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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 190159
Area Forecast Discussion
National Weather Service Honolulu HI
359 PM HST Sun Mar 18 2018
High pressure will strengthen far to the north of the state over
the next couple days, allowing moderate trades to increase to
locally breezy levels by Tuesday and Wednesday. The trades will
ease Thursday through next weekend as a front approaches from the
west and slowly pushes eastward across the island chain. A band of
clouds and showers will push southward through the islands tonight
and Monday, keeping showery weather in place in windward areas,
with a stray shower reaching leeward areas from time to time.
Drier conditions are then expected to overspread the entire state
beginning Monday night and continuing through Wednesday night.
Rain chances should then be on the increase late in the work week
and over the upcoming weekend as the weakening front shifts
eastward through the islands.
Currently at the surface, a 1037 mb high is centered around 1600
miles north-northwest of Honolulu, with a 1002 mb low around 1125
miles west-northwest of Kauai. Meanwhile, a weakening or
dissipating front is located around 450 miles west of Kauai.
Satellite imagery shows mostly cloudy skies in place across the
state, with radar imagery showing persistent showers rolling into
windward areas on the trades, with some leeward spill over and
convective development over the Big Island. Main short term
concern revolves around rain chances over the next 24 to 36 hours.
Tonight through Monday night,
High pressure well north of the state will lift northward and
merge with another high lifting northwestward out of the western
Pacific over the next couple of days, with the resultant high
reaching 1045 mb near the central Aleutians by Tuesday morning.
This will keep a moderate trade wind flow in place across the
Low clouds and showers will remain over the smaller islands
tonight, and will push into the Big Island early this evening.
Showers will be most prevalent over windward and mauka areas, but
a few showers will spill over into leeward areas of the smaller
islands from time to time. Across leeward sections of the Big
Island, convective showers should end for the most part during
the evening, but an isolated shower or two can't be ruled out
overnight along the Kona coast.
There are some mixed signals in the guidance for Monday, with the
ECMWF showing a drying trend over the smaller islands during the
afternoon, while the GFS shows a wave of showers associated with a
shortwave trough moving through. Will keep some showers in the
forecast for all areas of the smaller islands Monday to account
for this possibility, then show conditions drying out Monday
Showery low clouds are expected to hang up over the Big Island on
Monday, then begin to shift south of the state Monday night.
Expect a wet day across windward sections of the Big Island, with
shower coverage trending down Monday night as the deepest moisture
shifts south of the state. Mainly dry conditions are expected over
leeward sections of the Big Island Monday morning, with convective
showers then re-developing Monday afternoon. The convective
showers should largely end in the evening, but a stray shower may
affect the Kona coast even after midnight.
Tuesday through Wednesday night,
Both the GFS and ECMWF are in good agreement showing the strong
surface high over the Aleutians weakening as it drops south-
southeastward and closer to the islands through the period. This
will keep a moderate to locally breezy trade wind wind flow in
place across the state. There is some disagreement however with
respect to the eastward extent of the deeper moisture associated
with the front west of the state. The GFS shows the deepest
moisture remaining west of the state while the ECMWF shows it
extending eastward and over Kauai and perhaps as far east as Oahu.
Since this is the first run that the ECMWF has shown this deeper
moisture extending farther east, will favor the drier GFS
solutions until details become more clear. As a result, we expect
scattered mainly windward and mauka showers through the period,
with mainly dry conditions in leeward locales.
Thursday through next Sunday,
The latest GFS and ECMWF runs are in good agreement showing a
weakening front approaching the state from the west Thursday and
Friday, then slowly pushing eastward across the island chain over
the weekend. Kauai will remain in closest proximity to the front
and its associated deep moisture Thursday through Friday, so this
is where the best chances for rain are expected. Elsewhere, fairly
dry conditions are expected to continue through Friday. More
unsettled conditions are then expected Friday night and over the
weekend as the front and band of deep moisture shift eastward
across the island chain.
Surface high pressure building far north of the state will
maintain moderate trade winds through tonight. The leading edge
of a diffuse band of low clouds and showers extends as far
eastward as Maui this afternoon. The clouds and showers within
this band will continue to spread southeastward before reaching
the Big Island early this evening. These low clouds and showers
will produce periods of MVFR ceilings / visibilities, particularly
along north and east facing slopes of most islands.
AIRMET Sierra remains in effect for mountain obscuration across
windward Kauai...Oahu...Molokai...and Maui. This AIRMET will
likely need to be extended to include windward Big Island this
evening as the band of low clouds and showers continues to spread
across the state.
The latest ASCAT pass showed an a small area of 25 kt winds north
of Kauai. This, along with the latest hi-res models, led to the
issuance of a Small Craft Advisory (SCA) for the Kauai northwest
waters this afternoon. Trades will continue to increase tonight
and Monday, and are forecast to reach Small Craft Advisory (SCA)
levels in the windy zones around Maui County and the Big Island,
as well as the waters around Oahu and the Kaiwi Channel Monday. An
SCA has been issued for the aforementioned areas beginning at 6
am Monday. These elevated trades are expected to persist through
Tuesday afternoon, before shifting out of the east southeast late
Tuesday through the second half of the week as a front approaches
from the west and stalls west of Kauai. The front may also bring a
period of wet weather during the second half of the week and next
A more northerly, short-period swell will fill in tonight. Small,
long-period south swells will keep surf just above flat for south
facing shores over the next few days. Surf heights will remain
below advisory levels on all shores through the early part of this
A broad low in the northeast Pacific will direct a large swath of
gale force winds towards the islands early this week. This should
translate to a building north-northeast swell late Tuesday, and
could bring advisory level surf to north and east facing shores
through at least Thursday, possibly longer for east facing shores.
Small Craft Advisory from 6 AM Monday to 6 AM HST Tuesday for
Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters-Oahu
Leeward Waters-Kaiwi Channel-Maalaea Bay-Pailolo Channel-
Small Craft Advisory until 6 AM HST Tuesday for Kauai Northwest
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office