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Hawaiian Islands Synoptic Discussion and Guidance

771
FXHW60 PHFO 022000
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST Thu Apr 2 2020

.SYNOPSIS...
Deep low pressure developing far northwest of the state will
cause trade winds to veer out of the east-southeast today,
leading to a somewhat unstable land and sea breeze regime that
will produce spotty showers, some briefly heavy, over the islands
this afternoon. Winds will turn more southerly on Friday and the
weekend, likely drawing up deeper moisture that will fuel
increased shower activity and possibly thunderstorms. Wet
conditions are expected to linger over portions of the state
through the first part of next week.

&&

.DISCUSSION...
Mostly clear and rather dry conditions prevail across much of the
state this morning, but as sea breezes overpower a shifting east
to east-southeast trade wind flow, spotty showers will develop
this afternoon over mainly interior sections. Trade winds remain
rather weak due to deep low pressure developing far north-
northwest of the state and weak surface troughs passing just north
of the islands. A broad upper level trough covers much of the
central north Pacific and has essentially wiped out the inversion
according to the overnight soundings and recent aircraft data. As
the deep low continues to drop southward toward the islands today,
background winds will shift more southeasterly, especially around
Kauai and Oahu, and sea breezes will take over. This will produce
interior clouds and spotty showers, a few briefly heavy, this
afternoon that will diminish rapidly toward sunset.

As the deep low settles about 850 miles northwest of the state on
Friday, shower chances will increase as southeasterly winds draw
deeper moisture over the islands. The state will be under the
outer portion of the upper level trough associated with the low,
possibly leading to enough instability to trigger isolated heavy
showers and a few thunderstorms, mainly during the weekend. As a
front associated with the low edges closer to the state on Sunday,
shower chances should increase, especially across the western end
of the state. In addition, the southerly flow will push dew
points up to around 70, bringing an increase humidity that we have
not experienced for a while.

Early next week, the front will likely morph into a convergence
band and push across portions of the island chain. The latest runs
of the GFS and ECMWF show this feature maintaining wet conditions
for the western end of the state on Monday but have diverged on
motion of the convergence band on Tuesday and Wednesday.
Confidence is therefore low in the extended forecast, though
portions of the island chain will experience active showers
through the middle of next week.

&&

.AVIATION...
A trough to the northwest of Kauai will allow for a light flow
pattern across the islands again today. Thus, sea breeze
development will lead to shower development this afternoon mainly
over island interior and areas of elevated terrain. Sea breezes
will diminish late this afternoon, along with must shower
activity.

No AIRMETS are in effect at this time.

&&

.MARINE...
A surface high far northeast of the area and a surface trough
northwest of Kauai is producing gentle to locally fresh east to
southeast winds across Hawaiian waters today. As this trough
strengthens later today into tonight, winds will veer further
toward the south. Over the western coastal waters, winds are
expected to become southerly by early Friday. Meanwhile over the
eastern coastal waters, winds will become southeast and is
expected to strengthen on Friday. Winds could even reach Small
Craft Advisory speeds tomorrow over the Big Island windward waters
and Big Island leeward waters off South Cape.

Over the weekend, the trough is expected to shift further east and
will bring unsettled weather over the Hawaiian waters. This
unsettled weather pattern looks to continue into the first half of
next week.

The current small northwest swell will continue subsiding through
tomorrow night. A new swell building Saturday will produce moderate
surf along north and west shores over the weekend, but is expected
to remain well below the advisory threshold. As the weekend swell
subsides, another northwest swell will slowly build, peaking around
the middle of next week. Surf from that swell will also remain below
the advisory level. Swells from the southeast Pacific are partly blocked
by the islands of French Polynesia, but our current southeast swell
is coming through a narrow swell window. This swell will maintain elevated
surf on the south shores through tomorrow, but surf will remain below
the advisory threshold. Surf along east shores will slowly subside as
trade winds weaken.

&&

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

&&

$$

DISCUSSION...Wroe
AVIATION...Thomas
MARINE...Kino

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



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