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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 111332

Area Forecast Discussion
National Weather Service Honolulu HI
332 AM HST Sat Jul 11 2020

Trade winds will continue through next week as a ridge of high
pressure stays far north of the main Hawaiian Islands. The current
windy trades will be trending softer as we head into a new week,
with speeds reaching light to moderate by Thursday. Showers riding
in with the trades will favor the windward and mountain areas
with a few drifting into the lee side of the smaller islands.


A broad 1027 mb surface high pressure system is located 1040
miles north- northeast of Kauai this morning, moving slowly east.
A ridge extends southwestward from the high to just north of
Midway Atoll, located 1340 miles west-northwest of Kauai. By the
middle of next week, the ridge is expected to drift northward from
Midway and get stretched further as the parent high becomes
stationary off the west coast. This mean trades will be with us
through most if not all of next week.

The trades are currently blowing moderate to strong island-wide,
possibly enhanced by the presence of a westward moving weak
disturbance located far south of the islands. By tonight, this
disturbance will be further away from the islands, resulting in
the start of a softening process for the trade winds. By Monday or
Monday night, trades will blowing in mainly the moderate range.
Light to moderate trades should be in place by Thursday, lasting
into Saturday.

The frequency of these trade showers may vary from day to day,
depending on the low cloud coverage upwind of the islands.
Commonly though, the frequency will increase overnight, then
diminish by noon. The windward and mountain areas will take in
most of these trade showers, leaving the lee areas of the smaller
island dry. Except for the lee side of the Big Island, where the
wind sheltered area promote a day time sea breeze that will lead
to afternoon/evening clouds and showers.

An upper level low is forecast to drop southwestward to point 500
miles north of the islands by Sunday night. The low then drifts
westward through the middle of next week to a point 420 miles
northwest of Kauai. The low is currently half way between the
islands and the west coast. At this range, it will be closely
enough to enhance the trade showers between Sunday night and
Wednesday. Models also show two pools of low level moisture with
the first coming in Sunday night and a higher dosage on Tuesday.
With this said, this boost in trade showers will be enhanced by
the upper level low. This is a few days out, and this may change.
So, more to come.

Current satellite imagery showers mainly stable stratocumulus
clouds passing through the islands from east to west. Radar is
picking up a few showers embedded within these clouds, and is
drier compared to a day or two ago. So, we expect a relatively
dry morning to say two days ago.


High pressure north of the state will keep moderate to locally
breezy trades in place through the weekend. Some MVFR cigs/vsbys
may periodically affect windward areas, with VFR conditions
expected in leeward locales.

AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of the terrain. This AIRMET is expected to
continue through the weekend.

No other AIRMETs are in effect and none are expected through the
daylight hours Saturday.


A breezy trade wind pattern will likely hold into Monday before
trending down slightly. A Small Craft Advisory (SCA) remains
posted for the typically windier waters around the Big Island and
Maui County, as well as for windward Maui waters and the Kaiwi
Channel. The windward Maui waters and Kaiwi channel SCA goes
through this afternoon and the SCA for the waters around Maui
County and the Big Island through Sunday. This may need to be
extended into Monday.

No significant swells are due well into next week. A mix of
background south and southeast swells will continue to produce
small surf along south facing shores through next week. Trade wind
swells will hold into early next week, producing choppy surf
along east facing shores. Additionally, TC Cristina in the far
East Pacific could produce a small moderate-period east swell the
middle of next week. Surf along north facing shores will remain
nearly flat through next week.


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

Small Craft Advisory until 6 PM HST this evening for Kaiwi
Channel-Maui County Windward Waters.



H Lau/Jelsema/Powell

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