Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 200306 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
506 PM HST Mon Mar 19 2018

Trade winds will increase to locally breezy levels by midweek,
then ease toward the weekend as high pressure strengthens to our
north before getting edged out by a front approaching from the
west. Trade winds will push clouds and showers across the
islands, focusing rainfall mainly across windward and mauka
areas, with only light amounts expected. Showers will increase
late in the work week and over the upcoming weekend as the front
passes eastward through the islands, with locally heavy rainfall


Currently at the surface, a 1040 mb high is located around 1850
miles north of Honolulu, with the ridge axis extending southward
from the high, and running roughly northwest to southeast around
250 miles northeast of the state. The resulting gradient is
driving moderate trades across the island chain this afternoon.
Satellite imagery shows mostly cloudy skies in place across the
state, with radar imagery and rain gauge observations indicating
shower coverage is on the decline over windward areas of Oahu and
Kauai, with numerous to widespread showers continuing to roll into
windward sections of Maui and the Big Island. Additionally,
scattered thunderstorms have developed over the Big Island this
afternoon. Main short term concerns revolve around thunderstorm
potential through this evening and rainfall trends as drier air
works its way into the state from the north.

A pair of strong highs to the north of the area will merge into a
1045 mb high just south of the Aleutian islands by Tuesday
morning. This will shift the band of showery low clouds affecting
mainly windward areas of Maui and the Big Island, with improving
conditions expected later this evening across Maui, and by
Tuesday morning across the Big Island. A shortwave impulse will
pass over the Big Island this evening, so will continue to mention
a chance of thunderstorms in the forecast. After midnight things
should stabilize sufficiently that thunder will no longer be a
threat. Moderate trade winds are expected to prevail through the
overnight hours.

Tuesday through Thursday night,
Model solutions are in good agreement showing mid and upper level
ridging holding in place during the mid week time frame. The
source airmass through this period is over the far northeast
Pacific and will have advected southwestward over the islands due
to the large high north of the state and the persistent low to the
northeast. As a result, we expect a stable weather regime over
the island chain through the period, with dewpoints generally in
the upper 50s and lower 60s. This will greatly limit shower
activity across the state, with spotty light showers primarily
confined to windward areas. Moderate trade winds will strengthen
on Tuesday, and reach locally breezy levels Wednesday through

Friday through next Monday,
The 12Z runs of the GFS and ECMWF are in reasonably good
agreement showing shortwave energy digging to the west of the
state Friday and Friday night, with a surface low developing near
or over the islands along a lingering surface trough over the
weekend. The surface low is then expected to lift northeast and
away from the state early next week.

As for sensible weather details, deep tropical moisture is
expected to begin advecting over the western islands on Friday,
then shift slowly eastward Friday night and over the weekend
resulting in a wet conditions statewide. Conditions should begin
to improve early next week as the surface low pulls northeast and
away from the state, dragging the deepest moisture east of the
island chain. There is the potential that heavy rainfall and flash
flooding could affect all or portions of the state over the
weekend, but it remains too early to tell how much rainfall will
occur or where flooding, if any, would threaten. This will
continue to be monitored closely as the week progresses. The wind
forecast will be a bit problematic as it will depend greatly on
the location, track and intensity of the surface low late week
through the weekend. Will take a blended approach to the winds for
the time being, although there will likely be some significant
changes as details become more clear.


A surface high far N of the area will maintain NE surface flow
over the main Hawaiian Islands. These NE winds will continue to
push showery low clouds from the trailing end of a front over the
islands. MVFR ceilings will likely be extensive enough to require
AIRMET SIERRA for MTN OBSC to remain up throughout the night for
NE sections of most islands.

A sharp short-wave trough aloft is enhancing shower activity over
the islands today. Isolated thunderstorms have already developed
over the Big Island interior and waters west of the Big Island
and south of Maui. Thunderstorm activity will likely continue
into the evening hours but taper off after midnight.


The morning ASCAT pass showed a wide swath of 40-50 knots
associated with the strengthening low northeast of the state.
Wavewatch guidance shows the swell associated with this low
exceeding 10 feet from the northeast, peaking Wednesday/Wednesday
night. Surf will likely reach warning levels along east-facing
shores (with advisory along north-facing shores) during this
period, then gradually diminishing through the week with the
potential for advisory surf through Friday.

Fresh to strong trades associated with a ridge to the north will
drop a bit early on Tuesday, then ramp up again late
Tuesday/Wednesday as the high far to our north starts to move
southeastward and tighten the gradient. A surface trough
associated with a low northwest of the state will start to edge
eastward late in the week, gradually weakening the winds and
turning them more southeasterly. Portions of the small craft
advisory that drop below the wind threshold of 25 knots on
Tuesday will see seas building above 10 feet Tuesday night.


Small Craft Advisory until 6 AM HST Tuesday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha



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