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Hawaiian Islands Synoptic Discussion and Guidance

229
FXHW60 PHFO 261953
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
953 AM HST Wed Apr 26 2017

.SYNOPSIS...
Moderate easterly trade winds will persist today and tonight
as a surface ridge holds north of the state. The atmosphere will
remain stable, with pockets of moisture affecting mainly windward
slopes. Trades will ease and shift out of the southeast on
Thursday as a front approaches the state, leading to spotty
afternoon showers. On Friday, the front will bring showers to
Kauai and possibly Oahu, while prefrontal showers, some heavy,
develop across the rest of the island chain. Wet and unstable
conditions are expected over portions of the state through the
weekend as a late season low drops south near the state.

&&

.DISCUSSION...
A ridge of high pressure north of the state is providing moderate
easterly trade winds to the islands with passing windward showers.
Overnight soundings from Hilo and Lihue show a stable atmosphere
above the boundary layer with a temperature inversion capping
cloud tops at 8 to 10 thousand feet. The CIMSS MIMIC satellite
product shows higher moisture levels near 1.4 inches of precipitable
water around the smaller islands with drier conditions near the
Big Island with PW's near 1.1 inches.

Trade winds will continue today before diminishing tonight as a
late season cold front, now 1000 miles to the NW, approaches the
state. The front will weaken winds and turn the general flow out
of the southeast Thursday with widespread leeward sea breezes
expected in the afternoon. Light/moderate trade winds will hold
over windward areas from Maui to the Big Island into Thursday
night. The upper level trough driving the front will erode the mid
level ridge over the islands, slowly destabilizing the atmosphere
and causing the inversion to lift and weaken. Expect an increase
in shower activity, including over leeward areas during the
afternoon hours as sea breezes develop.

The front is expected to reach Kauai sometime Friday with
increasing clouds and showers accompanying the front. Models are
depicting a shallow front with the majority of rainfall on Kauai
falling over the northern slopes and coast into Friday night. The
atmosphere will become more unstable as the upper trough brings
colder temperatures over the state. Prefrontal convergence and the
unstable atmosphere will bring an increase in shower activity
over the rest of the state with some pockets of heavier showers
expected.

The shallow front with light north winds is expected to reach
Oahu Saturday morning and then stall in the vicinity of Oahu and
Maui Saturday afternoon. Meanwhile, the core of the upper trough
will be moving toward the islands from the north bringing
increasing instability over the area. This will lead to the
potential for heavy, flooding rainfall over portions of the state
this weekend, and possibly continuing into next week. Models are
showing the upper low over the islands Sunday but the exact
location is illusive and remains difficult for the models to
resolve. The models do agree on a convergence band forming along
the old frontal boundary located someone near Maui County Sunday,
and becoming the focus for the heavy rain. The upper low will
bring very cold temperatures aloft and thunderstorms will be
likely under the core of the low.

&&

.AVIATION...
A moderate to locally breezy trade wind regime will continue
today. AIRMET TANGO for low level turbulence over and immediately
over and south through west of the mountains remains posted today.
Winds will begin to ease starting tonight as a front approaches
from the northwest, with localized sea breezes becoming likely
Thursday.

Clouds and showers will favor windward and mountain areas. Some
showers will introduce passing periods of MVFR conditions.
However, widespread MVFR is not expected today. Most terminals
should see prevailing VFR conditions.

&&

.MARINE...
The high pressure ridge maintaining the moderate to locally breezy
trade winds is about 400 nm north of the state. An overnight ASCAT
pass confirmed the strongest winds remain over the typically
windier locations near the Big Island. The high resolution models
show an increase in winds through the Pailolo Channel, so it has
been added to the Small Craft Advisory, which is in effect
through this afternoon.

Winds will begin to weaken tonight into Thursday as a late-season
cold front approaches from the northwest and erodes the ridge.
The front is forecast to stall over the mid-section of the state
Friday night and Saturday, with breezy north winds causing SCA-
level seas across the western coastal waters.

The current northwest swell will peak this morning, before slowly
fading through Thursday. This swell will be followed by a moderate
north-northwest swell arriving late Friday and into Saturday.
Small surf is expected along south facing shores through mid-week,
with a new, long-period south swell forecast to arrive from the
south- southwest Thursday. This swell could raise surf along south
facing shores to advisory levels, and will persist into the weekend.
Surf along east facing shores will begin to drop off Thursday and
Friday as the ridge to the north is eroded and winds diminish.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Pailolo
Channel-Alenuihaha Channel-Big Island Windward Waters-Big Island
Leeward Waters-Big Island Southeast Waters.


&&

$$

Morrison...Discussion
Eaton...Aviation
TS...Marine

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



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