Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 250652
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
852 PM HST Wed May 24 2017

.SYNOPSIS...
Trade winds will gradually weaken through Friday, with light and
variable winds expected during the Memorial Day weekend. This
will produce muggy conditions, and allow the development of local
afternoon sea breeze and nighttime land breezes this weekend. A
weak upper level trough passing over the state this weekend may
provide some enhancement to the afternoon showers over leeward and
interior sections. In addition, volcanic haze may spread over
some of the smaller islands starting Sunday.

&&

.DISCUSSION...
The pressure gradient south of a 1036 mb surface high located near
39N 166W, or less than 1300 miles north-northwest of Honolulu, is
maintaining a relatively tight pressure gradient across the
Hawaiian Islands after sunset this evening. As a result, locally
breezy trade winds continue across parts of the state. Aloft, a
weak middle tropospheric trough is above the state. This
continues to maintain slight instability across the islands, with
no significant low-level trade wind inversion evident in the
early afternoon soundings from Hilo and Lihue.

Satellite imagery and radar reflectivity data show broken low
clouds and scattered showers associated with an old frontal
boundary are being transported by the low-level trade wind flow
over the windward sides of Kauai and Oahu. Additional scattered
to broken low clouds and isolated showers are also moving toward
the central and eastern parts of the island chain. Low clouds,
which developed earlier today, are beginning to dissipate over the
leeward Big Island.

A surface low, which formed about 1060 miles north-northeast of
the islands, is beginning to diminish the pressure gradient over
the state. As a result, expect the trades to gradually weaken
later tonight and Thursday. The guidance also indicates the old
frontal boundary will continue to be a source of moisture in the
form of low clouds and showers moving mainly into the western end
of the state on Thursday before eventually dissipating Thursday
night. Windward facing sections of the state will continue to
have some showers through Friday.

The surface winds will become light and variable by this weekend
as a front approaching from the northwest turns the background
flow out of the southeast. The light southeast flow will allow
local afternoon sea breeze and nighttime land breeze circulations
to develop over the leeward and interior sections of the islands
from Saturday into early next week. Weak middle to upper
tropospheric troughs are expected to move over the islands this
weekend. As a result, a stacked low aloft is forecast to form near
the southern end of the troughs. This is expected to produce some
atmospheric instability over the state this weekend. The
instability may enhance rainfall during daytime heating, which
could allow for brief localized heavy downpours to occur Saturday
and Sunday afternoons.

Note that the light winds will also bring muggy conditions over
the Memorial Day Holiday weekend. In addition, the weak
southeasterly background flow may bring volcanic haze, or vog,
to the smaller islands from sources on the Big Island by Sunday.
Once this haze arrives, it is expected to linger into early next
week.

&&

.AVIATION...
High pressure north of Hawaii will keep the islands in a
northeasterly trade wind regime. A remnant cold frontal
convergence boundary over Kauai and Oahu will continue to drift
southeastward across the islands over the next 24 hours producing
scattered showers and periods of MVFR conditions. Clouds and
showers along this band will move into Maui county later this
evening. Expect mostly VFR conditions with scattered MVFR
ceilings and visibility along the convergence band.

AIRMET Sierra remains in effect this evening for mountain
obscuration over Oahu and Kauai with low cloud ceilings.

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. Wind speeds will decrease over the
next 24 hours.

&&

.MARINE...
The latest surface analysis showed a large area of high pressure
(1039 mb) encompassing much of the northern Pacific that continues
to support fresh to strong trade winds across the local waters. An
ASCAT pass earlier today reflected slightly higher winds than
expected across the northwest Kauai waters (25 kt) near the remnants
of an old frontal boundary lingering near Kauai. These strong
trades, however, are forecast to weaken through the second half of
the week and over the upcoming holiday weekend as a weakness within
the ridge develops in response to low pressure that has developed
well northeast of the state. Land/sea breeze conditions will become
a possibility over the upcoming weekend, which will allow the seas
to trend down locally. The current forecast remains on track this
evening with no significant changes expected in the evening update.
The only change possible in the later forecast package overnight
would be to extended the small craft advisory across the typical
windier marine areas through Thursday, if observations support it
overnight.

Surf along east facing shores will begin to trend down through the
second half of the week and over the weekend as the trades relax. A
small north-northeast swell, however, will support small surf
continuing at the exposed spots each day into the weekend.

Surf along north facing shores will steadily trend down into the
weekend with mainly surf continuing at spots exposed to north-
northeast energy. For the long range, models are hinting at another
small to moderate north-northwest (330 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.

The largest south swell this season is forecast to fill in late
Thursday into Friday, peak Friday night through Saturday well above
advisory levels, then slowly ease into early next week. This swell
peaked at the Samoa buoy early Tuesday, which should correspond to a
peak locally along south facing shores Friday night into Saturday. A
high surf advisory for south facing shores will likely be issued
Thursday afternoon, which will continue Friday through the Memorial
Day weekend.

In addition to the expected advisory-level surf through the Memorial
Day weekend along south facing shores, impacts from coastal flooding
that has already begun each afternoon with the high tide cycles will
likely become enhanced Friday and Saturday evenings due to a
combination of record level spring (king) tides (especially for Maui
County and Oahu) and large surf expected. These waters levels could
break the late April record that occurred last month at Honolulu on
Friday and Saturday. Flooded beaches, vulnerable low-lying roads,
boat ramps and docks will be impacted during these hours surrounding
the high tides each day. Water levels are forecast to trend back
down toward normal through the upcoming week.

The following are the daily high tide times for Honolulu which will
correspond to the coastal flooding impacts each day through the
weekend:

5/25 Thursday   4:20 PM HST

5/26 Friday     5:07 PM HST (peak water levels/surf expected)

5/27 Saturday   5:55 PM HST (peak water levels/surf expected)

5/28 Sunday     6:46 PM HST

5/29 Monday     7:40 PM HST

For more detailed information on surf for Oahu, see the latest
Collaborative Nearshore Swell and Wind Forecast that was issued
Wednesday 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.

&&

$$

Discussion...Houston
Aviation...Bohlin
Marine...Gibbs



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