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RH16 %
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Hawaiian Islands Synoptic Discussion and Guidance

732
FXHW60 PHFO 250207
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
407 PM 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 00z soundings show the capping inversion over the state has
risen to about 9000 feet, with about 1.2 inches of precipitable
water at both sites. However, 700 mb temperatures are still in the
double digit range, warmer than originally forecast by the models,
indicating that weak mid-level features are not having as much of
an effect at destabilizing the airmass as originally expected.
Nevertheless, a few showers have popped up over leeward areas this
afternoon.

Sunday looks similar to today. Leeward clearing in most areas
tonight should be followed by some afternoon cloudiness and
limited hit-or-miss shower activity as sea breezes and diurnally
driven upslope flows take over. Windward areas will likely see
more sunshine than leeward areas in the afternoon, but with the
background flow still gentle trades, we can't rule out nighttime
or morning showers in this hybrid pattern.

Our weak trades right now are being driven by a faraway high
pressure area about 1600 mi to the NNE of Hawaii. A surface ridge
extending SW from this high will be pressing SE toward the islands
in the coming days, tightening the pressure gradient. This will
lead to a return to locally breezy trades for the eastern islands
starting as early as Monday, with breezy to locally windy trades
statewide by Tuesday. 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
incoming trade wind showers by next weekend.

&&

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

Beginning tonight, tides and surf along south facing shores will
lower Sunday into the upcoming week. However, a background
southeast swell will continue into mid-week, keeping south shore
surf bumped up slightly.

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. Regardless, rough surf
seems likely 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 tonight.
However, we expect development of afternoon sea breezes and cloud
build ups along western and mountain sections of the islands on
Sunday. 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