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Hawaiian Islands Synoptic Discussion and Guidance

310
FXHW60 PHFO 211339
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
339 AM HST Thu Sep 21 2017


.SYNOPSIS...
The moderate to locally strong trades will be turning slightly
softer this evening as the surface high weakens far NNE of the
islands. Gentler trade winds will prevail through early next week
with clouds and showers favoring the windward and mauka areas.
The showers will be most frequent during the nights and mornings.

&&

.DISCUSSION...
The locally strong trades will be trending slightly softer this
evening as the surface of 1032 mb starts weakening while drifting
east, further away from the islands. The focus will change from
the high to its associated ridge currently located 800 miles north
of Kauai. This ridge will move south over the weekend to about
360 miles N of Kauai by early Tuesday morning. Moderate trade
winds will rule the region during this stretch, and is expected to
be strong enough to ward off or limit the daytime sea breezes.

The upper low located 700 miles NE of the Big Island this morning,
is forecast to remain stationary there through early next week.
The low may have deepened the marine layer to its current 16k feet
over the Big Island, but it has not really affected the islands
negatively. And none is expected as long it does not come any
closer to the islands. In fact, mid level ridging briefly builds
over the Big Island Saturday night, but weakens by Saturday
afternoon. Therefore, clouds will continue its presence over the
Big Island summits on a daily afternoon bases through early next
week.

The GFS has now picked up on the mid layer moisture between
15k and 20k feet over the Big island. The overcast patch which
covered the Big Island on last evening's satellite imagery, has
pretty much broken up, but there is more of it just south of South
Point, and can easily surge up over the Big Island at any time. A
short wave trough now approaching the Big Island from the N
should keep these clouds at bay or even disperse it as the day
progresses.

Precipitable water content is relatively moderate at best this
morning, at between 1.2 and 1.6 inches, Lihue and Hilo,
respectively, with the highest amount over the Big Island. Little
change is expected through early next week with no additional
influx.

Satellite imagery shows an area of broken low clouds with embedded
showers approaching the central islands. They will be affecting
Molokai and Maui first by 4 am, and Oahu between the 7 am and 9
am. The area is breaking up as it near these islands. These clouds
will bring a period of passing showers lasting a couple of hours.
A field of scattered low clouds will continue to push through
Kauai through the rest of the morning hours. The windward and
mountain areas will absorb most of these showers. More clouds and
showers appear headed for the Hamakua coast, likely making a
showery day there.

.AVIATION...
High pressure north-northeast of the State will continue to drive
a moderate to breezy trade wind flow across the islands through
tonight. Clouds and showers will favor windward and mauka areas,
with a stray shower reaching leeward locales from time to time.
Predominantly VFR conditions are expected at the terminals through
12Z Friday, but some brief MVFR cigs/vsbys will be possible in
windward areas as trade showers move through.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain of all islands. The AIRMET will likely
continue through the day on today.

AIRMET Sierra may be needed for mountain obscuration across portions
of the island chain later this morning.

&&

.MARINE...
Fresh to locally strong trades associated with 1032 mb high
pressure north-northeast of the state across the eastern Pacific
will hold through this afternoon before trending down over the
upcoming weekend and early next week. Strongest winds are expected
across the Pailolo and Alenuihaha Channels, Maalaea Bay and the
waters south of the Big Island through this time. Trades should
reach the light to moderate range by the end of the weekend as
high pressure shifts eastward and weakens in response to a frontal
boundary passing far north of the state. This pattern should hold
through the first half of the upcoming week (potential for
land/sea breezes for some leeward areas setting up Monday through
Tuesday).

A combination of moderate to strong breezes locally and upstream of
the islands south of high pressure will continue to generate rough
surf along east facing shores into Friday. Surf along east facing
shores will trend down over the upcoming weekend as the trades
weaken locally and upstream. Guidance suggests a weak or broad
surface trough developing east of the islands Friday due to a
broad upper low centered northeast of the state, which will weaken
the easterlies upstream of the state and lower the trade wind
swell.

Surf along south facing shores will remain small through early next
week with mainly background southeast and southwest swell energy
expected. The next potential long-period pulse out of the southwest
from the Tasman Sea will be possible by Sunday, which should lead to
a slight increase in surf. For the extended, strong- to gale-force
southwest winds associated with a low that has been impacting the
Tasman Sea over the past 24 hrs may lead to another small southwest
(210-220 deg) swell Tuesday through midweek.

A small northwest (310 deg) swell associated with broad and complex
low pressure that has been centered over the central Pacific near
the Date Line over the past couple of days is expected to fill in
locally Thursday and hold into the weekend before easing. The latest
surface analysis showed 989 mb low centered near the Aleutians
around the Date Line with a decent pocket of strong- to near gale-
force winds focused within the 320 to 330 deg band relative to
the islands. This may end up reinforcing what is left of this
weekend's 310 deg energy by Monday, before completely fading
Tuesday through Wednesday. As a result, a slight increase in surf
is expected along north and west facing shores beginning Thursday,
which should hold into early next week before dropping off.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Maalaea Bay,
Pailolo and Alenuihaha Channels, and waters south of the Big
Island.

&&

$$

Lau/Jelsema/Gibbs

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



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