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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 211931

Area Forecast Discussion
National Weather Service Honolulu HI
931 AM HST Mon Jan 21 2019

Stable conditions will prevail through Thursday as a surface
ridge remains just north or over the islands. While windward Big
Island will likely experience typical shower activity, most of the
state will be under a stable land and sea breeze pattern,
featuring afternoon clouds and a few showers over island terrain
and partial clearing at night. A front is expected to bring an
increase in showers Friday and Saturday, with cooler conditions
moving in behind it.


A stable southeast to south flow will produce limited showers today.
A passing North Pacific front, currently about 225 miles north of
Kauai, has pushed the subtropical ridge over Kauai. This is
resulting in moderate southerly winds over the Garden Isle and
lighter southeast to south background flow elsewhere. A ridge
aloft is maintaining stable conditions with an inversion around
5,500 ft, and precipitable water is near to just below seasonal
normal. A few showers have been forming in the convergent
southerly flow near Kauai, where shower chances will be a bit
higher today. Elsewhere, morning land breezes and mostly clear
skies will gave way to afternoon sea breezes with clouds and a few
spotty showers forming over island terrain and interior sections.
That said, rainfall should be minimal.

A stable and somewhat dry weather pattern will persist through
Thursday. As the front stalls and dissipates tonight, the surface
ridge will shift northward slightly. This will allow the gentle to
locally moderate background winds to become more easterly near the
Big Island and more southeasterly elsewhere. This subtle change
will not translate into a significant change in the weather. A
land and sea breeze regime will continue to produce clouds and a
few showers over island terrain and interior sections each
afternoon, followed by mostly clear conditions overnight and in
the morning hours. The exception will likely be over windward Big
Island and east Maui, where the more easterly flow will deliver
modest showers. A mid level ridge will remain overhead,
maintaining stable conditions.

The GFS and ECMWF models have come into better agreement on a weak
frontal passage for Friday and Saturday. Although this feature
looks to be weak, expect showers to increase, especially along
windward areas and north facing slopes. Cool northerly winds
behind the front may push dew points into the upper 50s, giving a
cooler feel to the air mass.


A mostly dry and stable atmosphere will continue over the main
Hawaiian islands today with a deep layer ridge over the region. A
front north of Kauai is expected to stall through the night. The
pressure gradient between the front and the ridge is producing
some stronger southerly winds over and near Kauai. Kauai's
proximity to the front also increases the possibility for shower
activity. Will continue to monitor wind speeds over this area
to determine if an AIRMET is necessary.

Elsewhere, winds will remain light with VFR conditions
prevailing. Some afternoon clouds and isolated showers are
possible over interior locations under the light wind regime.


Light to moderate southeast to south winds are expected to hold
through mid-week as a cold front stalls and diminishes north of
Kauai and the ridge remains near the islands. Pockets of fresh,
southeast breezes are likely in the typically windier areas, with
moderate to fresh southerly winds continuing today over the Kauai
waters. Elsewhere, land/sea breezes near the coasts are expected.
Another front may push through the islands toward the end of the
week with gusty northerly winds behind it into the weekend.

Surf along north and west facing shores will gradually rise
later today through Wednesday as moderate to large, overlapping
west-northwest swells fill in. This will translate to an extended
period of advisory-level surf for north and west facing shores,
late Tuesday into this weekend. The peak of the event is expected
Wednesday through Thursday with surf nearing warning levels for
north and west facing shores of the smaller islands. This direction
(300-320 deg) will also make it into the leeward Big Island coast
from Kona to South Point late Tuesday through midweek, likely nearing
warning levels there as well. The last reinforcement in the series
is expected Friday through Saturday, which may bring the surf back
to around warning levels Friday night. Surf will gradually lower
through Sunday.

A small increase in surf is expected along east facing shores
Wednesday into the second half of the week due to a pocket of
fresh to strong winds upstream of the state. Surf along south
facing shores will remain near the seasonal average through the
week with mainly background short-period southeast trade wind
swell energy.





AVIATION...M Ballard

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