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Hawaiian Islands Synoptic Discussion and Guidance

831
FXHW60 PHFO 260653
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
853 PM HST Sun Oct 25 2020

.SYNOPSIS...
A surface ridge over the islands will produce locally breezy
southeast winds around the Big Island and gentle and variable
winds around the smaller islands through tomorrow. Land breezes
will bring partial clearing tonight, while sea breezes will
produce spotty showers over interior sections tomorrow. A strong
low aloft is forecast to form northwest of the area tonight. The
atmosphere over the islands will become very moist and unstable.
A period of wet weather may start as early as tomorrow night and
continue through the week.

&&

.DISCUSSION...
A surface ridge to our northeast is driving southeast flow near
the Big Island and Maui. The Big Island and Maui partly block the
southeast flow so winds are gentle and variable over the rest of
the islands. Southeast flow will continue to push clouds and
isolated showers over the Big Island and Maui through tomorrow.
Land breezes will clear leeward areas of Maui and the Big Island
and the islands from Molokai to Niihau overnight. Clouds and
spotty showers will form over interior areas during the afternoon
again tomorrow.

Models show an upper low forming northwest of the main Hawaiian
Islands tonight, then dropping south. This will make the
atmosphere increasingly unstable. The atmosphere is already rather
moist with PW around 1.5 inches. The atmosphere will become even
more moist as the upper low pulls tropical air over the islands.
The moisture and instability will likely produce a period of wet
weather, but we have low confidence on the details. Models have
been inconsistent with each other and have been much different
from run to run. At this time, the best chance for heavy rain
appears to be over Kauai and Oahu, but there could be heavy rain
state-wide. Because of the model differences, confidence is low
concerning the the duration of the heavy rain, which could
continue over some areas through the week.

&&

.AVIATION...
A low pressure system and cold front approaching Hawaii from the
north-northwest will bring periods of showers and MVFR conditions
to the western half of the state over the next several days.
Isolated IFR conditions are also possible in heavy showers. This
low pressure system will also keep light and variable winds in the
forecast for all islands. On-shore sea breezes will form over each
island in the day with off-shore land breezes developing in the
overnight hours.

Low cloud ceilings will likely product Tempo Mountain Obscurations
each day, mainly over the western islands of Kauai and Oahu.

No AIRMETs are in effect.

&&

.MARINE...
A ridge near Kauai will bring southeast flow across the area with
the strongest winds east of Maui and the Big Island. A front
approach the area from the west Tuesday through Thursday with
unsettled weather a possibility.

A High Surf Advisory is currently posted for most north and west
facing shores from Maui county westward as a moderate size long
period northwest swell moves through the area. The swell will
lower Monday through Wednesday. Another moderate size long period
northwest swell may arrive late Saturday.

A small, medium period northeast swell is expected to fill in
Monday and linger through Tuesday. There will also be a series of
small long period south swells through the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Monday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

&&

$$

DISCUSSION...Powell
AVIATION...Bohlin
MARINE...Morrison

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



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