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

&&

$$

DISCUSSION/MARINE...Foster
AVIATION...Eaton

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



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