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Hawaiian Islands Synoptic Discussion and Guidance

839
FXHW60 PHFO 200149
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
349 PM HST Thu Apr 19 2018

.SYNOPSIS...
Showery weather and abundant cloud cover will remain in place
through Friday, particularly over windward areas. Some showers
will be heavy at times with the potential for isolated
thunderstorms. A more typical trade wind pattern will return
Friday night with showers favoring windward and mauka areas. High
pressure north of the state will maintain moderate to breezy
trade winds through the weekend, before strengthening to locally
windy levels next week.

&&

.DISCUSSION...
Healthy trade winds and atmospheric instability will bring wet
weather to areas of the state through Friday with a focus over
windward and mountain locations. A upper level trough moving in
from the NW will bring increasing instability, while an old
frontal boundary riding the trades will push in enhanced low level
moisture from the east. The expected wet weather, along with the
copious amounts of rain in the recent past, have prompted a flash
flood watch through Friday afternoon. Mostly cloudy to overcast
conditions will prevail across the state, with an extensive shield
of mid and high cloud currently extending well southwest of the
island chain. These layered mid and high clouds will push east
bringing increasing chances of winter weather at the Big Island
summits. A winter storm warning is in effect through Friday
afternoon with up to a foot of snow expected above the 12 thousand
foot elevation.

Current radar shows low level showers pushing into windward areas
and isolated convective showers popping over the leeward slopes of
the Big Island. High resolution models are keying off the
increasing low level moisture and instability, showing a steady
increase in showers this evening, and continuing into Friday
morning. Expect showers to favor windward and mountains areas
through the night but leeward locations will see their fair share
of rainfall. The upper level trough will push from west to east
tonight across the islands allowing for some isolated
thunderstorms to form along with locally heavy rainfall. Areas
such as the North Shore of Kauai and East Oahu will be more
susceptible to flash flooding with the recent record rainfalls.

By Friday morning, the threat for heavy rain will focus over the
eastern half of the state with the upper trough over Maui County
and the Big Island. Kauai and Oahu will likely see some clearing
during the morning hours with more normal trade wind weather
returning. By Friday evening, some lingering enhanced showers
will remain over windward Maui and the Big Island, but drier air
from the east and increasing atmospheric instability will
gradually decrease shower intensity and coverage.

A surface ridge to the north will maintain moderate to locally
breezy trade winds Friday night through the weekend. High pressure
will strengthen Monday bringing trades to locally windy levels
through Wednesday. Overall, a fairly typical trade wind pattern
is expected through the period, with bands of enhanced clouds and
showers drifting through the islands from time to time. Showers
will focus over windward and mauka areas with some showers riding
the strong trades to leeward locations.

&&

.AVIATION...
Moderate trade winds will continue across the Hawaiian Islands
producing moderate low level mechanical turbulence over and
downstream of mountains for the next 24 hours. Low cloud ceilings
and showers will linger over mountain areas of all islands.

A band of unstable clouds and showers will enhance shower
activity across all islands with MVFR ceilings along windward
slopes producing widespread mountain obscurations.

In addition to low level impacts previously described, also
expecting some upper level moderate turbulence due to the jet
stream between FL280 and FL400. Also there is a chance of
thunderstorms continuing over Hawaii into Friday morning bringing
the potential for icing and convective turbulence in some spots.

&&

.MARINE...
High pressure north of the area will maintain a trade wind pattern
over the region, with a nearby upper level trough helping to
enhance shower activity. There remains a possibility for locally
heavy downpours and a chance of thunderstorms through Friday.
Gusty winds are also possible near any thunderstorms.

The Small Craft Advisory (SCA) has been trimmed back to the more
typically windier areas near Maui County and the Big Island
tonight. The SCA continues through Friday, but could very easily
be extended further with winds expected to remain elevated in
these areas.

Surf is expected to remain below High Surf Advisory (HSA)
thresholds along all shorelines through this weekend. Rough,
moderate surf is expected to persist along most east facing shores
through Sunday night due to the trade wind swell. Surf heights
will increase again along east facing shores early next week as
trade winds increase. Surf may reach the HSA threshold along east
facing shores starting late Monday, and persist into the middle
of next week.

The current moderate northwest swell is expected to peak Friday,
and then gradually subside this weekend. A slightly larger
northwest swell is expected to spread down the island chain late
Sunday, peaking Monday. This swell will gradually subside from
Tuesday through Wednesday.

A series of small swells from the southern hemisphere will
maintain modest surf along most south facing shores across the
island chain through this weekend.

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

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Friday afternoon for Niihau-Kauai-Oahu-
Molokai-Lanai-Kahoolawe-Maui-Kona-South Big Island-Big Island
North and East-Kohala-Big Island Interior.

Winter Storm Warning until 6 PM HST Friday for Big Island
Summits.

Small Craft Advisory until 6 PM HST Friday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

Morrison...Discussion
M Ballard...Marine
Brenchley...Aviation

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