Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 211958

Area Forecast Discussion
National Weather Service Honolulu HI
958 AM HST Wed Mar 21 2018

Cloudy conditions with breezy trades will continue through the
second half of the work week. Although most of the shower coverage
will focus over windward and mountain areas through Friday, a few
passing leeward showers, mostly light, will remain possible.
Light winds along with increasing rain chances are expected late
Friday through Saturday. Improving conditions will be possible
Sunday into early next week.


Cloudy and breezy conditions continue across the state this morning
due to strong high pressure to the north. Water vapor imagery
shows an upper trough positioned near the dateline with plenty of
middle to upper level clouds drawn up over the state from the
southwest along the trough`s eastern flank. At the low levels,
radar and visible satellite show broken to overcast clouds and
isolated showers continue to carry in with the trade winds,
focusing over the typical windward and mountain locations. A few
showers will pass over to leeward sides of the smaller islands at
times due to the breezy trades.

Model consensus supports breezy trades holding as strong high
pressure far north of the area shifts southeastward across the
northeast Pacific over the next several days. The aforementioned
upper trough to the west is forecast to slowly move east toward
the state through the weekend, passing over the islands on Sunday.
The upper pattern will continue to support mid and high clouds through
Saturday, then decreasing Sunday as the upper trough goes by. The
overall weather pattern will stay similar through Thursday.

Conditions will begin to evolve Friday into the upcoming weekend
as the upper trough draws closer, bringing deeper layered
moisture and more instability in the atmosphere. Despite the
model spread between various solutions that has been shown in each
cycle over the past few days, most indicate weak surface low
pressure developing along an approaching trough in the vicinity of
the islands Saturday with an abundance of deep moisture being
drawn northward (2 inches precipitable water values). A
combination of falling upper heights, instability, lift, and
moisture will lead to a wet weekend with a few thunderstorms,
especially late Friday through Saturday. Trade winds will also
weaken and shift out of the southeast as the low develops and
lifts away north of the state.

Sunday through early next week, improving conditions are expected
from west to east as drier air fills in and the upper trough and
surface low lift northeastward and away from the area. Guidance
depicts light west winds, potentially giving way to overnight land
breezes and afternoon sea breezes. Forecast confidence, however,
remains low through this extended period provided the spread
between solutions shown.


A surface ridge NE of the area will maintain low-level ENE trades
across the state today. Winds will increase through day as the
pressure gradient tightens slightly. Clouds and light showers will
persist over N and E sections of the islands, with tempo MVFR
ceilings and vis continuing through tonight.

AIRMET SIERRA for mountain obscuration for N and E sections of all
islands expected to remain in place through tonight.


Windward coastal buoys are all showing rapidly rising wave heights
as a large NE swell fills in across the state. The swell has
already peaked at Buoy 51100 about 250 statute miles to the
northeast of Maui. Thus, the peak of the swell should arrive
across the island chain today then slowly decrease over the next
couple of days. Expected surf heights in the range of 20 to 25
feet for north and east facing shores prompted the issuance of a
High Surf Warning yesterday which remains in effect through
Thursday afternoon. This surf event is especially of concern to
east-facing shores which do not typically receive waves at these
sizes. Strong surges are also forecast for harbors exposed the

No other significant swells are expected through the rest of the
forecast period. A small to medium sized northwest swell is
expected Sunday and into Monday with surf well below advisory
level. South-facing shores can expect to see a slight increase in
surf this weekend as a small southwesterly swell arrives.

Breezy trades will continue for another day. These winds combined
with the large NE swell will maintain Small Craft Advisory
conditions across all of the coastal waters through Thursday.


High Surf Warning until 6 PM HST Thursday for north and east
facing shores of Niihau, Kauai, Oahu, Molokai, Maui and the Big

Small Craft Advisory until 6 PM HST Thursday for all Hawaiian



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