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Hawaiian Islands Synoptic Discussion and Guidance

968
FXHW60 PHFO 241346
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
346 AM HST Sat Feb 24 2018

.SYNOPSIS...
Moist and somewhat unstable conditions will persist across
portions of the state through the weekend. East to southeast
winds will focus rainfall over windward and southeast slopes. The
greatest threat for heavy showers and thunderstorms will be over
the Big Island tonight, then shift to the western end of the
island chain over Kauai and Oahu on Sunday. Drier conditions will
push back over the state early next week as potentially strong
east to east- southeast winds develop across the entire state.

&&

.DISCUSSION...
Light to moderate showers continue to spread across the eastern
coast of the Big Island this morning. Satellite imagery shows
bands of heavier showers and thunderstorms lingering 200 miles
west of Kauai. Elsewhere scattered light showers are forming over
typical windward and mountain slopes. The threat for flash
flooding has diminished over the Big Island and the Flash Flood
Watch for Hawaii County was cancelled. The Winter Storm Warning
for the Big Island Summits was also cancelled with the morning
forecast package.

The large scale weather picture continues to show an upper level
closed low roughly 900 miles northwest of Kauai and a strong 1040
mb high center far to the northeast of the state. The interaction
between these two features continues to draw tropical moisture
northward over the Hawaiian Islands keeping us in moist and
moderately unstable low level environment. Morning upper air
balloon soundings in Lihue confirm a weak high level subsidence
inversion setting up around the 10000 to 12000 foot level. Model
guidance indicates this weak inversion should help to keep showers
mostly confined to windward and mountain areas today for the
smaller islands. While deeper moisture and instability will
linger over the Big Island today with elevated chances for shower
activity along the eastern and southeastern slopes.

The interaction between the ridge and a surface trough west of
Kauai has increased winds over the saddle and lower slopes of the
Big Island. A wind advisory remains in effect for sustained winds
forecast to range from 15 to 30 mph with locally higher gusts up
to 50 mph for the Big Island through the afternoon hours.

In the short term, we continue to see the upper low drifting
slowly northwest away from the state as the strong high pressure
ridge gradually builds in a more stable atmosphere from the
northeast. On Sunday, forecast guidance continues to show a shift
in shower activity towards the west with increasing rainfall
chances over Kauai and portions of Oahu as an upper level
negatively tilted trough sweeps across the western edge of the
state. Kauai will see the best chances for heavier showers and
thunderstorms, Oahu will also see some shower and thunderstorm
enhancement. A Flash Flood Watch may be needed for these western
islands.

A drier trend develops next week as the strong ridge builds in
over the state. Enough moisture and instability will linger over
the windward sections of the Big Island for continued wet trade
wind showers, however the rest of the state will see a drying
trend from Monday into Tuesday. Stronger winds are expected on
Tuesday afternoon and Wednesday where additional wind advisories
may be needed as wind speeds ramp up across the state.

&&

.AVIATION...
High pressure northeast of the state and a trough of low pressure
to the west, will keep a gusty east to southeast flow in place
across the island chain through tonight. A moist and unstable
airmass will remain over the island chain, particularly the E and
SE areas of the Big Island, where widespread MVFR and isolated IFR
conditions are expected. Across the smaller islands isolated MVFR
conditions will be possible mainly in windward areas. Afternoon
shower development will also be possible over interior and leeward
sections of the islands as a result of daytime heating, bringing
some brief MVFR conditions which should end during the early
evening hours.

AIRMET SIERRA remains in place for mountain obscuration across
E to SE facing slopes of the Big Island, and NE thru SE facing
sections of Kauai and east Maui. No other AIRMETs are in effect.

&&

.MARINE...
An unstable airmass will continue the possibility for isolated
thunderstorms over all coastal waters today, before shifting to
the western half tonight and tomorrow.

Winds have backed off a touch in the waters near the Big Island,
so the Gale Warning has been cancelled with the morning package. A
Small Craft Advisory (SCA) remains posted for most waters,
particularly on the east sides of the islands due to the strong
winds and elevated seas. Strong high pressure far to the north-
northeast will maintain this breezy east to southeast wind pattern
through the remainder of the weekend, before an uptick in winds
during the first half of the new work week. Its possible the SCA
will need to be expanded next week as the winds increase, and
additional gale force winds are possible.

A long and rather broad fetch of easterly winds on the south side
of the high will continue to produce advisory level surf along
east facing shores into early next week. The latest model guidance
shows surf could reach warning levels as early as Monday night or
Tuesday. This combined with high astronomical tides could cause
some coastal inundation issues along some low lying areas exposed
to east.

The current small northwest swell will continue to lower
gradually today. Otherwise surf along north and west facing
shores will be well below normal for this time of year during the
Sunday through Thursday time period. A series of small, mainly
background south swells can be expected through the weekend and on
into the middle of next week.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Monday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Wind Advisory until 6 PM HST this evening for Kona-South Big
Island-Big Island North and East-Kohala-Big Island Interior.

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

&&

$$

DISCUSSION...Bohlin
AVIATION...Jelsema
Marine...M Ballard

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



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