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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 241403 CCB
Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
403 AM HST TUE MAY 24 2016
The trade winds will give way to a hybrid mix of light trades
with daytime sea breeze and night time land breeze for the second
half of the week. An upper level trough will move through the
islands wednesday and help boost the afternoon and evening
showers. The mugginess level will also rise under this light wind
The weather will be changing during the next 18 hours as a surface
trough forms around Kauai later today. The trough will alter the
wind flow that will result not only in lighter winds but winds with
a slight south of east component for most of the islands. At the
upper levels, a short wave trough will be approaching the islands
from the northwest. The combo of this light wind and approaching
trough, will lead to a spat of enhanced showers for the Koolau
Mountains and windward communities of Oahu this early evening.
Otherwise...land breeze will likely take over the lee side of
the smaller islands leading to some fair weather there. Over on
the windward side...there is enough strength in the easterly wind
flow to carry in a few showers.
On Wednesday, we are anticipating maximum convergence over all the
islands. The combination of the convergent sea breezes along,
daytime heating, and ample low level moisture, will result in some
scattered convective showers. As per GFS and ECMWF, the upper level
disturbance, accompany by a pool of marginally cold air of minus 9
to 11 degree C, will help enhance a few of these showers to locally
heavy. This language is not yet part of the forecast due to its
marginal status but will be included if the models continues at
this temperature range or colder.
The air mass will regain some stability after the passage of the
upper level disturbance on Thursday. The surface winds will tether
between east and slightly south of east, or both, as we head into
the weekend at light to moderate speeds, with the lightest winds
over on kauai. At this strength, local sea breezes will result
in afternoon and evening clouds and spotty showers over the
interior and leeward areas. At night, the windward areas may receive
a stray shower or two due to the background easterly winds. But
other parts of the islands may have an off shore breeze leading to
fair skies there.
Depending on which model you choose, the trade winds will be
disrupted once more on Sunday, as per GFS, when it gives way to
light southerly winds. This is due to a front approaching the
islands from the northwest. The ECMWF has a different solution
by keep the front just out of reach of the islands and thus
maintaining an easterly wind flow Sunday through Monday. The GFS,
on the other hand, has the frontal boundary moving down the island
chain Monday and Monday night. If we take a blend of these two
models, which is preferred solution, the wind turns southerly
Sunday, then backs to the east Monday night as light trades. There
is an upper level trough accompanying the front. The dynamics,
however, are not there. Instead of a pool of cold air, both the
ECMWF and GFS are indicating warm temps of minus 3 to 4 degrees C
at 500 MB. The GFS's RH cross section reveals moisture depth to 12
to 15k feet. All highly questionable forecast for Memorial Day at
this point. Almost a sure bet, though, the winds will be light and
rather humid on Monday.
Currently, the central islands consisting of Maui County and Oahu...
are in an area of rather dry air and minimal showers. There is a
small pocket of enhance moisture about to reach Kauai that will give
the island a dosage of showers for a good part of this morning. The
afternoon hours may have more showers especially on the western half
of Kauai as remnant moisture mixes in with day time heating.
The leading edge of a low cloud field is now affecting windward
Big Island. There are other shower bearing clouds advancing westward
toward Maui. However, they are finding this hard as they appear to be
dissipating as they near Hana. Never the less, we are maintaining
scattered showers for the windward and mauka areas of the smaller
islands through out the day today. The Kona slopes will have their
usual afternoon and evening clouds with scattered showers.
Clouds and showers will continue to favor windward and mountain
areas under a weakening and veering light to moderate trade wind
regime. Some showers at times will likely carry MVFR conditions to
the smaller islands. AIRMET Sierra for mountain obscuration has
been posted from Laupahoehoe to Apua Point, including Hilo, on the
Big Island. Showery low clouds will continue to stream over the
Big Island likely into mid-morning. PHTO will see a range of
conditions from VFR to IFR throughout this time. Most other
terminals should remain VFR.
Sea breezes along the leeward areas on the Big Island will
introduce convective showers along the slopes this afternoon.
Expect periods of MVFR conditions to be embedded within these
Showers along windward areas are forecast to increase later
tonight as enhanced moisture and a surface trough move into the
area, bringing more prolonged periods of lowered cigs/vis.
There are currently no marine advisories or warnings in effect for
the Hawaiian waters now, and none is expected through Sunday.
The winds will remain below SCA level for a few days, at least
Small surf will dominate on most shores the rest of this week.
The one exception is a small to moderate southwest swell that will
build on Thursday, peak Friday, then decline during the weekend.
This swell will originate from the Tasman Sea, which tend to be
inconsistent for the Hawaiian Islands. This is due to blockage
from other Pacific islands while enroute to Hawaii. Please refer
to the Collaborative Nearshore Swell and Wind Forecast (SRDHFO)
for more details.
Public and Marine...Lau
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office