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Hawaiian Islands Synoptic Discussion and Guidance

271
FXHW60 PHFO 191357
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
357 AM HST Tue Mar 19 2024

.SYNOPSIS...
A high pressure system will slowly build north of the islands
over the next few days producing moderate to locally breezy trade
winds through Thursday. A weakening cold front caught between the
building high and the Hawaiian Islands will delay breezy trade
winds until Friday. These stronger trade winds will last through
Sunday. Decreasing wind speeds to more moderate levels will return
early next week as the high center drifts eastward away from the
island chain. Brief passing shower activity will favor windward
and mountain areas with increasing shower trends from Thursday
onward.

&&

.DISCUSSION...
The infra-red satellite imagery this morning shows a weakening
cold front roughly 400 miles northwest of Kauai. This front will
pass north of the island chain later this week and briefly block
stronger trade winds from a building high pressure system that
will also pass by north of the islands over the next few days.
High cirrus clouds associated with a subtropical jet stream are
filling in over the state today and will continue to produce
extensive high cloud cover across all islands through Wednesday.
Fairly stable pillow shaped stratocumulus clouds are moving into
the island on the east-northeasterly trade winds this morning.
These stable clouds are riding in under a stable subsidence
temperature inversion height of around 5,000 feet elevation,
keeping showers to a minimum through Wednesday.

By Wednesday night into Friday, weather conditions begin to shift
a bit as forecast guidance shows an upper level trough moving in
from the northwest and deepening over the islands. Colder air and
upper level divergence associated with this trough pattern will
decrease stability a bit, raising the temperature inversion
heights, producing deeper more unstable clouds, and increasing
shower trends. These showers will tend to favor the overnight to
early morning hours with the highest shower coverage expected over
the typical windward and mountain areas. Trade winds also
increase in strength to breezy levels from Friday through Sunday.

The weather forecast for this weekend is complicated by a small
upper level low drifting into the Hawaii region. This system may
enhance shower activity over some islands as model guidance tends
to be less certain on these smaller scale features this far out in
time. For now we are forecasting a normal trade wind shower
pattern with scattered to numerous showers at night favoring
windward areas. These showers could intensify for some islands
this weekend if this low tracks closer to the state than our
recent forecast model and model blends suggest. Stay tuned.

.AVIATION...
Moderate easterly trades will persist, with clouds and light
showers favoring windward and mauka areas overnight through the
morning hours. VFR conditions will persist, with only a slight
chance of MVFR CIGs/VSBYs for brief periods in showers.

&&

.MARINE...
Trades have generally eased back to more moderate to locally fresh
magnitudes as a result of high pressure settling in north of the
coastal waters. There will be a slight pick up in trades through
the notoriously windier eastern island channels and bays around
Maui County and south of Big Island this afternoon. This has
prompted a Small Craft Advisory for these waters through Wednesday
afternoon. The next high developing farther north of the region
Wednesday will build south Thursday and Friday. This will tighten
the regional pressure gradient across the local waters and
strengthen trades back to fresh to locally strong levels going
into the weekend.

The early week northwest (310-320 degree) swell is continuing to
fall through the day. It will be followed by a slightly larger
size northwest (310-330 degree) swell building in later tonight.
This swell's forerunners are now passing around the far northwest
offshore buoys during the pre-dawn morning hours. This times the
swell to fill in through the afternoon hours and peak along
Kauai-to-Maui's better exposed north and west-facing coasts from
late tonight through Wednesday. This high end moderate size, long
period swell will likely produce High Surf Advisory (HSA) level
north and west-facing shore surf tonight and Wednesday. North and
west shore surf will fall below HSA heights Thursday. The next
moderate size, medium to long period northwest (310 degree) swell
is expected to arrive this weekend. The weak pulse of early season
south southwest (200 degree) swell will fade out through the day.
This inconsistent swell will keep south-facing shore surf slightly
elevated through tonight with a gradual decline in surf as the
swell fades Wednesday. East shore surf will remain small through
Wednesday. There will be a slight pick up in east chop late this
week in response to strengthened trades.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory from noon today to 6 PM HST Wednesday for
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Leeward Waters-Big Island Southeast Waters.

&&

$$

DISCUSSION...Bohlin
AVIATION...Gibbs
MARINE...Blood

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