Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 230608

Area Forecast Discussion
National Weather Service Honolulu HI
808 PM HST Thu Mar 22 2018

Cloudy conditions with breezy trades will continue through
Friday, with showers focusing over windward and mauka areas. The
highest chances of rainfall will persist across the western
portion of the state. An upper trough will approach from the west
introducing a threat for heavy rainfall and a few thunderstorms
across all islands from late Friday through Sunday afternoon.
Improving conditions are expected from Sunday night into early
next week.


The big picture analysis shows a strong 1031 mb surface high
pressure center northeast of the state with a 1013 mb low center
roughly 1000 miles northwest of Kauai. A surface cold front
extends south of the low just a few hundred miles northwest of
Kauai. Cloud bands moving across the islands are developing under
a subtropical jet stream with embedded troughs moving through a
weak upper level ridge. These upper level disturbances are
enhancing shower activity mainly across the western islands this
evening. The surface cold front will likely move north of the
islands with little impact.

Meanwhile a sharp upper level trough west of the islands appears
in the GFS 500 mb streamlines. This trough will deepen as it
moves towards the island on Friday, destabilizing the atmosphere
and developing a weak surface low by Friday night enhancing shower
activity statewide. Atmospheric instability and forcing from this
upper level trough will lead to heavy rainfall and probably a few
thunderstorms through the weekend. A Flash Flood Watch was issued
across all islands starting Friday evening through Sunday
afternoon due to the strong potential for heavy rain and flash
flooding. We recommend postponing outdoor activities this weekend
near streams and low lying flood prone areas until this flash
flood threat passes.

The low center will drift north the islands on Sunday with
improving weather conditions as drier air filters into the state
through the first half of next week. Due to the position of this
surface low north of the islands we expect land and sea breezes to
develop on Monday and Tuesday with interior afternoon clouds and
a few showers. Weak easterly trade winds should return from
Wednesday onward.

The current forecast grids look good. No evening updates.


Plenty of low level moisture is combining with an approaching
upper level trough to induce showers and steady rain, mainly over
the western portion of the island chain. AIRMETs for TEMPO
mountain obscuration remain in effect for Kauai and Oahu, and, to
a lesser extent, for the N thru E sections of Molokai, Maui, and
the Big Island. There is also a chance of light icing in the
layered clouds around Kauai and Oahu.

The area of multilayered cloudiness and showers will move slowly
east over the next day or two.


The large northeast swell continues to gradually drop early this
evening according to the nearshore buoys. A High Surf Warning
remains in effect for most north and east facing shores through
early Friday morning. With the current trend in the observations,
we expect to issue a High Surf Advisory for most of these same
north and east facing shores starting around daybreak Friday

The gusty trades and elevated, rough seas will persist through
tonight. Therefore, a Small Craft Advisory (SCA) remains in effect
for all Hawaiian waters through early Friday morning. With the
diminishing trades across most of the coastal waters, and the
subsiding northeast swell, conditions will be generally improving
over most of the Hawaiian waters by daybreak Friday. Therefore,
the SCA will only be in effect during the day on Friday for the
typically windy waters adjacent to the islands of Maui County and
the Big Island. Even though the winds and seas will continue to
improve by late Friday, mariners who are making plans for boating
activities this weekend should be aware of the threat of
inclement weather. An area of low pressure is forecast to move
across the state, which could produce an extended period of
widespread precipitation, with a chance of thunderstorms and
locally heavy rainfall. Westerly winds are expected to spread
across the state early next week after this area of low pressure
departs the region.

Once the northeast swell subsides early this weekend, no other
significant swells are expected through early next week. A small
to medium north-northeast swell is expected from Saturday night
into Monday. A small northwest swell is also expected from Sunday
night through Tuesday, followed by another small northwest swell
during the middle of next week. Small south-southwest and
southwest swells are also expected from Friday into early next


Flash Flood Watch from Friday evening through Sunday afternoon
for all Hawaii islands.

High Surf Warning until 6 AM HST Friday for Niihau-Kauai
Windward-Oahu North Shore-Oahu Koolau-Olomana-Molokai-Lanai
Makai-Maui Windward West-Maui Central Valley-Windward Haleakala-
Big Island North and East.

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

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



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