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Hawaiian Islands Synoptic Discussion and Guidance

375
FXHW60 PHFO 220141
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
341 PM HST Tue May 21 2019

.SYNOPSIS...
A front to our northwest will continue to put a dent in the ridge
to our north through this week. Winds will remain light east to
southeasterlies through Friday across the smaller islands, while
stronger residual easterlies continue around the Big island.
Cloud cover and rainfall will favor mauka areas during afternoon
and early evening hours, with clearing expected each night. Trade
winds are forecast to return Memorial Day weekend and last through
the first half of next week.

&&

.DISCUSSION...
There is no change in forecast philosophy from this morning. A
front far northwest of the main Hawaiian Islands marks the western
edge of the subtropical ridge north of the chain. The orientation
of the front and ridge is such that winds continue to blow from
the east southeast across local waters. Wind speeds are light
enough, especially across the smaller islands, to allow
development of local land and sea breezes. Satellite loop shows
scattered to patchy broken low clouds across interior sections of
the smaller islands, with broken to overcast low clouds noted
across most shores and slopes of the Big Island. Radar shows
these cloudier areas have scattered moderate showers. Cloud and
shower coverage will likely decrease across interior sections of
the islands as evening cooling sets in.

Models show little or no pattern change across the islands
through the rest of the work week as we remain stuck between high
pressure to our northeast and a front to our west and northwest.
The front will continue to move toward the islands this evening,
but then stall far enough away to spare us any direct effects. The
upper trough supporting the front will also remain far enough
west of the islands to prevent significant destabilization. Models
show the ridge will build back westward this weekend as troughing
to our west lifts northward. This will allow winds to back a bit
to east northeasterlies once again, restoring trade flow and the
associated pattern of windward and mauka showers. This restored
trade flow should then last into next week.

So, expect a mild convective pattern of afternoon and evening
clouds and showers across mauka areas, with nighttime clearing,
through Friday. Showers may become moderate at times. Expect a
shift back to a trade wind pattern this weekend into next week,
with clouds and showers shifting to windward and mauka areas.

&&

.AVIATION...
Weak southeast flow will continue into Wednesday due to a high
pressure system to the far northeast and a front approaching from
the northwest. Afternoon clouds and showers will thin overnight,
but expect additional clouds and showers mainly over interior
sections tomorrow afternoon. VFR conditions are expected to
prevail. Weak inversions on the afternoon soundings around 7000
feet will allow cloud tops to reach higher than normal. No
AIRMETs are expected.

&&

.MARINE...
A cold front approaching from the northwest will stall just northwest
of the offshore waters tonight. A ridge near Kauai will lift north
and will slowly push the front to the north over the next several
days.

East southeast flow caused by the approaching front will keep winds
light over much of the coastal area due to blocking from the Big
Island and Maui. Fresh easterly trade winds will persist for waters
surrounding the Big Island tonight with winds reaching Small Craft
Advisory (SCA) criteria by Wednesday morning. The SCA has been
posted for Wednesday and Wednesday night.

Small to moderate surf will continue along south facing shores through
Friday. A fairly large swell from southeast of New Zealand is forecast
to reach the coastal waters Saturday. This swell may produce surf near
the advisory level along south facing shores through Memorial Day.
Surf along north facing shores will be quite small through Thursday.
A moderate northwest swell arriving Friday will peak Saturday then
slowly subside through Memorial Day. Surf along east facing shores
will remain small through the middle of the week. As winds strengthen
northeast of the area during the second half of the week, surf will
build along east facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory from 6 AM Wednesday to 6 AM HST Thursday
for Alenuihaha Channel-Big Island Windward Waters-Big Island
Leeward Waters-Big Island Southeast Waters.

&&

$$

DISCUSSION...Powell
AVIATION...M Ballard
MARINE...Foster

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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