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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 261955
Area Forecast Discussion
National Weather Service Honolulu HI
955 AM HST Sun Feb 26 2017
Light to moderate trade winds and mostly dry weather will prevail
through Monday, with land and sea breezes developing over most
island areas. While high clouds will continue to blanket skies over
Maui county and the Big Island, they will stay clear of Kauai and
Oahu today, only to thicken again on Monday. Unsettled weather is
expected Tuesday and Wednesday, with the potential for heavy
showers, and a chance of thunderstorms, as a low pressure area
develops nearby. The low will move north of the area on Thursday,
and island weather will become more settled, but light winds are
expected to continue through next weekend.
Latest surface analysis features lows centered to the distant NE and
NNW of the islands, with a surface ridge just N of Kauai extending
from a high far N of the area. Water vapor imagery shows a broad
trough aloft over the area, with a swath of broken to overcast mid-
and high-level clouds blanketing island skies from Oahu eastward.
With this setup, light winds and dry conditions prevail over land,
light to moderate E to SE winds prevail over water, and cloudy skies
rule over Maui county and the Big Island.
The island atmosphere is fairly stable, and thus the short term
forecast anticipates little in the way of rainfall. As sea breezes
develop this afternoon, interior clouds may lead to isolated showers
on Kauai and Oahu, but high cloud cover elsewhere will likely act to
limit the strength of the sea breeze, and the subsequent low
cloud/shower development. Little overall change to the benign
weather pattern is expected tonight, with high clouds persisting
over the E half of the state while light surface winds allow land
breezes to clear most areas of low clouds. However, there is a
chance that showers start to increase over windward portions of the
Big Island and Maui later tonight.
Latest guidance continues to indicate that a trough aloft will
deepen NW of Kauai through Tuesday before moving over the islands
Tuesday night and Wednesday. This development will support
thickening high clouds over the whole state on Monday, while also
bringing increased instability and moisture. Additionally, as the
sharp trough aloft passes over the islands, guidance indicates the
development of a weak surface low near the islands on Wednesday. We
therefore anticipate a period of unsettled weather, but the
specifics of where remain uncertain, with some uncertainty as to
when as well. Latest thoughts are that the best chance for wet
weather is Tuesday night and Wednesday, with potential for winter
weather over the Big Island summits, and the potential for
thunderstorms, but little confidence as to which areas may be
impacted. With confidence increasing that at least somewhere in the
islands will receive heavy rainfall, a statewide Flash Flood Watch
may be issued later today or tomorrow, and a Winter Storm Watch for
the Big Island summits may also need to be issued. However, these
are usually issued when forecast confidence exceeds 50% that
conditions will be met within 48 hours, and those criteria have yet
to be met.
A trend toward drier weather is expected Wednesday night/Thursday as
the trough aloft and associated surface low moves N, and a surface
ridge develops over the islands. Light winds and mostly dry weather
are then expected to persist through next weekend, although the flow
around the ridge may advect vog from the Big Island over the smaller
islands, bringing hazy conditions Friday through Sunday.
Gentle to moderate east southeast flow expected at the low levels
through tonight. Aloft, a 120 knot subtropical jet will bring
moderate turbulence over most of the state today. AIRMET Tango is in
effect statewide for this hazard between FL200 and FL350.
Currently, SIGMET Romeo is also in effect for moderate to severe
turbulence concentrated over Oahu and Maui County between FL200 and
FL270. Models show winds aloft over the state will begin to decrease
later today and this evening, and the mid/upper level turbulence
will likely diminish at that time.
The jet stream is responsible for the mid and high level clouds over
the eastern half of the state today. These clouds may cause some
icing concerns for aircraft at the midlevel, and AIRMET Zulu has
just been posted for Molokai to the Big Island.
No AIRMETs are in effect for mountain obscurations at this time, but
the potential threat for this will be across the Big Island summits
as these mid and high clouds build lower today and tonight.
The current west-northwest swell (300-310 deg) will continue to
slowly lower into the early portion of the week. A small
reinforcement out of the west-northwest is forecast to fill in
Tuesday through mid week. Surf along exposed north and west facing
shores will remain below high surf advisory levels through this
time. A small to moderate north-northeast swell (030 deg) due to a
pair of compact lows to the north of the islands will fill in across
the local area tonight through Tuesday night, peaking early Monday.
This swell could produce surf near high surf advisory threshold (8
ft faces) for exposed east facing shores around the peak of the
swell. For the extended, the latest model guidance depicts a gale to
storm-force low developing and tracking a few hundred miles north of
Midway Wednesday through Thursday, sending a large west-northwest
swell toward the local waters for the upcoming weekend.
Light to locally fresh breezes out of the east-southeast will
continue through Monday night, strongest across the channels and
windward zones. Models continue to depict a developing area of low
pressure near the state by mid-week, lifting northward across the
state through Thursday. Plenty of moisture and instability
associated with this feature will lead to increasing
shower/thunderstorm chances through this period.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office