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Hawaiian Islands Synoptic Discussion and Guidance

767
FXHW60 PHFO 221347
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
347 AM HST Mon Apr 22 2019

.SYNOPSIS...
Moderate to locally breezy trade winds will continue through
the work week, bringing periods of clouds and showers to windward
areas, while leeward areas remain mostly dry. The trades will
begin to ease a bit on Friday, with a further decrease over the
weekend allowing for the development of sea and land breezes. This
will favor showers developing over interior and mountain areas
during the day, and in windward areas near the coast at night.

&&

.DISCUSSION...
Currently at the surface, a 1032 mb high is centered around 1550
miles northeast of Honolulu, with the ridge axis extending west-
southwestward to a location around 500 miles north of Kauai. The
resulting pressure gradient is driving moderate to locally breezy
trade winds across the island chain early this morning. The
airmass remains dry and stable, with precipitable water (PW)
values ranging from 1.0 to 1.2 inches and inversion heights
hovering around 6 to 7 kft based on the morning PHTO and PHLI
soundings. Infrared satellite imagery shows clear to partly cloudy
skies in most areas, with some patches of more extensive cloud
cover in some windward locales. Radar imagery shows a few light
showers moving into windward areas, with mainly rain free
conditions in leeward sections. Main focus revolves around the
potential for some unsettled weather over the weekend.

Quiet trade wind weather is expected to prevail through much of
the week, with a ridge of high pressure holding in place to the
north of the state. Moderate to locally breezy trades should
persist through the work week, although a slight easing of the
trades is expected on Friday as an approaching front weakens the
ridge to the north of the islands. The airmass will remain dry and
stable with PW values holding in the 0.9 to 1.3 inch range, while
inversion heights hover between 5 and 8 kft. As a result, we
should see a continuation of the dry trade wind weather through
the work week, with rainfall amounts remaining on the lighter side
and confined primarily to windward and mauka areas, particularly
at night and during the early morning hours. An isolated shower or
two may also reach leeward areas of the smaller islands during
nights and early mornings, with generally dry conditions each
afternoon. Leeward sections of the Big Island may see a few
showers develop each afternoon/evening, with mainly dry
conditions late at night and during the morning.

Model solutions continue to exhibit some differences over the
weekend, and this is where forecast confidence decreases. Both
the GFS and ECWMF do however show a deep upper level trough
developing to the northwest of the state and digging southeastward
towards islands during the weekend. This results in a further
erosion of the ridge to the north of the state as a surface cold
front edges closer to the area from the northwest, with surface
troughing developing in advance of the front and in the vicinity
of the island chain. This is expected to shift the background
boundary layer flow around to the southeast and south over the
weekend, with surface winds becoming light and variable allowing
sea and land breezes to develop in many areas. Additionally, the
low level inversion will erode as the upper level trough moves
into the area, and a modest increase in PW values to above normal
levels is expected as the boundary layer winds shift southerly.
Overall, we should see a more convectively driven weather pattern
over the weekend, with showers favoring interior and mountain
areas during the afternoon and early evening hours and windward
and coastal areas favored under a background east-southeasterly to
southeasterly boundary layer flow. There is the potential that
some of the shower activity could be locally heavy and a
thunderstorm can't be ruled out at this time. Given the
differences in the models and being so far out in the forecast
period, will hold off on adding any mention of thunder to the
forecast until details become more clear.

&&

.AVIATION...
A ridge of high pressure will remain far northeast of Hawaii and
drive fresh trade winds over the next 24 to 36 hours. AIRMET
Tango remains in effect for moderate turbulence below 6000 feet
over and downwind of the terrain due to the trade winds. These
conditions are expected to hold in place through today, possibly
longer.

Trade winds will carry in periodic clouds and showers that will
focus over windward and mauka sections. Brief MVFR ceilings and
visibilities, and mountain obscurations may occur within the
showers.

&&

.MARINE...
A ridge north of the state will maintain fresh to locally strong
trade winds through Thursday. A front is expected to stall north
of the state Friday into the weekend and cause the ridge of high
pressure to weaken. This will result in a decrease of winds
Friday through the weekend and may cause sea breezes to develop
near coastal areas by Saturday afternoon. The current Small Craft
Advisory (SCA) is in effect for the typical windy areas around
Maui and the Big Island through Wednesday.

Surf along all shores is expected to remain below advisory levels
for this week. A few small west-northwest and north-northwest
swells will provide small surf along north and west facing shores
throughout the week. A series of small south swells will provide
small surf starting today and will likely continue through most
of the week and into the weekend. Local PacIOOS/CDIP buoy at
Barbers Point is starting to show an uptick in energy across the
16 to 18 second bands this morning. This swell should continue to
fill in through the day today and will likely peak tonight and
slowly drop over the next couple of days. Several reinforcing
south swells are expected towards the latter half of the week.
Surf along east facing shores will hold steady in the small to
moderate range through Thursday due to the breezy to locally
strong trades.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Powell
MARINE...Kino

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

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