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Hawaiian Islands Synoptic Discussion and Guidance

442
FXHW60 PHFO 130138
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
338 PM HST Tue Nov 12 2019

.SYNOPSIS...
Light trade winds will prevail through tomorrow, then shift out
of the south Thursday and Friday as a cold front advances toward
the state. Light showers will favor the typical windward slopes
into Thursday, though afternoon sea breezes may produce an
isolated showers over interior sections. Rain chances will trend
up this weekend as the front approaches the western end of the
state and a weak trough moves in from the east.

&&

.DISCUSSION...
A stable and weak trade wind flow will persist through tomorrow.
The subtropical ridge driving the trades remains weakened and
displaced southward to about 225 miles north of Kauai, where it
will generally remain through tomorrow. As a result, trades will
be light enough to be overpowered by local overnight land breezes
and afternoon sea breezes. Modest showers will favor windward
slopes, while afternoon sea breezes produce clouds and a few
showers over interior sections. A ridge aloft is producing rather
stable conditions with a strengthening inversion noted in the
afternoon soundings. For tonight, expect windward and north
facing slopes of Kauai to experience scattered light showers,
while little shower activity is anticipated elsewhere.

Winds will gradually veer southerly on Thursday and Friday as a
front advances toward the state. A somewhat dry and stable land
and sea breeze pattern should persist on Thursday, with spotty
afternoon showers favoring interior sections. Shower chances will
increase on Friday over portions of the island chain. Expect
prefrontal showers to develop ahead of the front near Kauai, and
a trough moving in from the east may provide a boost to rainfall
over the Big Island later in the day.

Wetter, and possibly unsettled, conditions are expected during
the weekend, though details remain somewhat uncertain. The front
will stall and dissipate near Kauai, likely providing an increase
in showers for the western end of the state. Meanwhile, moisture
associated with the surface trough will spread westward, producing
increased rain chances for the Big Island and Maui. An upper
level trough will dig toward the state on Saturday and possibly
form a cutoff low overhead on Sunday. This will likely destabilize
the atmosphere and could lead to heavy showers and thunderstorms.
Given uncertainty in the long range regarding the position and
strength of the upper low, we have decided to hold off on
introducing heavy rainfall into the forecast at this time.

Unsettled conditions could into early next week as trade rebuild.
The GFS and ECMWF show the upper low slowly filling and drifting
westward, raising the possibility that unsettled conditions could
continue.

&&

.AVIATION...
Kauai continues to be the winner in terms of clouds and showers
today thanks to increased low level moisture in the vicinity.
Elsewhere, sea breezes have taken over across the island chain
this afternoon with interior cloud build ups becoming more evident
on visible satellite. These have not produced much in the way of
shower activity, but some isolated showers will remain possible
before land breezes clear things out overnight. Brief periods of
MVFR conditions are possible with heavier showers that do develop,
but VFR is expected overall.

There are no AIRMETs in effect.

&&

.MARINE...
Light easterly winds will prevail through Wednesday as a weak
area of high pressure passes eastward just north of the area.
Light southerly flow will gradually develop later in the week as a
front approaches from the northwest. Winds are expected to swing
around to a more northerly direction on Saturday as the front
moves over portions of the forecast area. Wind speeds will remain
below Small Craft Advisory levels through Saturday. Winds are
expected to swing around to the east on Sunday and may become
strong enough to reach Small Craft Advisory levels over the
typically windy waters around Maui County and the Big Island.

The current northwest swell will continue to lower tonight and
Wednesday. A larger north northwest swell is expected to arrive on
Thursday, peak Thursday night, then lower gradually Friday and
Saturday. A High Surf Advisory will likely be issued for this
swell for most north and west facing shores.

A larger northwest swell is expected to arrive on Saturday, peak
Saturday night, then lower gradually Sunday and Monday. Surf
heights may reach low end warning levels during the peak of this
swell.

A series of small southerly swells are expected through the
remainder of the week and on through the weekend. Surf will remain
small along east facing shores due to the light winds over and
upstream of the state. Surf heights will likely rise by early next
week as high pressure build in to our north.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Wroe
AVIATION...Dye
MARINE...Burke

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



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