Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 251409
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
409 AM HST Thu May 25 2017

.SYNOPSIS...
Low clouds and showers associated with a weakening frontal
boundary will continue to move across the state today, especially
over the western islands. Trade winds will gradually weaken
through Friday, with light and variable winds expected during the
Memorial Day Holiday weekend. This will produce muggy conditions,
and allow the development of local afternoon sea breezes 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, especially over leeward and interior
sections. In addition, volcanic haze may spread over the smaller
islands starting Sunday.

&&

.DISCUSSION...
The pressure gradient south of a 1039 mb surface high located
near 44N 177W, or almost 1900 miles northwest of Honolulu.
However, a surface low located almost 1100 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
today.

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 morning soundings from Hilo and Lihue. Satellite imagery and
radar reflectivity data show broken to overcast low clouds and
scattered showers associated with an old frontal boundary are
being transported by the low-level trade wind flow over Kauai,
Oahu and Molokai. Additional scattered to locally broken low
clouds and isolated showers are also moving toward the eastern
half of the island chain.

The forecast guidance 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 today,
before eventually dissipating later tonight. Windward facing
sections of the state will continue to have some showers through
Friday.

The background synoptic flow is expected to shift out of the
southeast this weekend. Since the Big Island typically blocks most
of the low-level winds from this direction across the smaller
islands, expect the local winds to become light and variable. This
light wind regime will allow local afternoon sea breeze and
nighttime land breeze circulations to develop over the leeward and
interior sections of the individual islands from Saturday into
early next week. Weak middle to upper tropospheric troughs are
also expected to move over the islands this weekend. In addition,
a low aloft is forecast to form over the state. This feature is
expected to produce some atmospheric instability over the region
this weekend. The instability may enhance rainfall during daytime
heating, which could allow for brief localized heavy downpours to
occur Saturday and Sunday afternoons, especially over leeward and
interior sections of the island chain.

Note that the light winds will also bring muggy conditions during
most of the Memorial Day Holiday weekend. In addition, the weak
southeasterly background flow may transport 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 moving over Kauai...Oahu and Molokai this
morning will continue to drift southeastward across the islands
over the next 24 hours. Expect scattered to numerous showers and
periods of MVFR conditions along this convergence band. Expect
scattered MVFR ceilings and visibility along the convergence
band.

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

AIRMET Tango continues for mechanical turbulence for Kauai over
mountains and south through east sections due to the breezy trade
winds. Wind speeds will continue to decrease this morning likely
ending the turbulence AIRMET.

&&

.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 trade winds across the local waters. An ASCAT
pass overnight reflected moderate to fresh trades over the area.
As a result, the small craft advisory will be dropped in the
morning package. Trades are forecast to weaken further into 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 weekend, which will allow the seas to trend down locally.

Surf along east facing shores will trend down into 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 later
today through Friday, peak Friday night through Saturday 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 this afternoon, which will continue 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 have 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 that
will correspond to coastal flooding impacts each day through the
weekend:

5/25 Today      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...
None.

&&

$$

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



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