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Hawaiian Islands Synoptic Discussion and Guidance

538
FXHW60 PHFO 181338
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
338 AM HST Mon Mar 18 2019

.SYNOPSIS...
Trade winds will continue to blow across the islands along with
slightly cooler temperatures lasting through Tuesday. Warmer
temperatures will return on Wednesday with light winds as another
cold front approaches Hawaii from the northwest, briefly
weakening the high pressure ridge north of the state. Breezy trade
winds are forecast on Thursday and strong trade winds develop on
Friday as a high pressure system builds in north of the Hawaiian
Islands. Another shallow cold front moves into the northwestern
islands of Kauai and Oahu from Friday night into Saturday morning
with increasing clouds and showers over both islands. Clouds and
showers along the frontal band will then spread southeastward
down the rest of the island chain.

&&

.DISCUSSION...
Satellite imagery this morning shows an extensive stratocumulus
cloud bank north of the state. Most of these clouds will decrease
in coverage after sunrise due to atmospheric heating and mixing.
The shallow cold front has drifted east of the Big Island. Large
scale atmospheric subsidence will keep stable conditions in the
forecast with only isolated showers possible along windward
mountain areas through Tuesday.

A surface high will migrate eastward over the next few days
passing just north of the island chain with moderate northeast trade
winds forecast through Tuesday. As the high moves into the East
Pacific another shallow cold front will approach the islands from
the north.

On Wednesday, a broad trough associated with a cold front moving
into the Central Pacific will weaken the high pressure ridge north
of the state. These weaker pressure gradients will produce light
and variable winds with some onshore sea breezes developing over
the western islands. The atmosphere will remain fairly stable
through the middle of the week as strong downward atmospheric
motions (Subsidence) will keep cloud heights capped at around 6000
feet resulting in a dry weather pattern through Friday afternoon.

Long range weather models continue to show a high pressure ridge
building into the region by the end of the week. Increasing trade
winds will drive a shallow cold front into the northwestern half
of the state from Friday night into Saturday morning. Expect
increasing clouds and some showers over Kauai and Oahu during this
time period. Clouds and elevated showers along the frontal band
will then spread southeastward to Maui County on Saturday, and the
Big Island on Sunday morning. The American (GFS) model cross sections
are showing deeper cloud heights, as compared to the previous
frontal system, with cloud tops rising up to the 10,000 to 12,000
foot level. These deeper cloud heights will allow more collision
of cloud droplets within the cloud layer, thereby increasing
rainfall chances across the state. Strong trade winds will help
lift clouds over windward mountain ranges, enhancing rainfall
chances along north and east sections of all islands. Some of the
stronger shower bands will likely bring at least scattered
rainfall into leeward areas with the passage of this cold front.

Model solutions differ a bit on the movement of this frontal band,
expect some timing changes as the Friday to Saturday forecast
period gets closer. We increased precipitation chances for the
Friday through Sunday time period to account for the next cold
frontal passage. A blended approach was used between the American
(GFS) model and the European (ECMWF) models with additional
weighting towards the GFS solution. Breezy to strong trades will
continue through the weekend into the first half of next week as a
1030 MB surface high lingers north of the Hawaiian Islands.

&&

.AVIATION...
In the wake of a passing shallow cold front, winds have veered
northeasterly at around 10-15 KT. These winds will continue to
veer east northeasterly this morning and into this afternoon.
Limited shallow moisture will leave the area mostly dry with the
exception of ISOL -SHRA along the windward coasts and slopes.

No AIRMETS are in effect and none are expected during the period.

&&

.MARINE...
Fresh to strong north-northeast winds associated with high
pressure building to the north will shift out of the east tonight
into Tuesday. A brief period with light and variable winds will
become a possibility through the midweek time frame as the ridge
shifts south over the area due to a passing front to the north. A
return of fresh to strong trade winds will be possible Thursday
into next weekend as high pressure builds north of the area in the
wake of the passing front.

Surf along north and west facing shores will hold above warning
levels today, then drop to advisory levels Tuesday as the large
north-northwest swell gradually eases. For more details regarding
the potential impacts associated with the large surf see the
latest High Surf Warning that remains in effect until 6 PM HST
today.

The active pattern across the northern Pacific will continue
through the week with overlapping long-period northwest swells
expected Wednesday, Friday, and Saturday night. These
reinforcements will be similar in size, which will keep the surf
around advisory levels for north and west facing shores each day.
The northwest swell Wednesday and Wednesday night will be the
largest of the three, which could drive the surf toward warning
levels around its peak.

Surf along east facing shores will remain small through midweek,
then rise Friday into the weekend as strong trades return locally
and upstream.

Surf along south facing shores will remain small through the week
with mainly background southerly pulses moving through.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST this evening for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala-Big Island North and East.

High Surf Advisory until 6 PM HST this evening for Kona.

Small Craft Advisory until 6 AM HST Tuesday for all Hawaiian
waters-

&&

$$

DISCUSSION...Bohlin
AVIATION...JT
MARINE...Gibbs

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



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