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Hawaiian Islands Synoptic Discussion and Guidance

565
FXHW60 PHFO 181952
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
952 AM HST Fri Aug 18 2017

.SYNOPSIS...
Trade winds will decrease a bit late this weekend as a weakness
develops in the ridge to our north, allowing development of local
seabreezes across leeward areas. Leeward areas will see an
increase in clouds and showers each afternoon and evening, while
diminished trade flow will continue to supply both to windward and
mauka areas. Moisture from an old tropical cyclone will increase
shower coverage and bring muggy conditions to the islands late
Saturday through the first half of next week. Expect drier and
windier conditions to return late next week as the ridge rebuilds.

&&

.DISCUSSION...
Upper troughing along 150W is beginning to put a dent in the
surface subtropical ridge to our north. The ridge remains strong
enough to drive locally breezy trade winds across local Hawaiian
waters this morning, but this will gradually change over the next
few days. Satellite loop shows scattered to patchy broken low
clouds embedded within trade flow moving across the islands, with
little change noted upwind as far as 150W. Radar shows only
isolated showers moving across the islands within this cloud
cover, with an area of scattered showers noted moving west
southwestward through the Alenuihaha channel. Overnight soundings
showed a dry and stable airmass, with less than 1.25 inches of PW
and positive lifted index values. Strong subsidence inversions are
noted between 6000 and 7000 feet. It seems that the upper trough
to our north has yet to raise the inversion or destabilize our
airmass.

Models show the upper trough to our north will dig southwestward
over the next few days, placing the base of the trough over the
islands, and the trough axis near Kauai and Oahu, by Sunday. This
roughly coincides with the arrival of an influx of moisture from
now-defunct Jova and a general decrease in background trade flow.
While trade flow may rebound slightly Monday and Tuesday around
the Big Island, the overall statewide result will likely be
increased trade shower coverage across windward and mauka areas,
the introduction of afternoon seabreeze clouds and showers across
leeward areas, and an overall muggy feel to the air from late
Saturday through Tuesday. GFS wants to bring enough cold air down
aloft to trigger a thunderstorm threat across the western islands
on Monday, but ECMWF brings this cold air down too far west of
the main islands to be threatening during the same time frame. We
will keep thunderstorms and heavy rainfall out of the forecast
for now, but will reconsider if and when model solutions converge.

By Wednesday, both models show the subtropical ridge recovering,
with trade winds increasing back into the moderate to locally
breezy range once again. Drier conditions will move in as well.

&&

.AVIATION...
A ridge of high pressure north of the state will keep a moderate
to locally breezy trade wind flow in place through tonight, with
clouds and showers favoring windward and mauka areas. Brief MVFR
cigs may affect windward areas as showers move in with the trades,
particularly during the overnight and early morning hours.
Predominantly VFR conditions are expected at the TAF sites through
18Z Saturday.

AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of the mountains of all islands. This AIRMET
will likely remain in effect through the day.

&&

.MARINE...
A Small Craft Advisory (SCA) for Big Island and Maui County waters
will remain up through this afternoon due to locally strong trade
winds. Winds are forecast to subside through the weekend as a
trough aloft moves over the area. The trough may make showers more
active. Wind speeds will increase again next week.

Surf along south facing shores will rise this morning, then peak
later today due to a swell generated in the Southern Hemisphere.
Surf is expected to remain below the High Surf Advisory threshold.
Surf will gradually subside over the weekend.

The peak high tides for the month will occur over the next few days.
Water levels are running around .8 ft above the predicted levels due
to an eddy moving westward through Hawaiian waters. Coastal flooding
is possible around the times of the high tides through the
weekend. See the Special Weather Statement for more details.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Powell
AVIATION...Jelsema
MARINE...Donaldson

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



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