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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 230639
Area Forecast Discussion
National Weather Service Honolulu HI
839 PM HST Wed Nov 22 2017
Mostly dry and windy conditions are expected to continue through the
holiday weekend. Passing showers will favor windward and mauka
areas, leaving leeward locations mostly dry each day. An increase
in moisture will be possible through the second half of the
weekend and early next week.
The latest short term (rest of tonight - Saturday) guidance remains
in good agreement and supports dry and windy conditions prevailing
over the state into the holiday weekend. The anomalous pattern
across the northern Pacific featuring dry middle- to upper-level
northerly flow across the eastern Pacific due to a blocking ridge
that remains established over the central Pacific is projected to
continue. Strong surface high pressure (1033 mb) northwest of the
state will become reinforced Friday through Saturday as a 1040 mb
high builds south-southeast from the Aleutian Islands to an area
north of the state.
This pattern will support breezy to windy trade wind conditions
prevailing across the state due to a strong pressure gradient south
of the ridge axis. The evening observations reflect this and are
reporting 20 to 30 mph with higher gusts nearing 40 mph at the
typically windier locations. The current wind advisory supports this
and remains in effect through the holiday and may need to be
extended. Showers that develop will favor windward and mauka areas
each day, with very little spilling over to leeward areas due to the
dry airmass in place. Model precipitable water (PW) values are
expected to remain around an inch, which is below average for this
time of year.
For the extended (Sunday through Wednesday, guidance supports a
trend toward more of a wet trade wind pattern setting up across the
state, especially over Maui County and the Big Island. This combined
with falling upper heights due to a shortwave trough diving
southward into the area will support increasing shower coverage by
early next week. Model PWs are forecast to climb above average
through this time with the greatest source of deeper moisture
becoming focused over the southeastern end of the island chain
(where PWs near/reach the 2" mark by Sunday night). Additionally,
trades will likely remain breezy through this period.
Trade winds will increase to strong levels across the state
overnight into Thursday, as high pressure builds northwest of the
area. AIRMET Tango has been issued for moderate low level
turbulence over and downwind of the higher terrain. An AIRMET for
strong surface wind speeds will probably also be necessary on
Thursday for Lanai and the North Kohala area of the Big Island.
Trade wind showers will be most prevalent over windward
areas, and AIRMET Sierra for mountain obscuration is currently in
effect for windward Maui, where the showers have been particularly
persistent. We may need to expand Sierra to windward parts of the
Big Island or possibly other islands later tonight or early
Thursday morning. While some of the showers will reach leeward
areas at times due to the strengthening trades, expect VFR
conditions to prevail there.
A High Surf Warning is in effect through Friday morning for the
north and east facing shores of all islands due to a very large
north swell plus high wind waves due to strong to near-gale force
northeast trade winds. Wave heights have been running quite a bit
higher than forecast by the Wave Watch III model, and we have
adjusted our forecast heights up again. There will be large waves
breaking in north facing harbor entrances and possibly surges in
those harbors, so a Marine Weather Statement is in effect. The
swell will remain quite high through Thanksgiving, then slowly
subside through Saturday. A new north swell starting on Sunday
will peak on Monday and subside through Tuesday. Surf from this
swell may reach advisory levels. A northwest swell, and yet
another north swell are expected next Wednesday.
A Small Craft Advisory (SCA) is in effect through Friday afternoon
for strong to near-gale force northeast trade winds and large seas
due to a combination of large wind waves and the large north swell.
Trade winds are expected to remain strong through the first half of
next week, so the SCA will likely be extended.
High Surf Warning until 6 AM HST Friday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Olomana-
Molokai-Maui Windward West-Maui Leeward West-Maui Central Valley-
Windward Haleakala-Big Island North and East.
Wind Advisory until 10 PM HST Thursday for Lanai-Big Island
North and East-Kohala.
Small Craft Advisory until 6 PM HST Friday for all Hawaiian
REST OF DISCUSSION...Gibbs
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office