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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 221347
Area Forecast Discussion
National Weather Service Honolulu HI
347 AM HST Sat Jul 22 2017
Some what of an unsettled trade wind regime will be with us for
the next couple of days as a nearby upper level trough add a boost
to the trade wind showers. Then...tropical moisture associated
with weakening and dissipating Tropical Storm Fernanda moves in
early next week prolonging the unsettle weather. Drier weather
with lower humidity and strong trades will return by Wednesday.
An upper level trough lies just west of Kauai this early
Saturday,morning. A low is expected to evolve just southwest of
Kauai Sunday night. It pretty much stays there through the first
half of the new week before weakening. This trough is expected to
weaken the low level inversion today and allow low level moisture
to rise from 10 to 13k feet to 15 to 20k feet by this afternoon.
It will also bring a slight chance of a thunderstorm to the
interior and summits of the Big Island this afternoon.
The low level moisture now over the islands is not the deep
tropical moisture associated with Fernanda. Fernanda is still a
plus 500 miles east of the Big Island. It is captured moisture
originating from off Baja Californin which is of stable
stratocumulus. This moisture is much shallower and is holding
precipitable water values (PW) of between 1.50 and 1.70w inches
across the smaller islands. PW is higher over the Big Island at 2
inches, which has been rather wet during the past 24 hours or so.
Any ways, the frequency of the showers aren't as high as earlier
in the evening. And in short, the POPs were lowered for
particularly Kauai and Oahu. Windward Big Island should remain
showery today as there is quite a bit of low level moisture upwind
of the island.
Models are in agreement of a dry patch glancing the islands
tonight and Sunday. Hence, POPs have been lowered for the islands
for Sunday for especially the Big Island. This dry patch is the
subsidence region located to the west of any tropical cyclone.
Up to Sunday, the trade winds will be modest in speed, but will
turn lighter on Monday. The deep tropical moisture from dying
Fernanda should be bleeding onto the Big Island as early as Sunday
evening, and gradually spread westward across the rest of the
islands through Monday night, raising the discomfort level. This
injection of additional, deeper moisture, will result in some
spotty heavy showers especially Monday afternoon, aided by daytime
heating, light winds, and an already established unstable air
mass brought on by the upper level trough.
The EC model appear to have slowed the clearing process of the
tropical air mass. GFS, the preferred choice, has the moist humid
air mass exiting Kauai by Tuesday evening. The drier air will be
accompanied by locally strong trades.
Clouds and frequent showers will continue to affect mainly
windward areas of the state this morning. Periods of MVFR
conditions will be likely, with isolated/brief IFR possible.
AIRMET Sierra for mountain obscuration is currently in effect for
windward sections of Oahu, Molokai, Maui and the Big Island.
Conditions should improve across windward sections by midday.
VFR conditions will prevail over leeward areas.
This afternoon, some heavier showers will be possible as an upper
trough digs south toward the state. An isolated thunderstorm will
even be possible over the interior Big Island. Localized MVFR/IFR
conditions may be produced by this activity.
Moderate trade winds will continue over the state today. Recent
raob and VWP data indicate that winds have decreased enough such
that AIRMET Tango for moderate low level turb will not be
necessary for today.
The Small Craft Advisory has been extended to until noon today
due to the locally strong trade winds blowing around the waters of
Maui County and the waters south of the Big Island. SCA will
likely return on Tuesday of next week in tandem with the return of
the locally strong trades.
The High Surf Advisory remains 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 noon Monday for east facing shores of
Kauai, Oahu, Maui, Molokai, and the Big Island.
Small Craft Advisory until 6 am Sat for Maalaea Bay, Pailolo
Channel, Alenuihaha Channel, and waters south of the Big Island.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office