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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 161330
Area Forecast Discussion
National Weather Service Honolulu HI
330 AM HST Mon Jul 16 2018
High pressure far to the north of the Hawaiian Islands will
continue to produce moderate trade winds across the area through
this week and into next weekend. Clouds and scattered showers will
tend to favor windward and mauka areas with just isolated showers
possible over some leeward locations. A trough passing south of
the state on Wednesday will produce increased showers along
windward slopes of the Big Island and Maui. Otherwise a drier
trade wind pattern will prevail through Saturday.
This mornings satellite imagery continues to show an active
Tropical Upper Tropospheric Trough (or TUTT) roughly 800 miles
northwest of the state with several upper lows embedded in the
trough axis. High level cirrus clouds are drifting from west to
east across the region due to divergent upper level westerly
winds. Bands of more stable stratocumulus and closed cell cumulus
clouds are drifting across the islands with the trade winds.
In the big picture, a high pressure ridge remains in place far to
the north of islands through this week and into next weekend.
Expect a decrease in shower activity today and Tuesday as a
building high pressure ridge aloft produces more subsidence and
stability over the islands. The trade wind subsidence inversion
will range from 5,000 to 7,000 feet over the next two days keeping
scattered showers along windward and mountain slopes with only
isolated showers possible over leeward areas.
On Wednesday, a weak disturbance in the deep tropics passing
south of the islands will push higher moisture amounts up into the
eastern half of the state with enhanced windward showers forecast
over the Big Island and Maui. Otherwise, a drier and more stable
trade wind pattern will prevail with scattered showers forecast
along windward and mountain slopes with isolated showers possible
over leeward areas from Thursday through Saturday.
Over the next three to seven days the deep tropics southeast of
the islands will become more active with several disturbances
developing in the long range model guidance. These troughs may
bring periods of increased moisture amounts up into the island trade
wind flow as they drift westward, resulting in periods of higher
chances for rainfall mainly along windward slopes. These higher
moisture amounts will reach the southeastern half of the state
more easily and that means both Hawaii and Maui Counties will have
a higher chances for enhanced shower activity starting on Sunday.
This far out in the extended forecast period we have lower
confidence in the arrival timing of these increasing shower
trends. Stay tuned as this upcoming active long range trade wind
weather forecast evolves over time.
Moderate trade winds will persist through tonight. Low clouds and
showers carried by the trade winds will remain concentrated over
windward and mountain areas of the state. This will likely produce
brief periods of MVFR conditions along windward sections. A few
light showers may be carried over to leeward sections of some of
the smaller islands this morning, but VFR conditions are expected
to prevail there.
AIRMET Sierra remains in effect for mountain obscuration on
Kauai, Oahu, Molokai, Lanai, and Maui. This AIRMET may be
continued for windward Kauai beyond 6 AM HST this morning, but
conditions appear to be improving across the other islands.
High pressure north of the islands will continue to generate
trade winds across the islands this week. Locally breezy trades
has led to a Small Craft Advisory (SCA) for the typically windy
waters around Maui County and waters south of the Big Island. This
SCA is good through Tuesday afternoon. Additional extensions of
the SCA are likely to follow. An area of low pressure associated
with a low pressure system will be passing south of the islands
at midweek, which may bump up the trades another notch or two as
well the need to expand the coverage of the SCA.
Trade winds will produce surf mainly in the moderate range along
the east facing shores this week. The strengthening trades at
midweek will bump up the surf to near advisory level for the east
facing shores. The current south southwest swell will maintain
small to moderate surf along the south and west facing shores.
This swell will slowly fade through Monday night. A small to
moderate south swell with a similar size is due in Wednesday
night. An even longer period of 21 seconds and a 2 foot south
swell, will arrive over the upcoming weekend, resulting near
advisory level surf for the south facing shores.
A slight chance of thunderstorms remains across the northern
offshore waters through tonight due to the presence of an upper
level trough. The low pressure area passing south of the islands
may bring a slight chance of thunderstorms to the far southern
offshore waters around mid week.
Small Craft Advisory until 6 PM HST Tuesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office