Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 271957

Area Forecast Discussion
National Weather Service Honolulu HI
957 AM HST WED JUL 27 2016

High pressure building north of the state will keep breezy trade
winds in place through the week. Relatively dry and stable trade
wind weather will prevail through Friday, with a more showery
trade wind pattern expected this weekend into early next week.


Water vapor imagery shows an upper level ridge just south of the
Big Island, with southwesterly upper level flow across the
state. At the surface, high pressure is located far northeast of
the state, with weakening troughs to the northwest. 12z soundings
from Lihue and Hilo show inversions less than 4kft and 6kft
respectively. Precipitable waters were just under 1.5 inches,
which is somewhat above normal for this time of year. Early
morning MIMIC total precipitable water imagery shows similar
moisture values extending 300 miles upstream to the northeast of
the state, before dropping off steadily toward the subtropical

Conditions are stable across the islands, which is reflected in
both the cloud field over and upstream of us and in the overnight
rainfall amounts. Only a couple gages saw any measurable rainfall,
and those that did just received a hundredth or two. This stable
pattern with lower than normal inversion heights will continue at
least through Thursday night. Models show precipitable water
dropping below normal, but this may be attributed to mid-level
drying above the inversion; forecast soundings/cross sections show
low-levels remaining nearly saturated. The current forecast has
lower than normal rainfall chances, especially during the
afternoon, which looks good given the shallow cloud layer.
Otherwise our currently breezy trade winds will pick up a bit as
the high shifts westward and strengthens. These breezy winds will
continue into Friday before the next shift in the weather pattern.

The 12z GFS/ECMWF continue to be in good agreement with respect to
the subtropical high, which remains entrenched far north-
northeast of the state into next week. The remnant low that was
Georgette will dissipate well east of the area, though moisture it
leaves behind will get picked up in the trades. By later
Friday/Friday night, we see an increase in moisture depth
spreading from east to west across the state. MIMIC precipitable
water imagery shows higher TPW values extending a few hundred
miles west of former Georgette (in addition to the clouds in this
area having a more showery/unstable appearance in visible
imagery), which accounts for the initial increase in moisture
depth by Friday night. This surge in moisture combined with the
locally breezy trade winds will lead to a wet trade pattern, with
most rainfall across windward sections but more showers affecting
leeward sides of the smaller islands as well.


High pressure far NE of the state will support moderate to breezy
NE to E trade winds. AIRMET TANGO for low level turbulence over
and immediately S through W of the mountains is posted for all
islands. The 12Z soundings from PHTO and PHLI show a strong low
level inversion sloping from 6 kft at PHTO to 3.5 kft at PHLI.
Morning AMDAR reports in and out of PHNL suggest the inversion
over Oahu is between 3 and 3.6 kft, with winds near the inversion
level reaching upwards of 30 kt.

Conditions will remain fairly dry and stable, any passing showers
will be brief and light. An overcast stratocumulus deck extending
from the California coasts is pushing towards the state. These low
stable clouds could introduce periods of MVFR cigs, mainly along
the windward coasts, as they pass. Otherwise, expect VFR


High pressure far northeast of the state will strengthen and shift
westward, keeping trade winds breezy across the area. A small
craft advisory is currently in effect for a somewhat larger than
typical area, a sign of the stronger gradient across the islands.
The advisory may need to be expanded to additional areas tonight
as the high strengthens, though we will wait to see how the
current forecast verifies against the morning ASCAT pass to make
that determination.

Breezy trade winds will result in choppy, short-period surf along
east facing shores, but surf heights are expected to remain below
advisory levels. There will also be a series of small southerly
swells through the weekend and into next week.


Small Craft Advisory until 6 PM HST Friday for Oahu Leeward
Waters-Kaiwi Channel-Maui County Windward Waters-Maui County
Leeward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-
Big Island Windward Waters-Big Island Southeast Waters.



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