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Hawaiian Islands Synoptic Discussion and Guidance

344
FXHW60 PHFO 260145
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
345 PM HST Sun Feb 25 2018

.SYNOPSIS...
A trough of low pressure west of the area will allow for some
locally heavy showers and isolated thunderstorms over Kauai and
possibly Oahu through tonight. Mostly cloudy skies with passing
showers can be expected elsewhere across the area. High pressure
will build in northeast of the area this week with generally drier
weather expected. Showers will tend to favor windward and mauka
areas. Rather breezy to locally windy conditions can be expected
toward the middle of the weak as the high moves closer to the
state.

&&

.DISCUSSION...
A rather complex weather pattern is in place across the area. At
the surface, a rather strong area of high pressure is located far
to the north northeast of the state with a surface trough located
several hundred miles to the west of Kauai. This places the
islands in a moderate to locally breezy east southeast wind
environment. Aloft, a broad upper trough is centered several
hundred miles to the west northwest of the area. Destabilization
caused by the trough along with abundant low level moisture will
allow for the possibility of heavy showers and isolated
thunderstorms through tonight especially over the west end of the
state. A Flash Flood Watch remains in effect through tonight for
Kauai since they will have the greatest chances of flash flooding
since they are closer to the trough axis and soils are rather
saturated from recent rains.

Models are in good agreement on the synoptic weather pattern this
week. Both the surface and upper level troughs will gradually
weaken and move a bit further west of the area. In the meantime,
an upper level ridge will build in from the north and the strong
surface high will move southward just a bit taking up a position
far northeast of the area. The upper ridge will allow for the
airmass to become more stable with time decreasing the threat of
heavy showers. Shower activity will tend to be focused over
windward and mauka areas for the remainder of the week. As the
surface high gets closer to the state, the result will be for
rather breezy to locally windy conditions across much of the area.
The wind direction will be slight south of due east. Downsloping
effects may allow the winds to be strong enough for Wind
Advisories needing to be posted for some areas.

&&

.AVIATION...
High pressure far NE of the area and a broad surface trough W of
Kauai, will keep locally strong E to SE winds over the islands
into tonight. Model guidance is hinting at the winds backing a to
a more E direction tomorrow. AIRMET TANGO for low level
turbulence is posted over the entire state due to the stronger
winds.

Wet weather will continue for most areas tonight as moisture
continues to be drawn up from the SE. Windward and SE areas will
be the most favored. An upper level disturbance W of Kauai will
contribute to potential thunderstorm development over Kauai and
Oahu tonight.

AIRMET TANGO for upper level turbulence is posted over the state.
AIRMET SIERRA for mountain obscuration is posted across Kauai and
portions of Oahu, Molokai, Maui, and the Big Island. Any layered
clouds within the area have the potential for light icing.

&&

.MARINE...
A moist and unstable airmass will keep a chance of thunderstorms
in the forecast for the western half of the coastal waters tonight.
Mariners should be alert for strong, shifting winds and frequent
lightning with any thunderstorms that develop.

A Small Craft Advisory (SCA) remains posted for most waters,
including all windward areas, due to strong east-southeast winds
and elevated seas through Tuesday. The Sunday morning ASCAT pass
covered the eastern waters and showed 25 kt winds widespread over
windward areas, with areas of 30 kt winds to the north and south
of the Big Island. Gales will be possible in some areas around
Maui County and the Big Island during the middle and later part of
the week.

A long and rather broad fetch of easterly winds on the south side
of a distant subtropical high will continue to produce at least
advisory level surf along east facing shores through the next
several days. The High Surf Advisory for those shores continues
through Monday afternoon. Beyond then, the latest model guidance
continues to show that surf could reach warning levels on those
shores as early as Monday night or Tuesday. The large surf
combined with high astronomical tides could cause some coastal
inundation issues along some low lying areas exposed to the east.

Small surf is expected along north, west and south facing shores
this week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through late tonight for Niihau-Kauai.

High Surf Advisory until 6 PM HST Monday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

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

&&

$$

Synopsis/Discussion...Burke
Aviation...Eaton
Marine...Foster

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