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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 140145

Area Forecast Discussion
National Weather Service Honolulu HI
345 PM HST Mon Jul 13 2020

Trade wind speeds will fluctuate somewhat Tuesday and Wednesday, but
likely remain locally gusty in the afternoons. Winds will weaken
later Wednesday, then remain on the lighter side for several days
before strengthening by Sunday. The trade winds will deliver periods
of clouds and showers that will primarily favor windward areas
during nights and mornings, but the lighter winds will allow some
afternoon clouds and showers to develop over leeward areas.


Trade wind speeds will fluctuate between moderate to locally breezy
Tuesday and Wednesday, then trend toward the lighter end of the
spectrum from Thursday into Saturday, with wind speeds then
increasing by Sunday.

Latest guidance supports much of the ongoing forecast philosophy,
with some slight changes through the short term. These include the
overnight arrival of a showery cloud area that will fuel an increase
in windward showers through tomorrow morning, and a slower trend
toward weaker winds. While these are not especially impactful
forecast trends, they "cloud" the picture as to the extent of
afternoon leeward convection that can be expected on the smaller
islands the next 2 days.

Although trade winds are expected to diminish slightly tonight into
Tuesday, forecast models now indicate a slight increase in wind
speeds later Tuesday into Wednesday. Winds will diminish and veer
toward the ESE later Wednesday into the weekend, even though a
surface high pressure cell far NE of the islands will move/change
very little. Instead, the decrease in winds will be due to weak low-
level features passing between the islands and the high, acting to
loosen the low-level pressure gradient.

A weak low-level trough to the N of the islands (associated with a
low aloft) will move W and sharpen to the NW of the islands from
Tuesday into Thursday. A band of showery low clouds that appears to
be loosely related to this feature will traverse the islands tonight
through early Tuesday, bringing windward showers. Additionally, a
trough marking the remnant of TS Cristina will move between the
islands and the high later in the week, while a front will dig
toward the islands from the NW. These features will act to keep the
flow over the islands veered and diminished for the second half of
the week.

The lighter winds will allow afternoon leeward sea breezes to drive
cloud and shower formation over leeward areas that will diminish
after sunset. Warming mid-level temperatures are expected as the low
aloft to the N moves W the next couple of days, with a stabilizing
mid-level ridge prevailing into the weekend. Shower coverage and
intensity should be limited as a result, especially since most of
the moisture associated with the remnants of Cristina will pass N of
the islands. As the low aloft moves to a position NW of Kauai, it
will continue to deliver some periods of high clouds. Latest
guidance has the bulk of the high clouds over Kauai and Oahu,
potentially becoming thick at times from Tuesday into Thursday.


A high pressure system far to the north northeast will drive
moderate to fresh trade winds across the state through at least
Tuesday. Passing clouds and showers will impact mainly windward
portions Kauai this evening with brief mountain obscurations
possible. Incoming showers expected to move in across the other
islands later tonight. Mountain obscurations, occasional MVFR
ceilings and visibility are expected around these showers. AIRMET
Sierra will be likely for some windward sections later tonight as
the showers fill in.

AIRMET TANGO remains in effect below 8000 feet for south through
west of mountains on all islands for tempo moderate turbulence due
to trade wind speeds.


High pressure far north-northeast of the islands will maintain
moderate to locally fresh trades through the middle of the week. A
slight decrease of trade winds is expected tonight, so the Small
Craft Advisory (SCA) for the windier waters around Maui County and
the Big Island has been cancelled. A slight increase of trades
Tuesday afternoon may bring a brief return of SCA winds for these
areas. A weak disturbance passing north and northeast of the area
Thursday and Friday should weaken the trade winds down to light to
moderate speeds by Friday. Trades should then ramp back up to
locally strong levels next weekend as high pressure strengthens far
to the north of the island chain.

Overall, no significant swells are expected through the forecast
period. Small background surf will continue along south facing
shores over the next several days. A small south-southwest swell is
possible over the weekend. East facing shores will continue to
experience small short period surf produced by the trade winds
locally and upstream of the state. An increase is expected over the
weekend as the breezy trade winds return. Surf along north facing
shores will remain nearly flat through the period.






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