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Hawaiian Islands Synoptic Discussion and Guidance

568
FXHW60 PHFO 060629
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
829 PM HST Sun Apr 5 2020

.SYNOPSIS...
A low to our northwest and a displaced ridge to our east will
keep winds light across the islands through the week. Showers
will favor mauka areas during the day and areas near the coast at
night. Increased moisture will slide northward tonight through
Tuesday, bringing the potential for locally heavy downpours and a
few thunderstorms.

&&

.DISCUSSION...
Currently at the surface, a 994 mb low is located around 950
miles northwest of Kauai, with a high pressure ridge axis around
1000 miles northeast of Hilo. These features have left a weak
pressure gradient across the islands, with sea breezes beginning
to transition to land breezes. Infrared satellite imagery shows
partly to mostly cloudy conditions across the state. Radar
imagery shows some decaying showers over the islands, with showers
increasing over the coastal waters off to the south and southwest
of Kauai. Main short term concerns revolve around the potential
for heavy rainfall and thunderstorms during the next couple days.

The area of low pressure northwest of the islands will gradually
lift northeastward and further away from the state during the next
couple days, while the ridge of high pressure holds in place
northeast of the islands. A weak frontal boundary will move into
the western islands Monday night, then gradually weaken as it
shifts slowly eastward across the island chain, and dissipate over
the eastern islands by late in the week. This will keep a weak
pressure gradient in place across the state through the period,
with daytime sea breezes and overnight land breezes dominant.

As for weather details, showers will continue to favor areas near
the coast at night, and locations over the islands during the day
through the next week. Some higher precipitable water values in
the 1.5 to 1.7 inch range will get drawn northward over the
islands tonight through Tuesday. This in combination with some
shortwave energy moving overhead will lead to an increase in the
coverage and intensity of showers statewide, with some locally
heavy rainfall and even a few thunderstorms possible. The latest
model trends suggest that shower coverage will decrease over the
western islands Wednesday through late in the week as the deepest
moisture associated with a slow moving and weakening/dissipating
frontal boundary shifts into Maui County and the Big Island.
Another weakening cold front may move into the western islands
next weekend.

&&

.AVIATION...
A large low pressure system with a low level trough north of the
islands will keep humid and unsettled weather in the forecast for
the next few days. A light background south to southeasterly wind
flow will continue to allow land and sea breezes to form over
most airfields with above normal variability in wind directions
over the next two days.

Mountain obscurations with low cloud ceilings and showers may
develop during this time period. A subtropical jet stream aloft
will also keep upper level turbulence over the islands.

AIRMET Sierra remains in effect for portions of Maui, Lanai and
the Big Island.

AIRMET Tango remains in effect for tempo moderate turbulence
between FL230-300 across the state.

&&

.MARINE...
The current light to locally moderate wind regime is expected to
persist through most of this week. The overall background flow
will generally be from the south as a boundary slowly approaches
the area from the northwest. As it approaches, a broad area of
convergent southeast to south flow is expected to set up over the
state tonight, extending through Tuesday. This will likely result
in a period of unsettled weather with locally heavy rainfall, and
possibly thunderstorms. With seas and swells expected to stay
below Small Craft Advisory thresholds for much of the upcoming
week, the main threat to mariners will be associated with any
thunderstorm development.

The current north-northwest swell will continue to decrease
tonight. As this swell lowers, a new 3-to 4-foot long-period
west-northwest swell will slowly begin to fill in tonight.
Observations from the PacIOOS Hanalei buoy have just started to
show small long-period forerunners arriving. This swell should
continue to fill in tonight and peak Monday night. This swell will
be followed by a slightly larger northwest swell arriving late
Tuesday. This second swell is expected to peak Tuesday night or
early Wednesday. Surf is expected to remain below the High Surf
Advisory criteria along north and west facing shores with these
swells.

A series of small south to south-southeast swells are expected
during the upcoming week. Surf is expected to reach the summer
average by Tuesday, followed by another small pulse later this
week. Surf along east facing shores will remain small through the
upcoming week due to the lack of trades.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Kino

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



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