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Hawaiian Islands Synoptic Discussion and Guidance

499
FXHW60 PHFO 191943
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
943 AM HST Mon Aug 19 2019

.SYNOPSIS...
A building ridge north of the state will maintain a trade wind
pattern through the upcoming weekend. Showers will be focused over
windward and mountains areas in this flow. Light to moderate trade
winds will persist with periods of moderate to locally breezy
trades in the middle of the week and again for the weekend.

&&

.DISCUSSION...
The overnight analysis depicts a 1017 MB high pressure system
about 575 miles northwest of Kauai, with a building east to west
oriented ridge. There is good model agreement with the weather
pattern near the islands into the weekend. The surface ridge to
the north strengthens, allowing the trade winds to rebound to
moderate to locally breezy levels Tuesday night and Wednesday.
Some weak troughing over the state could weaken winds a bit
Thursday and Friday, but expecting winds to rebound again this
weekend.

The background trade wind flow will focus shower activity over the
windward sides of the islands. Models show various batches of
moisture riding in on the trades through the week, so there will
be at times periods of increased shower activity. However, despite
the periods of increased activity, it should primarily be more
typical trade wind showers versus the drier than normal activity
we have seen as of late.

Towards the end of the week, moisture from an old front to the
northeast of the islands will move towards the islands. The models
have thus far been pretty consistent with the bulk of that
moisture staying north of the islands, with some of it skirting
Kauai and perhaps Oahu. Currently the forecast reflects a more
typical trade wind pattern, but as we get closer will make
adjustments based on the higher resolution models.

Longer term, we are seeing model differences and run to run
differences primarily with the GFS. The current forecast maintains
the trade wind flow for the start of the new week.

The forecast remains on track so no changes to the forecast at
this time.

&&

.AVIATION...
Once again, an elongated surface high to the north of the state
will act to maintain weak to moderate east northeast trades
through this afternoon. Conditions will be rather dry except for
isolated showers that develop either on the windward side of the
islands or over the mountain ridges. After sun down, showers will
increase in coverage along the windward slopes, possibly leading
to MVFR conditions at times.

No AIRMETs are currently in effect.

&&

.MARINE...
A nearly east to west oriented surface ridge located about 300 nm
north of Lihue is forecast to weaken and shift southward later
today. This will result in light to moderate northeast trade
winds across the Hawaiian coastal waters. As a new surface high
pressure system builds north of state from tonight through mid-
week, east-northeast trade winds will gradually strengthen. Based
on the latest forecast, trade winds are expected to reach the
Small Craft Advisory (SCA) threshold starting Tuesday night
across the typically windier waters adjacent to the islands of
Maui County and the Big Island. These SCA conditions may persist
over these waters through Wednesday night. A weak surface front
approaching the area from the north later this week is expected
to cause a slight disruption in the pressure gradient over the
region. Therefore, the trade winds are forecast to weaken
slightly from Thursday through Friday.

Surf along south facing shores will slowly build through the day
today as a new large, long-period south-southwest swell begins to
spread across the Hawaiian waters. Long-period forerunners are
starting to register on the offshore buoys at NOAA buoy 51002 and
51004, which should be filling in this afternoon. Surf is expected
to rise above the seasonal average by late this evening and is
expected to reach advisory levels by tonight. As this swell passed
through the PacIOOS buoy at Pago Pago several days ago, it
registered swell heights and energy levels slightly higher than
our recent mid-July 2019 warning-level event along south facing
shores. The possible limiting factor for this swell event in
Hawaii was that the storm was slightly more compact compared with
other high surf events. We will be monitoring observations in case
a High Surf Warning might be required during the peak of this
event on Tuesday or Tuesday night. The surf may not drop below the
HSA criteria along south facing shores until Thursday based on
the latest guidance.

The small west swell associated with recent tropical cyclones in
the Northwest Pacific will steadily trend down today. Surf along
north and west facing shores of the smaller islands may increase
from late Wednesday through Friday as small northwest swells
spread down the island chain. Small, choppy surf may trend up
slightly along east facing shores later this week as the trades
strengthen.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


DISCUSSION...M Ballard
AVIATION...Thomas
MARINE...Kino

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