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Temp-0.9 C
RH100 %
WindSW 13 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 030159

Area Forecast Discussion
National Weather Service Honolulu HI
359 PM HST Thu Apr 2 2020

Winds will decrease and veer southeasterly tonight as deep low
pressure intensifies far north-northwest of the state. Isolated
showers will diminish early this evening as land breezes develop.
High clouds will move up from the southwest and shower chances
will increase over the western end of the island chain late
tonight. Winds will turn more southerly on Friday and the weekend,
drawing up deeper moisture that will likely fuel increased shower
activity and possibly a few thunderstorms. Wet conditions are
expected to linger over portions of the state through the first
part of next week.


A somewhat unstable east to east-southeast low level flow is
producing spotty showers across mainly island interior sections
this afternoon. Low level background winds are decreasing and
veering due to deep low pressure developing far north-northwest of
the state. A broad upper level trough covers much of the central
north Pacific and is maintaining somewhat unstable conditions with
essentially no inversion according to the afternoon soundings and
recent aircraft data. For now, low level moisture remains below
April normal, and afternoon sea breezes have managed to trigger
only spotty, mainly interior showers, a few briefly heavy.

The deep low will continue to drop toward the islands tonight.
This will cause winds to continue to shift out of the southeast
and allow land breezes to clear out most areas this evening. Later
tonight, the deep low will cause the subtropical jet stream to
lift northward over the state, leading to increasing high clouds
and possibly generating showers over waters to the southwest of
the islands. Some of these showers could reach as far north as
Kauai and Oahu late.

As the deep low settles about 850 miles northwest of the state on
Friday, shower chances will increase as south to southeasterly
winds draw deeper moisture over the islands. The state will be
under the outer portion of the upper level trough surrounding the
low with the subtropical jet stream overhead, possibly leading to
enough instability to trigger isolated heavy showers and a few
thunderstorms, mainly during the weekend. As the low edges closer
to the state on Sunday, shower chances should increase, especially
in the moist, convergent flow across the western end of the
state. In addition, the southerly flow will push dew points up to
around 70, bringing an increase humidity.

Early next week, area of convergence on the flank of the low will
push to the east. 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 likely
experience active showers and no trade winds through the middle
of next week.


A trough to the northwest of Kauai will continue the light flow
pattern across the islands through the remainder of this
afternoon. Sea breezes will limit shower activity to the island
interiors. Elsewhere, expect partly cloudy skies and VFR
conditions. Sea breezes should dwindle by sundown, leaving light
and variable winds overnight with isolated showers.

No AIRMETS are in effect at this time.


A surface high far northeast of the area and a surface trough
northwest of Kauai will continue to produce gentle to locally
fresh east to southeast winds across Hawaiian waters through
tonight. A front slowly approaching the state Friday will cause
the wind direction to veer slightly more toward the south on
Friday and over the weekend. Over the western coastal waters,
winds are expected to become southerly by Friday. Meanwhile over
the eastern coastal waters, winds will remain out of the
southeast, but 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

Over the weekend and into early next week, the front or resulting
convergence zone is expected to stall somewhere over the western
coastal waters. This will bring the chances for heavy rain and
possibly a few thunderstorms over the area waters.

A new NNW swell building Saturday will produce moderate surf
along north and west shores over the weekend, but is expected to
remain below the advisory threshold. As the weekend swell
subsides, a west-northwest swell will slowly build, peaking
around Monday night followed by a northwest swell on Wednesday.
Surf from these swells 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 continue to
drop tonight through Friday. A small south-southeast swell is
possible early next week. Surf along east shores will slowly
subside as trade winds weaken.






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