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Hawaiian Islands Synoptic Discussion and Guidance

922
FXHW60 PHFO 061343
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
343 AM HST Mon Apr 6 2020

.SYNOPSIS...
A low to our northwest and a ridge of high pressure to our east
will keep winds light through the week. A weakening frontal
boundary moving into the islands will bring some locally heavy
downpours and a few thunderstorms today through Tuesday. Scattered
showers will continue across the state through the remainder of
the week, as the frontal boundary slowly dissipates over the
central and eastern islands. Showers will favor mauka areas during
the day and areas near the coast at night Wednesday through the
weekend.

&&

.DISCUSSION...
Currently at the surface, a 993 mb low is located around 1000
miles northwest of Kauai, with a high pressure ridge axis around
900 miles northeast of Hilo. Meanwhile closer to home, a weak
frontal boundary is located around 50 miles west of Kauai. These
features have left a weak pressure gradient across the islands,
with light southerly flow in place over Kauai and Oahu, and land
breezes dominant across Maui County and the Big Island. Infrared
satellite imagery shows variably cloudy conditions across the
state. Radar imagery shows numerous showers across Kauai, with
isolated to scattered showers over the other islands. 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 tonight, 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 in
most areas.

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 week. Some higher precipitable water values in the
1.5 to 1.7 inch range will move over the islands today through
Tuesday. This in combination with some shortwave energy moving
overhead will lead to an increase in the coverage and intensity of
showers statewide, which is already evident across Kauai early
this morning. Some locally heavy rainfall and even a few
thunderstorms will be possible at times through Tuesday. 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 over the 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. 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.
Expect increasing shower trends with potential for isolated
thunderstorms through Tuesday. Mountain obscurations will develop
over the islands as low clouds and showers move through the
region. A subtropical jet stream aloft will also keep tempo
moderate upper level turbulence across the state this morning.

AIRMET Sierra remains in effect for tempo mountain obscuration
for portions of Kauai, Lanai, Maui, and the Big Island. This
AIRMET may expand later this morning as additional showers
develop over Oahu and Molokai.

AIRMET Tango remains in effect for tempo moderate turbulence
between FL230-340 across the state. This AIRMET will diminish in
coverage by this afternoon.

&&

.MARINE...
A front located about 40 nm west of Kauai will slowly make its
way through the Hawaiian waters over the next few days, then stall
near the eastern waters during the second half of the week. This
will lead to a light to moderate wind regime with a southerly
flow expected ahead of the front and a westerly flow behind the
front.

Latest satellite imagery is showing a broad area of convergent
southerly flow near Kauai with isolated thunderstorms developing
over the northern waters. As a upper level short-wave passes
through the area, we should see an increase in showers and
possibly thunderstorms over the area today through Tuesday. 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.

Fore-runners from a small long-period west-northwest swell are
starting to register at the near shore buoys, but it looks to be
arriving slower than guidance. This swell should fill in through
the day today and peak tonight with a swell height of 4 to 5 feet.
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 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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