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Hawaiian Islands Synoptic Discussion and Guidance

804
FXHW60 PHFO 250517 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
717 PM HST Wed May 24 2017

.SYNOPSIS...
Breezy trades will persist today, pushing clouds and showers
across windward areas. Winds will gradually weaken through the
second half of the week, becoming light and variable through the
Memorial Day weekend. Daytime sea breezes and nighttime land
breezes will take over this weekend and bring warm and humid
weather with possible vog. A weak upper trough will pass over the
state this weekend and may provide some enhancement to afternoon
showers.

&&

.DISCUSSION...

Breezy trade winds have been pushing an old frontal boundary over
windward Kauai through the day bringing showers and moderate
rainfall. Showers will continue overnight on Kauai and increase
over windward Oahu as those same showers affecting Kauai sag
south. The boundary will continue bringing showers to the western
end of the state into Thursday before finally dissipating tomorrow
night.

A strong high pressure system centered far NW of the islands is
providing the breezy trade winds over the state. Trades will
weaken tonight and tomorrow as a surface low forms to the NE,
diminishing the pressure gradient over the state. Winds will
remain light and variable through the weekend as an approaching
front from the NW turns the synoptic scale winds out of the SE.
The light SE flow will allow for afternoon sea breezes and
nighttime downslope winds to prevail over the islands from
Saturday into early next week.

Windward and mountain areas will see the majority of showers
through Friday, but sea breezes over the weekend will bring
passing afternoon showers to interior and leeward areas. A weak
upper level low will bring some instability to the atmosphere
Saturday and Sunday. The instability will enhance rainfall,
allowing for some isolated heavy showers to occur.

The light SE flow will bring volcanic haze, or vog, up over the
smaller islands by Sunday and linger into the beginning of next
week. The light winds will also bring hot and humid conditions for
the Memorial Day weekend.

&&

.AVIATION...
AIRMET Tango continues for mechanical turbulence to the lee of the
mountains due to the breezy trade winds. Expect the turbulence to
continue through tonight. Winds are expected to continue to
decline over the next couple of days.

Clouds and showers continue to impact Kauai and Oahu this
afternoon and have made little progress southward. Expect these
clouds and showers to continue to bring periods of MVFR
conditions. As such, AIRMET Sierra continues for mountain
obscuration. Remaining areas should remain VFR through the first
part of the night.

&&

.MARINE...
The latest surface analysis showed a large area of high pressure
encompassing much of the northern Pacific. The high pressure is
driving moderate to fresh trade winds over the state. A Small
Craft Advisory (SCA) is in effect for locally strong trades in
the typical windy zones around Maui County and the Big Island
through tonight, and should begin to taper off Thursday.

Surf along north facing shores will steadily trend down tonight
as the north, short period swell declines. For the long range,
models are hinting at another moderate north northwest (340 deg)
swell filling in early next week due to a low tracking east of the
date line over the weekend and deepening as it passes well north
of the state on Monday.

Surf along south facing shores is expected to maintain through
Thursday night. The largest south swell this season is forecast
to fill in Friday, peak Friday night through Saturday night, then
slowly ease into early next week. A high surf advisory for south
facing shores will be likely beginning Friday through Memorial Day
weekend, before steadily easing into early next week.

In addition to the expected advisory-level surf through the
weekend along south facing shores, coastal flooding is anticipated
due to a combination of record level spring (king) tides
(especially for Maui County and Oahu) and large surf. See the
Special Weather Statement on our website for more details.
Similar to the April 28th through May 1st weekend, when the
highest tides on record were reported for Honolulu, impacts will
include beach flooding and standing water on roadways and low-
lying areas near the coast around the times of high tides each day
through the weekend.

The following are the daily high tide times for Honolulu which
will correspond to the maximum coastal impact:

5/25 Thursday 4:21 PM HST 2.4 feet

5/26 Friday 5:07 PM HST 2.5 feet (south shore surf expected)

5/27 Saturday 5:56 PM HST 2.4 feet (south shore surf expected)

5/28 Sunday 6:47 PM HST 2.3 feet

5/29 Monday 7:40 PM HST 2.1 feet

For more detailed information on surf for Oahu, see the latest
Collaborative Nearshore Swell and Wind Forecast that was issued
this afternoon (www.prh.noaa.gov/hnl/pages/SRF.php#Discussion).


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

Morrison...Discussion
M Ballard...Aviation
Foster...Marine

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