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Hawaiian Islands Synoptic Discussion and Guidance

939
FXHW60 PHFO 230154
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
354 PM HST Fri Feb 22 2019

.SYNOPSIS...
A surface trough near the Big Island will support increased
clouds and chances of showers over the eastern end of the island
chain through the weekend. Meanwhile, weak high pressure to the
northwest will support mostly dry weather over Kauai and Oahu. A
cooler northerly flow is expected across the state next week.

&&

.DISCUSSION...
The Big Island remains in the vicinity of a weak surface trough.
At the upper levels a zonal west to southwest flow is
transporting the higher level clouds off to the east and
northeast, while light winds at the surface are allowing sea
breezes to dominate.

The upper level jet has weakened enough to allow the winds to
weaken on the Big Island summits below wind advisory levels. Wind
speeds should remain below advisory tonight, but slight changes
in the jet position and strength over the next few days may
necessitate future wind advisories.

Expecting continued showery conditions around the eastern side of
Big Island into the evening. Instability is marginal for deep
convection and a few heavy showers, but also can't rule out a
slight chance for a thundershower in those same areas this evening.

This overall pattern appears more or less locked in through early
Sunday. An axis of higher humidity remains stuck near the Big
Island, and occasional upper level shortwaves bringing potential
bursts of deep convection and locally heavy rainfall Saturday and
Sunday to mainly the eastern half of Big Island. The western
portion of the state remains under a much drier and more stable
airmass during the weekend for quieter weather. The surface trough
begins to nudge east of the Big Island by Sunday afternoon as
light northerly flow starts to build in from west to east.

Medium range ECMWF and GFS guidance still shows another trough
developing far northeast of the state by the middle of next week.
This would put the state in a cooler northerly flow, but without
much moisture or dynamical support for rainfall. A possible
increase in showers may be felt Tuesday and Wednesday, if a
weak cold front reaches the state, but this feature may not make
it this far south intact. On the other hand, if subsequent
guidance shifts the trough and front closer to the state, that
would increase the threat for gusty north to northwest winds and
rain showers and summit snowfall around the middle to end of next
week. Will need to keep monitoring this period for shifts in
guidance.

&&

.AVIATION...
Patches of mid-level clouds continue to stream across the eastern
end of the island chain this afternoon. The background low level
flow will remain light as a weak surface trough near the eastern
Big Island tracks slowly westward. Sea breezes have allowed for
cloud build ups over the island interiors this afternoon, and some
showers to develop over Maui and the Big Island. AIRMET Sierra
has been posted for tempo mountain obscurations on both islands.
Clouds and showers may linger around Maui and the Big Island
tonight, but the remaining islands should clear out with the land
breezes. VFR conditions will prevail across the rest of the
islands.

&&

.MARINE...
Minimal marine hazards are expected through the weekend. A surface
trough near the Big Island and a weak high north of the area will
keep light northeast winds over the smaller islands through
Sunday. The winds will be light enough for sea breezes to develop
during the day. Light to moderate southeast to south winds should
be expected through the windward and southeast Big Island waters
tonight and Saturday. Periods of heavier showers and a slight
chance of thunderstorms are also expected over these waters
through tonight. Light north winds will fill in Sunday around the
Big Island. The surface trough will begin to shift east Sunday as
high pressure builds to the northwest and a front passes to the
north.

A declining east swell will keep surf along east facing shores
just below the advisory level tonight. This swell and surf will
continue to trend down through the weekend. Surf elsewhere will
remain quiet until the beginning of next week. A small northwest
swell is forecast to peak Monday night, bringing surf below
advisory levels. A hurricane force low is expected to develop off
of Japan this weekend and send a large, long period northwest
swell towards the islands midweek. The swell will begin building
Tuesday and peak Wednesday night, with surf likely nearing warning
levels.

Additionally, a cold front will approach and may move into the
area from the north Monday night through midweek, possibly
bringing fresh to strong north winds and rough seas (small craft
advisory conditions) across the waters Tuesday through midweek.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


DISCUSSION...Brenchley
AVIATION...Foster
MARINE...Eaton

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