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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 222030 CCA
Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
1010 AM HST Sat Jul 22 2017
Breezy trades will be more showery than normal today and tonight
due to an upper trough located to the northwest of the islands.
The upper trough will weaken on Sunday with fewer showers
expected. A trough of low pressure, the remnants of Fernanda,
will move across the islands Sunday night and Monday. This will
enhance the showers once again, and bring light winds and
uncomfortable humidity. Breezy trades and more settled weather
will start to return Tuesday as high pressure far to the northeast
regains control of our weather.
Infrared satellite imagery shows a gradual cooling trend in cloud
tops just S and SW of the islands with even an isolated CB now
with a top to 39000 ft about 125 mi SSW of Kauai. These showers
are associated with an area of moist tropical air which is riding
across the islands in the trades, and are being enhanced by a
weak but digging upper trough. The trough shows up quite well in
water vapor imagery about 200 mi W of Kauai. Radars confirm that
most of the enhanced (and locally heavy) showers are remaining
offshore. With the digging trough in our proximity, along with
abundant moisture (1.72 inch PW in the 12z Hilo sounding), we
could see some locally heavy downpours and even a rumble or two of
thunder over the interior of the Big Island this afternoon.
Otherwise, the added moisture will allow for extra clouds and
more-than-usual windward and mauka shower coverage this afternoon.
Trade winds are hanging on at breezy levels thanks to a nearly
stationary subtropical ridge about 850 mi N of Honolulu, and these
should hold through the weekend. What is expected to be the remnants
of Fernanda will track near the islands Sun night into Mon as a
trough that will slacken the pressure gradient enough to interrupt
the trades for about 24 hours. Breezy trades will make a
triumphant return on Tuesday and continue through the rest of the
work week as a sprawling surface high sets up shop over the
eastern Pacific well to our NE.
The nearby upper trough will continue to enhance showers W of the
islands through tonight. Will have to keep an eye on Kauai and
Oahu which are forecast to be on the edge of this area. Although
the airmass over the islands will remain quite moist through
Sun, the upper trough is expected to weaken and so there could be
a relative lull in the showers prior to the arrival of ex-
Fernanda. In some situations, the remnants of tropical cyclones
can bring copious amounts of rainfall, but this does not appear to
be one of those situations. Mid-level ridging is actually
forecast to start building over the islands about the same time
the surface trough passes over the islands. In addition, the GFS
and ECMWF remain insistent that the deepest moisture with ex-
Fernanda will skirt by just to the N of the islands. Nevertheless,
expecting some increase in shower coverage again Sunday night,
and the possibility of an active convective day on Monday. It does
not look nearly as bad as it could be if we had some significant
upper level support.
The airmass begins to dry out considerably from E to W on Tue and
fairly typical summertime trade wind weather is expected through
the rest of the forecast period as weak mid-level ridging keeps
the airmass stable.
A locally breezy trade wind pattern will start to trend down this
afternoon. The VAD wind profiles from the two Big Island radars,
PHWA and PHKM, still show some occurrences of 25 kt winds through
the low levels. This suggests pockets of isolated low level light
to moderate turbulence may be occurring around the mountains.
However, widespread tempo moderate turbulence is not expected as
the winds continue to trend down this afternoon. Therefore, no
AIRMET TANGO for low level turbulence is issued.
Enhanced moisture in the low levels will continue to move across
the state, with most clouds and showers favoring the windward and
mountain areas. A mid to upper level trough over the western
portion on the state will help enhance showers further. MVFR to
potential IFR conditions can be expected, mainly over the
windward areas of the islands. The forecast also paints a slight
chance of thunderstorms this afternoon along Big Island slopes.
AIRMET SIERRA for mountain obscuration is currently posted along
all north through east sections on the islands. Expect some
improvement into this afternoon, with more showers returning this
evening and overnight. AIRMET SIERRA will be tailored as needed
through the day and tonight.
Locally strong trade winds will continue to produce Small Craft
Advisory conditions for the typically windy waters around Maui
County and the Big Island through noon today.
The High Surf Advisory is now in effect for the east facing
shores of the Big Island, Maui, Kauai, Molokai and Oahu. It
remains posted through Monday morning due to a mid- to long-
period east swell associated with a fetch from when Tropical
Storm Fernanda was a major hurricane.
The largest high tides of the month will occur each afternoon
during the next few days. Note that these 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 today through Monday as
an upper trough and increased moisture from Fernanda remnants move
across the area.
High Surf Advisory until 6 AM HST Monday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.
Small Craft Advisory until noon HST today for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office