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Hawaiian Islands Synoptic Discussion and Guidance

880
FXHW60 PHFO 210134
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
334 PM HST Wed Nov 20 2019

.SYNOPSIS...
High pressure north of the islands will bringing increasing trade
winds to the region through the weekend. A wet trade wind pattern
is expected, with some rain reaching leeward areas at times with
the stronger winds.

&&

.DISCUSSION...
No major changes to the forecast in the near term. Have boosted
the QPF values for the first 18 hours or so to better reflect some
of the higher values we are seeing. Have also modified PoPs/Sky/Wx
and Wind grids for Monday night through Wednesday to bring the
forecast in line with a blend of the GFS and ECMWF which includes
southeasterly winds and increased PoPs.

High pressure north of the islands will be moving to the east
through tomorrow, and is helping trades to build back in across
the region. The high is expected to remain to the northeast
through the weekend, with a strong pressure gradient over the
islands. As such, expect winds to increase and then remain strong
through much of the weekend. It is quite possible a Wind Advisory
will be needed for some areas as early as Friday morning. Will
continue to monitor future model runs, particularly of the higher
resolution models to nail down timing and location.

The ECMWF and GFS both show an upper level trough at 250mb dropping
down over the islands over the next 24 hours or so, with the GFS
developing a low near Kauai at that level. Heading into the
weekend, the models differ on the placement and strength of the
trough/low, but both lift it out to the northeast by Sunday
afternoon. The models have some reflection of this upper trough at
the mid levels near 500mb, with a 500mb low moving to the west
during the weekend. High pressure remains at the surface
throughout, with strong trade winds. Anticipating a fairly decent
trade wind inversion to set up, with the upper level features
helping to enhance the trade wind showers. At this time, not
expecting any additional impacts.

Longer range, the GFS and ECMWF both indicate a cold front with
associated upper level trough approaching the islands from the
northeast next week. The GFS continues to be faster with the
system, and has a stronger system that gets closer to the islands
than the ECMWF. This set up could bring another round of
thunderstorms and/or heavy rain to the area. The current forecast
does reflect an upward trend in precipitation during this time,
but will need to make further refinements as the even draws
closer. As mentioned above, have made some adjustments to use a
blend of the models for Monday night through Wednesday, but given
the model differences, this may be washing out some of the
details, which still need to be worked out.

&&

.AVIATION...
East northeast trades will strengthen this evening as surface
high pressure continues to build north of the state. This will act
to concentrate most shower activity along the windward coasts and
slopes. Only isolated activity expected for the leeward sides.
Similar conditions will ensue late this evening and into tonight
as clouds and showers increase, especially along the windward
areas. Thick low clouds and greater SHRA coverage may lead to
MVFR conditions at times with periods of mountain obscuration or
low visibility.

No AIRMETs are currently in effect.

&&

.MARINE...
Trade winds are beginning to strengthen and will become breezy to
strong tonight into the weekend in response to strong high pressure
building far north and northeast of the islands. Combined seas are
expected to exceed the 10-foot Small Craft Advisory (SCA) threshold
in most coastal zones tonight into Thursday due to a building northwest
swell and/or strong trade winds. An SCA is in effect through Friday
at this time, but will likely be extended for many areas due to strong
winds continuing. Gale conditions are possible in some channels toward
the end of the work week.

A new long-period northwest swell is beginning to fill in this
afternoon at the Waimea nearshore buoy, but is running a bit slower
to arrive than expected. However, buoys 51001 and 51101 northwest
of the islands are now caught up to and following WW3 guidance, so
expect the same here locally later this evening. A High Surf Advisory
(HSA) for north and west shores of the smaller islands is in effect
through Thursday night. Due to the northwest buoys following guidance
at this point and WW3 running a bit smaller for the past few model
runs, have adjusted the swell and surf heights down slightly, though
still well above advisory threshold. Another, slightly smaller, NW
swell arriving this weekend may require a brief HSA.

Strong trade winds will likely drive a steady increase in short-period
wind waves tonight into the weekend, eventually leading to an HSA for
east facing shores. No other significant swells are expected. See the
Oahu Surf Discussion (SRDHFO) for details on swell sources.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Friday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

Small Craft Advisory until 6 PM HST Friday for all Hawaiian
waters except Maalaea Bay-


&&

$$


DISCUSSION...M Ballard
AVIATION...Thomas
MARINE...TS

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



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