Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 192142

Area Forecast Discussion
National Weather Service Honolulu HI
1142 AM HST Mon Mar 19 2018

Convection continues to develop off to the south of Maui County
and to the east of the Big Island this morning. The convection
will likely continue into the afternoon hours as a shortwave
trough tracks across the state. As a result, have updated the
forecast to include a slight chance of thunderstorms for the
coastal waters around the Big Island and south of Maui County for
the afternoon hours. Given some sunshine breaking through the
clouds over the Big Island, daytime heating combined with
instability aloft may be enough to spark a thunderstorm or two
this afternoon, so have added this to the forecast here as well.


Trade winds will increase to locally breezy levels by midweek,
then ease toward the weekend as high pressure strengthens to our
north before getting edged out by a front approaching from the
west. Trade winds will push clouds and showers across the
islands, focusing rainfall mainly across windward and mauka
areas. Rainfall will increase late in the work week and over the
upcoming weekend as the front passes eastward through the islands.


.PREV DISCUSSION.../Issued 956 AM HST Mar 19 2018/

Currently at the surface, a 1040 mb high is located around 1850
miles north of Honolulu, with the ridge axis extending southward
from the high, and running roughly northwest to southeast around
350 miles northeast of the state. The resulting gradient is
driving moderate trades across the island chain this morning.
Satellite imagery shows mostly cloudy skies in place across the
state, with radar imagery and rain gauge observations indicating
numerous to widespread shower activity continuing over windward
areas. Main short term concern revolves around rainfall trends
over the next 12 to 24 hours.

Today and Tonight,
A pair of strong highs to the north of the area will merge into a
1045 mb high just south of the Aleutian islands by Tuesday
morning. This will shift the band of showery low clouds affecting
windward areas slowly southward, with improving conditions
expected by this evening across Kauai and Oahu, and by Tuesday
morning across Maui County and the Big Island. A shortwave
impulse will pass overhead from west-northwest to east-southeast
this afternoon into this evening, and this may also lead to a few
showers in leeward areas as it moves through. Moderate trade winds
are expected to prevail through the period.

Tuesday through Thursday night,
Model solutions are in good agreement showing mid and upper level
ridging holding in place during the mid week time frame. The
source airmass through this period is over the far northeast
Pacific and will have advected southwestward over the islands due
to the large high north of the state and the persistent low to the
northeast. As a result, we expect a stable weather regime over
the island chain through the period, with dewpoints generally in
the upper 50s and lower 60s. This will greatly limit shower
activity across the state, with spotty light showers primarily
confined to windward areas. Moderate trade winds will strengthen
on Tuesday, and reach locally breezy levels Wednesday through

Friday through next Monday,
The 12Z runs of the GFS and ECMWF are in reasonably good
agreement showing shortwave energy digging to the west of the
state Friday and Friday night, with a surface low developing near
or over the islands along a lingering surface trough over the
weekend. The surface low is then expected to lift northeast and
away from the state early next week.

As for sensible weather details, deep tropical moisture is
expected to begin advecting over the western islands on Friday,
then shift slowly eastward Friday night and over the weekend
resulting in a wet conditions statewide. Conditions should begin
to improve early next week as the surface low pulls northeast and
away from the state, dragging the deepest moisture east of the
island chain. There is the potential that heavy rainfall and flash
flooding could affect all or portions of the state over the
weekend, but it remains too early to tell how much rainfall will
occur or where flooding, if any, would threaten. This will
continue to be monitored closely as the week progresses. The wind
forecast will be a bit problematic as it will depend greatly on
the location, track and intensity of the surface low late week
through the weekend. Will take a blended approach to the winds for
the time being, although there will likely be some significant
changes as details become more clear.

A surface high far N of the area will maintain NE winds near the
surface over the main Hawaiian Islands. These winds will continue
to push showery low clouds from the trailing end of a front over
the islands. MVFR ceilings will likely be extensive enough to
require AIRMET SIERRA for MTN OBSC to remain up through today for
NE sections of most islands.

A sharp short-wave trough aloft about 400nm NW of Kauai is
forecast to pass north of the main Hawaiian Islands today. This
feature may make showers more active along the old frontal band
and could possibly even produce isolated thunderstorms.


Small Craft Advisory until 6 AM HST Tuesday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha



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