Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 220632
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Wed Jun 21 2017

.SYNOPSIS...
High pressure passing far north of the islands will maintain gentle
to locally breezy trade winds the next couple of days. The trade
winds will deliver passing clouds and showers, mainly to windward
areas. A weak mid-level trough approaching the islands this weekend
and early next week may weaken the trades and increase showers.

&&

.DISCUSSION...
The afternoon soundings were relatively dry and stable, with
precipitable water of only 0.77 inch at Lihue and 1.16 at Hilo,
along with a stout equilibrium level near 7-8 kft. A surface
ridge about 300 mi N of the islands is keeping trade winds
blowing at locally breezy levels. Despite this, and to the chagrin
of forecasters due to their ephemeral nature, showery clusters
embedded in the trade wind flow are able to impact sensible weather.
Tonight is no exception as a random cluster of stable and showery
low clouds crossing parts of Maui county and Oahu has led to a
somewhat cloudy and damp evening, with some light rain even
spreading leeward on Oahu. Additionally, a well-developed eddy over
near-shore waters to the lee of the Big Island has been fueling
upslope flow that has supported shower development in the Kona
district, although this type of weather is well anticipated,
especially this time of year. The overnight forecast was updated a
short while ago to attempt to account for latest satellite and radar
trends.

Otherwise the ongoing forecast philosophy is unchanged. Trade winds
will remain in the gentle to locally breezy range through Sat as the
ridge to the N weakens and is overtaken by a high pressure area
which will pass about 1500 mi N of the islands Fri and Sat. Fairly
typical trade wind weather is expected through this period.

On Sun and Mon, the GFS shows subtle troughing in the surface
pressure and wind fields near the islands, in response to a
weakening mid-level low or trough passing E to W across the state.
According to the GFS, a weak surface trough would push W of the
islands Mon and veer the low level flow enough to bring the surface
winds from the ESE or even SE for a time. The 12Z ECMWF continues to
be less amplified with this feature, and the 00Z guidance has not
yet been received. The combination of the incoming mid-level system
and a possibility for veered low level flow will bring increased
shower chances this weekend. The best chance for additional showers
will be mostly windward and mauka, though possibly some sea breeze
or upslope showers as well if the winds weaken enough.

By Tue, the subtropical ridge will reestablish itself to the N
and NW. Models agree that the low level flow will return to breezy
ENE trades through midweek, along with drier and more stable air.

&&

.MARINE...
A long period SSW swell will continue to decrease through Thursday.
This swell will be reinforced by another above average south swell,
arriving Thursday afternoon, and peaking late Thursday night or
Friday. Surf observations from earlier in the day indicated a few
areas reaching high surf advisory levels, so will keep the ongoing
High Surf Advisory in place, as is, through Thursday. This will be
re-evaluated, though, as the surf may drop below advisory levels
late tonight and Thursday before building again as the next south
swell arrives.

In addition to the new southerly swell, tides have been running
about a half a foot above predicted levels. These elevated tide
levels will increase the risk for minor coastal flooding when
combined with the high surf. For the next few afternoons the
predicted high tides are around 2.5 feet, which does not account for
the extra 1/2 foot mentioned above.

Other wave trains are affecting Hawaii as well, with a small out-of-
season northwest swell diminishing into Thursday, a small trade wind-
generated swell and a short period southeast swell. The southeast
swell will continue into the weekend and will add a small component
to the wave heights along south facing shores.

In the longer range, models indicate a northeast swell developing
for the beginning of next week. This swell will need to be monitored
as it interacts with the trade wind swell, and a high surf advisory
may eventually be needed for east facing shores if the swell comes
in bigger than anticipated.

Moderate to fresh trade winds will continue through the weekend,
with a Small Craft Advisory soon to be issued through Thursday for
the windy zones around Maui and the Big Island, partially based on
an ASCAT pass from earlier today that indicated winds to 25 kt in
the Alenuihaha Channel.

For more details on the surf, please refer to the Oahu Collaborative
Surf Forecast (SRDHFO) prepared by Pat Caldwell under WMO header
FXHW52 PHFO.

&&

.AVIATION...
High pressure far NE of the state will maintain light to moderate
trade winds over the islands through the period. Clouds and showers
will tend to favor the windward and mountain areas, with some
showers introducing brief periods of MVFR. Otherwise, expect VFR
conditions. No AIRMETs are expected.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory for South facing shores of all islands until
6 PM HST Thursday.

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

&&

$$

Discussion...Birchard/R Ballard
Marine...Kinel
Aviation...Bedal/Eaton



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