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Temp10.1 C
RH31 %
WindNNE 11 mph
RoadOpen
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Hawaiian Islands Synoptic Discussion and Guidance

846
FXHW60 PHFO 242011
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1011 AM HST Sat Jun 24 2017

.SYNOPSIS...
A mid level trough will pass over the islands this weekend,
weakening the trade winds and increasing showers across the area.
The mid-level trough will shift west of the State by early next
week, with high pressure then building back northeast of the
islands. This will result in a drier and more stable trade wind
regime, with the trades increasing into the breezy to locally
windy range through much of next week.

&&

.DISCUSSION...
The overnight scatterometer passes, surface observations, and
nearby buoys all confirm that the background trade wind flow has
dropped off to gentle speeds near the islands. The weak trades are
being driven by a faraway high pressure area passing by about 1500
mi N of the islands. The background flow is too weak to overcome
local diurnal island circulations in many areas, so land breezes
last night cleared a lot of areas out. There are some exceptions,
however, such as the leeward slopes of the Big Island where an
area of enhanced low level moisture that moved across that island
yesterday is now pushing westward across the leeward coastal
waters.

It is difficult to discern in satellite imagery, but models are
showing a 700 mb trough drifting slowly over the islands from ESE
to WNW this weekend. At the same time, a weak digging 500 mb
trough is approaching from the NW. The combination of these two
features will allow for subtle destabilization over the islands
(although not as much as earlier model runs were showing). The
resulting weaker and higher inversion, and the combination of
lighter background flow, should lead to considerable afternoon
cloudiness and pockets of showers in leeward areas. These will
fade in the evening and many interior and leeward spots should
clear out overnight. A similar cycle should repeat itself Sun and
Mon. Made some tweaks to raise afternoon pops for some spots this
afternoon, but overall the grids look ok.

Later Mon into Tue, our weather will start to come under
increasing influence from a subtropical ridge that will be
pressing toward the islands from the N and NW. The pressure
gradient over the islands will tighten and the flow will start to
become more trade-like later Mon, with breezy to locally windy
trades for the rest of the week and into next weekend. Drier and
more stable air is expected to dominate our weather picture Tue-
Thu. Models are in fairly good agreement showing a stronger mid-
level feature may develop to the NE of the islands toward the end
of the week, possibly enhancing trade wind showers next weekend.

&&

.MARINE...
Gentle to locally moderate trade winds associated with high
pressure far north of the state will continue through the weekend.
Moderate to locally fresh trades are expected to return Monday,
becoming fresh to locally strong for Tuesday and beyond as the
pressure gradient tightens over the waters. Small craft conditions
are likely through this period over the typically windier channel
waters, Maalaea Bay and south of the Big Island.

Coastal flooding associated with king tides is a possibility once
again today, especially along south facing shores, where surf
will remain above normal. The greatest potential for coastal
flooding impacts will be during the mid to late afternoon hours,
when highest tides are expected. Visit tidesandcurrents.noaa.gov
for more specific information about forecast tides and water
levels in your area. Tides and surf along south facing shores will
lower Sunday into the upcoming week.

Surf along east facing shores will build early next week due to a
combination of a moderate northeast swell associated with a batch
of gales off the west coast and increasing trade winds locally and
east of the state. There still remains some uncertainty of the
size of this swell and whether or not a high surf advisory will be
needed for east facing shores next week.

For more details on the surf, please refer to the Oahu Collaborative
Surf Forecast (SRDHFO).

&&

.AVIATION...
East to east southeast trade winds will continue today. However,
we expect development of afternoon sea breezes and cloud build
ups along western and mountain sections of the islands. Tempo
showers and ceilings with isolated MVFR are possible and AIRMET
Sierra may be needed later for mountain obscuration.

No other AIRMETs are being considered or expected.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$
R Ballard/Stall/Powell

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