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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 180645

Area Forecast Discussion
National Weather Service Honolulu HI
845 PM HST Wed Jul 17 2019

A broad area of high pressure centered far north of the islands
will continue to feed the islands with a typical summertime trade
wind pattern through early next week. Showers riding in with the
trades will favor the windward and mountain areas, with the
highest frequency of these showers occurring during the nights and
mornings. Most lee areas of the smaller islands will remain dry,
while the lee side of the Big Island will have its usual clouds
and a few showers every afternoon and evening.


A 1032 mb surface high is centered about 1100 miles north of the
Big Island this evening, providing the islands with a nice
tradewind breeze. The high will meander around that area well into
next week. But the high will cycle through some weakening and
strengthening periods during this stretch. The high will weaken to
1029 mb about Friday, causing the trades to ease off slightly. The
high will then get reinforced around Monday night to 1031 mb,
resulting in a slight boost in the trades.

There is an upper level low at 500 mb over the central islands
this evening. It hasn't really done much, though. Low level
moisture appears limited, mainly scattered to locally broken
immediately upwind of the islands, as per latest satellite. The
locally broken applies to Maui and Molokai, The low inversion is
not all that strong; the bulk of the instability appears above 10k
feet. About 240 miles east of the Big Island is the leading edge
to a large field of low clouds consisting of a mix of cumulus and
stratocumulus clouds and possibly associated with a weak low level
trough. Roughly 200 miles further east is the beginning of a
large field of stable stratus and stratocumulus clouds, stretching
to Baja California. This area of enhance low clouds is picked up
well by both GFS and ECMWF models, which has it reaching the Big
Island early Thursday afternoon. The area then falls apart or is
blended in with the other trade showers Thursday night. We could
be a few hours too early with the arrival of this cloud field,
with the clouds and showers reaching windward Big Island at day
break and Oahu late tomorrow morning. So, we will be watching for
this progress of this field as the night progresses.

The upper low over the central islands will move west and stall
just west of Kauai Thursday night. The upper low then deepens some
400 miles north-northwest of Kauai Friday. A trough forms and
become oriented north to south over the weekend, leaving the
islands under a southerly wind flow aloft. This setup will probably
draw some high clouds over the islands from the south. Further
more, the GFS is showing the marine layer deepening from 6 to 7k
feet to 10-12k feet on Saturday afternoon over the smaller islands.
We may not see an increase in trade showers, but the showers will
be a little bit more robust or heavier with this profile. The BIg
Island could be just outside the range of this upper low, meaning
there is the threat of thunderstorms over the interior, higher
slopes, is nill at this time.


Relatively strong high pressure still sits well to the north of
Hawaii. This will continue the recent trend of moderate to breezy
northeasterly trade winds across the island chain. AIRMET Tango
remains in place for moderate turbulence below 7,000ft and this
AIRMET will most likely continue into Thursday. Expect lower
clouds and showers on the windward slopes therefore passing
periods of MVFR conditions are possible. Otherwise, VFR for the
rest of the state.


Breezy to strong trade winds will persist through the end of the
work week with high pressure north of the state. There remains
good model agreement with the high starting to weaken some this
weekend, which will ease the winds slightly.

The Small Craft Advisory (SCA) continues for all waters in
response to the trade winds, and it may be trimmed back this
weekend as the winds weaken.

Expect choppy surf along the east facing shores with the breezy to
strong trade winds through the end of the work week. Will continue
to monitor the surf heights, but at this time expecting surf to
remain below advisory levels. East facing shores will see surf
slowly lower over the weekend as trades ease.

Some small background south and southwest swells are expected
through the weekend. An increase in surf along south facing shores
is possible early next week as a couple southwest swells arrive
in the islands.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.


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

Small Craft Advisory until 6 PM HST Friday for all Hawaiian



H Lau/Chevalier/M Ballard

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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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