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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 261337
Area Forecast Discussion
National Weather Service Honolulu HI
337 AM HST Wed Apr 26 2017
Moderate easterly trade winds will persist today and tonight
as a surface ridge holds north of the state. The atmosphere will
remain stable, with pockets of moisture affecting mainly windward
slopes. Trades will ease and shift out of the southeast on
Thursday as a front approaches the state, leading to spotty
afternoon showers. On Friday, the front will bring showers to
Kauai and possibly Oahu, while prefrontal showers, some heavy,
develop across the rest of the island chain. Wet and unstable
conditions are expected over portions of the state through the
weekend as a late season low forms just north of the islands.
Passing showers have continued to move across parts of Kauai
during the night, but rainfall amounts have remained generally
light to moderate, with a couple of gauges recording over an inch
of rain during the past six hours ending at 2 AM. Elsewhere
around the state, radar detects scattered showers over the coastal
waters, but most of these are staying offshore with little if any
rainfall recorded on land during the night. Moderate to locally
breezy trade winds prevail over the islands, driven by a surface
ridge located about 450 miles north of Kauai. Mid level ridging
over the area is providing stable conditions. MIMIC-TPW satellite
imagery depicts an area of enhanced moisture embedded in the
trades over Kauai and the windward waters north and east of the
state, with a drier airmass over the eastern islands and the
leeward waters. The 12Z Lihue sounding shows the increased low
level moisture present there, with PW up to 1.32 inches and a
higher inversion near 9000 feet. On the other hand, the 12Z Hilo
sounding showed little change during the past 12 hours, with PW of
1.01 inches and the inversion based near 7000 feet.
Expect little change to the weather pattern today into tonight,
with windward slopes experiencing modest rainfall in passing
showers, and just isolated showers for leeward areas. Easterly
trade winds will hold at moderate strength while the mid level
ridge maintains stable conditions.
Late tonight through Thursday night, trade winds will ease and
veer around to the southeast, as a late season cold front
approaches from the northwest and the surface ridge to our north
weakens. A deep mid/upper level trough driving the surface front
will also advance eastward during this time, displacing the mid
level ridge initially over the islands. Available moisture will
remain quite limited through Thursday morning, but models show an
area of somewhat enhanced low level moisture moving into the state
from the east Thursday afternoon. This increased moisture and
decreased stability combined with an expected seabreeze regime
could allow for some spotty heavier showers to develop during the
afternoon, especially over leeward and interior areas.
The latest GFS and ECWMF runs remain consistent with earlier runs
in bringing the surface front to Kauai by Friday afternoon, while
the mid/upper level trough continues to deepen over the state.
Although moisture along the front itself may be rather shallow,
there will probably be ample moisture pooled ahead of the front,
combined with increasingly unstable conditions, to produce some
locally heavy downpours. Due to increased confidence in developing
instability, combined with afternoon sea breezes which could
provide the triggering mechanism, heavy showers were introduced to
the forecast for Friday afternoon, but thunderstorms have not
been included at this point.
The latest guidance continues to point toward an unstable weather
pattern for the weekend, with the potential for heavy and possibly
flooding rainfall. The GFS and ECMWF still show a compact but
potent late-season mid/upper level low developing north of the
islands on Saturday then dropping southward over or just west of
the state Sunday through Monday. The models are still in decent
agreement with the overall pattern, but there continues to be
a good deal of run to run inconsistency with the exact placement
of the cutoff low when it is closest to the state Sunday into
Monday. The models are also suggesting that a convergence band
will set up along the old front and become the focus for heavy to
possibly flooding rainfall. A blend of the last several GFS and
ECMWF runs points toward Maui County as the most likely position
for this band to set up, but it remains too early to have
confidence in those details. Locally heavy rainfall is mentioned
for much of the state through the weekend, and may need to be
added later at least for eastern sections on Monday as well. We
will also continue to evaluate whether the mention of
thunderstorms will need to be added for the weekend.
Unsettled weather may persist across the state into early next
week, as the latest models keep the mid/upper level low lingering
nearby, along with areas of above normal moisture.
With persistent high pressure ridging over the area, expect
moderate to locally breezy trade winds through today. Although
overall conditions will be VFR, isolated to TEMPO MVFR periods
(lower ceilings and visibilities in passing showers) are likely
in windward and mauka sections on the individual isles.
An AIRMET for TEMPO moderate low level turbulence to the lee of
the mountains is in effect due to the moderate to breezy trades.
The high pressure ridge maintaining the moderate to locally breezy
trade winds is about 400 nm north of the state. Winds remain
strongest over the typically windier locations near the Big Island
as evidenced by an ASCAT pass over the waters a few hours ago. The
high resolution models show an uptick in winds over the Pailolo
Channel today with winds funneling through the channel. As such,
Pailolo Channel is being added to the Small Craft Advisory this
morning. Winds are expected to begin to decline tonight, so the
the SCA only goes through today.
Winds will begin to weaken Wednesday night and into Thursday as a
late season cold front approaches from the northwest. The front
will begin to erode the ridge as it approaches the state. The
front is forecast to stall over the mid-section of the state with
breezy north winds causing SCA-level seas across the western
coastal waters Friday night and Saturday.
The current northwest swell should be nearing its peak over the
next couple of hours. This swell will be followed by a moderate
north- northwest swell arriving late Friday and into Saturday.
Small surf is expected along south facing shores through mid-week,
with a new, long period swell forecast to arrive from the south-
southwest Thursday. This swell could raise surf along south facing
shores to advisory levels, and will persist into the weekend.
Surf along the east facing shores will begin to drop off Thursday
and Friday as the ridge to the north is eroded and winds become
Small Craft Advisory until 6 PM HST this evening for Pailolo
Channel-Alenuihaha Channel-Big Island Windward Waters-Big Island
Leeward Waters-Big Island Southeast Waters.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office