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RH17 %
WindNNW 1 mph
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Hawaiian Islands Synoptic Discussion and Guidance

775
FXHW60 PHFO 262009
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1010 AM HST FRI AUG 26 2016

.SYNOPSIS...
Locally breezy trade winds will continue through early next week as
high pressure persists north of the islands. Passing low clouds and
brief showers will favor windward and mauka areas during nights and
mornings, but an increase in trade wind showers is expected from
Saturday night into Monday. Another increase in moisture, and
potentially an increase in winds, is possible starting Tuesday night
and Wednesday, depending on the movement of a developing tropical
disturbance now about 1350 miles southeast of the Big Island.

&&

.DISCUSSION...
A surface high centered near 47N 147W, or about 1900 miles NNE of
the islands, is supplying moderate to locally breezy ENE trade winds
to the islands. The trades are carrying randomly distributed low
clouds and showers in from the ENE, thereby stacking the bulk of
the moisture along the windward slopes and coasts of the islands. A
subsidence inversion seen in 12Z soundings from PHLI and PHTO is
keeping the bulk of the incoming clouds capped below 10 thousand
feet, which is sufficiently high to allow a few showers to develop,
but low enough to limit their intensity. A weakness in the low-level
pressure gradient on the SE side of the high (about 1300 miles NE of
Hawaii) is the result of a well-developed low aloft and it's weak
surface reflection. Meanwhile, a tropical disturbance located about
1400 miles ESE of the Big Island is becoming increasingly organized,
and a tropical cyclone may be in the process of developing. About
2600 miles E of Hawaii, Tropical Storm Lester has already developed,
and is tracking W.

While the center of the high to the NNE will meander around through
the forecast period, it will generally prevail to the N of the
islands, maintaining a trade wind flow through the forecast period.
This is generally expected to provide the islands with brief passing
windward showers, but there some caveats with the standard trade
wind forecast this time around. Initially, latest runs of the GFS
and ECMWF indicate that an area of moisture about 500 miles NE of
the islands will become enhanced just upstream of the islands, and
bring increased showers from Saturday night into Monday. Will be
increasing forecast PoPs for this time frame, especially for
windward areas, to be reflected in the next forecast package. After
this moisture clears the chain to the W, the next batch of moisture
in the trade flow is possible Tuesday night and Wednesday. This is
associated with the developing tropical disturbance to the SE of the
islands, but this is where the forecast becomes increasingly
uncertain due to significant model differences in the forward speed
of the disturbance. Additional wet weather may arrive as we approach
the labor day holiday weekend.

&&

.AVIATION...
VFR will prevail except for an area of low clouds moving toward the
SW over Kauai. AIRMET SIERRA is in effect for MTN OBSC on Kauai
through 22Z. Low-level NE winds will carry enough clouds and showers
over the other islands to produce isolated MVFR conditions over
windward areas today. There could be widespread-enough MVFR to
require additional AIRMETs for MTN OBSC at times over the next 24
hours.

&&

.MARINE...
A Small Craft Advisory (SCA) is in effect through early Saturday for
the coastal waters around Maui county and the Big Island, due to
locally breezy trade winds accelerating around the high terrain.
Winds may briefly drop just below SCA speeds Saturday and Sunday,
only to strengthen to SCA speeds again early next week.

No significant swells are expected through the middle of next week,
and biggest surf will be along E facing shores. However, a limited
fetch of trade winds upstream of the islands will preclude surf
heights from growing significantly.

Winds and seas, as well as surf along E facing shores, may increase
toward the middle and end of next week, when forecast uncertainty
increases dramatically due to the potential for tropical systems to
be traversing the central North Pacific.

&&

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

&&

$$

Discussion/Marine...Birchard
Aviation...Donaldson

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