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Hawaiian Islands Synoptic Discussion and Guidance

582
FXHW60 PHFO 101347
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
347 AM HST Fri Jul 10 2020

.SYNOPSIS...
A strong surface high centered far north of the main Hawaiian
Islands will keep breezy trade winds in the forecast into the
first half of next week. An upper level low lingering north of the
state and a few low level troughs will keep periods of showers in
the forecast mainly focused on Windward and Mountain areas, with
a few showers spilling into Leeward areas especially in the
evening and overnight hours. Day to day shower trends will vary a
bit as the low level troughs pass through each island.

&&

.DISCUSSION...
A low pressure system north of the islands shows up on satellite
along a narrow upper trough. This low will add to the large scale
instability across the region. A low level trough moving through
Kauai this morning will enhance shower activity this morning over
the Garden Isle. More stable stratocumulus clouds are moving in
from the east likely keeping showers more focused over windward
and mountain areas. Wind speeds will increase in strength today as
compared to yesterday.

Our large scale weather pattern shows a breezy trade wind pattern
continues into the first half of next week, courtesy of a strong
high pressure system locked in place north of the state. Meanwhile
an unstable upper low center will linger north of the islands as
several low level troughs drift from east to west across each
island. The upper low will bring added instability to the region
and as low level troughs move through each island the surface
convergence will trigger increasing shower trends. Day to day
weather changes are expected due to changes in low level stability
after each trough passes through the state. Trade wind speeds will
return to more moderate levels from Wednesday onward with periods
of increased shower trends remaining in the forecast through the
end of next week.

Wind speeds were increased today into the Small Craft Advisory
range over the Kaiwi Channel between Oahu and Molokai. Otherwise
the forecast grids looked good and only minor changes were made
to yesterdays forecast grids.

&&

.AVIATION...
Showers across the smaller islands which isolated in nature. Expect
mostly VFR conditions elsewhere across the state. Brief, passing
MVFR conditions are possible in windward mauka areas with a few
showers through the day. It will be breezy once again, so we are
anticipating some low-level turbulence most likely below 9Kft
through the day.

AIRMET Tango is in effect for low-level turbulence south thru
west of mountains.

&&

.MARINE...
High pressure far north and northeast of the state will maintain
fresh to strong trade winds into early next week. Expect minor
fluctuations, with trades increasing slightly into Saturday. A
Small Craft Advisory (SCA) remains in effect for the typically
windier waters around Maui and the Big Island through Sunday. An
overnight ASCAT pass across the islands prompted us to expand the
SCA to include the Kaiwi channel and Maui windward waters through
Saturday. The SCA will likely need to be extended into Monday.
Models suggest a brief wind decrease next Tuesday as a weak
disturbance passes north of the state. Winds could briefly drop
below SCA thresholds next Tuesday.

No significant swells are due through much of next week. A mix of
background south and southeast swells will continue to produce
small surf along south facing shores through next week. A small
south-southwest swell will likely taper off this afternoon. Trade
wind swell will gradually build through today, then hold into
early next week. Additionally, a small long-period swell from
tropical cyclone Cristina could arrive early next week. Surf along
north facing shores will remain nearly flat through next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
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 Saturday for Kaiwi Channel-
Maui County Windward Waters.

&&

$$

DISCUSSION...Bohlin
AVIATION...Chevalier
MARINE...Powell

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