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Hawaiian Islands Synoptic Discussion and Guidance

914
FXHW60 PHFO 212017
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1017 AM HST Fri Jul 21 2017

.SYNOPSIS...
A surface ridge of high pressure far to our north will keep
breezy trades blowing today. The trades will ease over the
weekend as weakening Fernanda approaches the waters to our
northeast. An upper level trough is expected to form close to the
islands today, bringing with it a period of unsettled weather
through early next week.

&&

.DISCUSSION...
The synoptic pattern is rather complex with several players that
look to be affecting our weather the next few days. The current
breezy trades are being driven by a nearly stationary surface
ridge about 850 miles or so N of Honolulu. Meanwhile, at 5 am,
CPHC analyzed the center of Tropical Storm Fernanda about 750 mi E
of Hilo, and continuing to move toward the W. A band of peripheral
moisture which has pushed far out ahead of Fernanda is currently
traversing the islands from Maui County to the Big Island. Finally,
a digging mid-level trough is evident on water vapor imagery about
700 mi NW of Kauai.

At the surface, the ridge to the N weakens and moves slowly S
over the next couple of days, replacing today's breezy trades with
more gentle to locally breezy winds this weekend. Based on the
latest CPHC forecast, Fernanda is expected to continue to move
toward the W or WNW and weaken, becoming a remnant low Saturday
afternoon. The latest track takes the remnant low just N of the
islands Sun night into Mon. It should be noted that the CPHC
forecast track is much closer to the ECMWF than the GFS, which
unrealistically takes a stronger Fernanda much farther N of the
islands. The CPHC forecast would likely imply a period of much
weaker winds as the remnant of Fernanda passes N of us. After
that, we should see breezy trades return, thanks to a large
surface high which at that time will be parked about 1400 mi NE of
Hawaii.

The CIMSS MIMIC Total Precipitable Water loop shows that the
aforementioned band of moisture affecting the islands this AM is
the leading edge of a fairly large area of very moist, tropical
air which extends all the way to Fernanda and beyond. It will get
more humid and uncomfortable today, then worsen over the weekend
and through Monday when dewpoints will reach the lower to middle
70s, TPW will climb to over 1.75 inches, and K-indices will exceed
30. The weak but digging mid-level trough to the NW of the
islands may help to put the moist air to work, with some localized
heavier showers expected. Will take a look at the new CPHC
forecast, as well as the latest model guidance, and may consider
adding heavy showers or thundershowers for what appear to be the
higher threat areas and time periods in time for this afternoon's
forecast package.

More settled trade wind weather should return about Tuesday, as
mid level ridging and drier air build in, in the wake of ex-
Fernanda.

&&

.AVIATION...
Locally strong trade winds will continue today. AIRMET TANGO for
low level turbulence remains in place over and immediately south
through west of the mountains. Winds will begin to ease up
slightly this evening into Saturday. AIRMET TANGO will need to be
revisited at that time.

The trade wind pattern will help focus clouds and showers over
the windward and mountain areas. Enhanced moisture moving through
the eastern islands this morning prompted an AIRMET SIERRA for
mountain obscuration along the north and east facing slopes of the
Big Island and Maui. We should start to see some improvement this
afternoon for these areas, as the moisture moves to the west.
AIRMET SIERRA may need to be expanded to cover the western islands
this evening/tonight.

&&

.MARINE...
Locally strong trade winds will continue to produce Small Craft
Advisory conditions through early Saturday morning for the
typically windy waters around Maui County and the Big Island.

The latest forecast for Tropical Storm Fernanda issued by the
Central Pacific Hurricane Center at 5 AM HST Thursday continues
to indicate the tropical cyclone will weaken to a remnant low that
may eventually move to a position northeast of the Big Island
Sunday afternoon. As this area of lower pressure approaches the
region, expect the trade winds to weaken this weekend. The
background flow will likely become much lighter early next week as
the remnants of Fernanda pass by to the north of the islands
beginning late Sunday and continuing through Tuesday.

A High Surf Advisory remains in effect for the east facing shores
of the Big Island and Maui through early Saturday morning due to
a mid- to long-period east swell associated with a captured fetch
from when Fernanda was a major hurricane. Wave model guidance
indicates reinforcing east swell energy from Fernanda arriving
along the east facing shores later today and tonight. Note that as
this reinforcing swell continues to spread westward to the
remainder of the smaller islands, a High Surf Advisory will likely
be needed for east facing shores of Molokai, Oahu and Kauai over
the weekend.

The largest high tides of the month (King Tides) will occur each
afternoon during the next few days. Note that these abnormally
high tides combined with wave run up due to the east swells
produced by Fernanda may result in coastal flooding in some areas,
through Monday.

Elsewhere, no significant swells are expected. Small south swells
will continue to produce background surf along south facing shores
through the weekend and on into next week.

Over the offshore waters, there will be a chance of isolated
thunderstorms over portions of the area from Saturday through
Monday as and upper trough, along with increased moisture from
Fernanda move across the area.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Saturday for Windward
Haleakala-South Big Island-Big Island North and East.

Small Craft Advisory until 6 AM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$
R Ballard/EATON/Burke

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



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