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Hawaiian Islands Synoptic Discussion and Guidance

428
FXHW60 PHFO 190640
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
840 PM HST Wed Apr 18 2018

.SYNOPSIS...
High pressure north of the state will keep a moderate to breezy
trade wind flow in place through the middle of next week. An old
frontal band combined with an upper level trough moving through
the state, will keep showery weather and abundant cloud cover in
place through Friday, particularly over windward areas. Some of
the showers may be heavy at times and an isolated thunderstorm
will be possible as well. A more typical trade wind pattern will
return Friday night through the middle of next week, with showers
favoring windward and mauka areas.

&&

.DISCUSSION...
Currently at the surface, an east to west oriented ridge of high
pressure centered around 600 miles north of Kauai, is driving
moderate trade winds across the island chain this evening. Aloft,
an upper level trough is located around 350 miles west-northwest
of Kauai, with an upper level ridge a couple hundred miles to the
east of the Big Island. Infrared satellite imagery shows mostly
cloudy skies across the state this evening, with an extensive
shield of mid and high cloud extending well to the southwest of
the island chain. Meanwhile, radar imagery shows numerous showers
moving into windward areas in association with an old frontal
band, with a few showers spilling into leeward areas as well. Main
short term concern revolves around the potential for heavy
rainfall and flash flooding.

Tonight through Friday,
A ridge of high pressure will remain to the north of the state
through the period, maintaining moderate to locally breezy trade
winds across the island chain. The remnants of an old front
currently moving into the state will get hung up over the island
chain tonight, while an upper level trough approaches from the
west. This will result in a destabilization of the airmass with
precipitable water (PW) values climbing into the 1.5 to 1.7 inch
range by daybreak Thursday. The upper level trough will then shift
slowly southeastward through the island chain Thursday through
Friday, before exiting southeast of the Big Island by early Friday
evening. Deep tropical moisture will increase statewide on
Thursday with PW values climbing into the 1.6 to 1.9 inch range,
then hold at these levels through Thursday night. PW values will
then decrease from northwest to southeast on Friday, ranging from
1.2 inches near Kauai to 1.6 inches over the Big Island by early
Friday evening.

A fairly wet night is expected over windward areas, with some
showers reaching leeward areas as well. As the old frontal
remnants and upper level trough begins to interact on Thursday,
shower coverage and intensity is expected to increase statewide,
with showery weather continuing Thursday night. Shower coverage
and intensity is then expected to decrease from northwest to
southeast down the island chain on Friday as deep moisture
decreases and the upper level trough shifts eastward. Locally
heavy rainfall will be possible across the entire state Thursday
through Friday. As a result, the Flash Flood Watch remains in
effect from 6 AM Thursday through 6 PM Friday. Thunderstorm
probabilities will remain low through the period, but given the
unstable airmass and deep tropical moisture in place, an isolated
thunderstorm can't be ruled out.

Additionally, with temperatures below freezing and deep moisture
in place over the Big Island summits, heavy snow will be possible
at elevations above 12000 feet. As a result, a Winter Storm Watch
remains in effect for the highest elevations of Mauna Kea and
Mauna Loa from 6 AM Thursday through 6 PM Friday.

Friday night through Wednesday,
A ridge of high pressure north of the state will maintain moderate
to locally breezy trade winds Friday night through the weekend.
High pressure will then strengthen as it shifts to the northeast
of the state Monday through the middle of next week, allowing the
trade winds to strengthen to breezy and locally windy levels.
Overall, a fairly typical trade wind pattern is expected through
the period, with bands of enhanced clouds and showers drifting
through the islands from time to time. Windward and mauka areas
will remain the primary targets for showers, with an occasional
shower spreading leeward due to the strength of the trades. Model
time/height relative humidity cross sections show the mid and high
cloud shifting eastward with the upper level trough, so more sunny
conditions are expected to return to the island chain through the
period.

&&

.AVIATION...
Moderate trade winds will continue across the Hawaiian Islands
producing moderate low level mechanical turbulence over and
downstream of mountains for the next 24 hours. Low cloud ceilings
and showers will linger over mountain areas of all islands.

A band of unstable clouds and showers will enhance overnight shower
activity across all islands with MVFR ceilings along windward
slopes producing widespread mountain obscurations.

AIRMET Sierra in effect for Tempo Mountain Obscurations over north
through east facing slopes of all islands for elevations above
2000 feet.

AIRMET Tango in effect for low level mechanical turbulence for all
islands over and south through west of mountains below 7000 feet.
Another AIRMET Tango issued for upper level moderate turbulence
due to the jet stream between FL250 and FL350.

&&

.MARINE...
A high far northeast of the area will maintain locally strong
trade winds over the area into Thursday. Therefore, a Small Craft
Advisory (SCA) remains in effect for most of the waters
surrounding the islands of Maui County and the Big Island through
Thursday afternoon. The trade winds are expected to weaken
slightly by Thursday evening as the high moves farther east and a
front approaches from the northwest. Mariners should also be
aware that an upper-level trough moving over the area combined
with abundant moisture will bring the threat of locally heavy
downpours and thunderstorms through Friday. The latest forecast
continues to indicate the trade winds will strengthen again
starting Friday. As a result, a SCA will likely be required for
the typically windy waters adjacent to the islands of Maui County
and the Big Island from Friday into early next week.

Surf is expected to remain below High Surf Advisory (HSA) heights
along all shorelines through this weekend. Rough moderate surf is
expected to persist along most east facing shores through Sunday
night. Surf heights will increase again along east facing shores
early next week as stronger trades return. Surf may reach the HSA
threshold starting late Monday, and persist into the middle of
next week.

A moderate north-northwest swell arriving late Thursday is
expected to peak Friday, with surf remaining below the HSA
threshold. This swell will gradually subside through this
weekend. A slightly larger north-northwest swell is expected to
arrive Sunday, and peak Monday. This swell will gradually subside
from Tuesday through Wednesday.

A series of swells from the southern hemisphere will maintain
moderate surf along most south facing shores through this
weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 6 AM HST Thursday through Friday
afternoon for Niihau-Kauai-Oahu-Molokai-Lanai-Kahoolawe-Maui-
Kona-South Big Island-Big Island North and East-Kohala-Big
Island Interior.

Winter Storm Watch from 6 AM HST Thursday through Friday
afternoon for Big Island Summits.

Small Craft Advisory until 6 PM HST Thursday for Kaiwi Channel-
Maui County Windward Waters-Maalaea Bay-Pailolo Channel-
Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward
Waters-Big Island Southeast Waters.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Houston

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