Current Conditions
Temp7.1 C
RH68 %
WindW 9 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

757
FXHW60 PHFO 230150
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
350 PM HST Tue Sep 22 2020

.SYNOPSIS...
Moderate to locally breezy trade winds will continue into the
weekend, with winds diminishing and becoming light Sunday and
Monday. An area of moisture will move over the islands later tonight
through Thursday morning, bringing an increase in low clouds and
showers, mainly to windward areas. Drier weather from later Thursday
into Sunday will likely continue into next week as winds diminish.

&&

.DISCUSSION...
Another mostly dry and hot day of trade wind weather. Despite Autumn
being officially underway, several locations reported high
temperatures in the lower 90s this afternoon. Trade winds will
persist into the weekend, supported by an E-W surface ridge along
30-35N. The trade winds are generally expected to deliver brief
passing showers to primarily windward areas, with little in the way
of significant accumulation. However, there is some good news for
those looking for accumulating rainfall, in the form of an area of
showery low clouds that currently lies about 100 to 600 miles E of
the Big Island. Latest UW-CIMSS MIMIC images indicate PWAT up to
1.6" within this area, which marks what remains of what was once
Tropical Storm Karina. This moisture will move into the Big Island
beginning tonight, then spread to the other islands from Wednesday
into early Thursday, bringing an increase in windward showers, with
windward Big Island likely receiving the bulk. Some of the showers
are bound to spread leeward on the smaller islands, but a mid-level
ridge will keep the low-levels capped, and shower intensity
moderated.

Drier air is expected to filter in from the E on Thursday, leading
to fewer trade showers into Sunday. Much lighter winds are expected
from Sunday onward as a cold front approaches and stalls just to the
NW of Kauai, and the surface ridge moves close to the islands and
weakens. The general expectation is for mostly dry weather to
persist into next week as a mid-level ridge continues nearby, with
the light winds potentially bringing a few afternoon and evening
showers over island interiors.

&&

.AVIATION...
Forecast for this period focused on increasing moisture approaching
Windward Big Island this evening. Prevailing VFR will hold through
the near term. By tonight, lowering ceilings and an expanding
coverage of showers may warrant AIRMET Sierra for windward sections
of the Big Island and Maui. Moderate easterly trades will bring the
northern periphery of this moisture plume to the remaining islands
during Wednesday morning leading to an increase in showers there as
well, particularly over windward areas.

No AIRMETs are in effect.

&&

.MARINE...
High pressure stationed northeast of the islands is producing a
relatively tight northeast-to-southwest pressure gradient across the
state. This will generate and maintain moderate to fresh trade winds
through Friday. Today's ASCAT passes have continued to show trades
just below Small Craft Advisory (SCA) criteria, but winds are still
expected to slightly pick up this evening and persist through
Friday. The SCA is in effect through Friday morning for the typical
windier channels and bays surrounding Maui County and for those
waters south of the Big Island. Trades will weaken this weekend as
the high is expected to back off towards the mainland and slacken
the gradient across the chain.

The arriving long period north to northwest swells will peak
tomorrow (Wednesday) night and Thursday. This will produce above
normal 8 to 12 foot surf Thursday, or below advisory levels, along
north-facing shorelines. These north to northwest swells will
gradually dampen out through Saturday. Small background south to
southwest swells will maintain near seasonal 3 to 5 feet surf along
south-facing shores through the middle of the week. Another small
south to southwest bump produced by a series of broad gale fetches
generated south of New Zealand within the past week will reach our
southern shores Friday into Saturday. Persistent moderate trade flow
through the week will keep the chop up along east-facing shores with
surf forecast to peak below advisory levels.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM Friday for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

&&

$$

DISCUSSION...Birchard
AVIATION...JVC
MARINE...Blood

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