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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 201322
Area Forecast Discussion
National Weather Service Honolulu HI
322 AM HST Tue Feb 20 2018
Dry and pleasant conditions will hold into midweek as the upper
low that brought the recent heavy rains across the state continues
to lift north and away from the area. A return of unsettled
weather is possible through the second half of the week and the
upcoming weekend as another upper low drops south into the area.
A dry pattern with hold today with dewpoints remaining in the low
60s. The upper low that brought the flooding rains and
thunderstorms across the state on Sunday continues to pull away to
the north this morning with plenty of middle- to upper-level dry
air filling in across the area. Upper air data and satellite-
derived precipitable water (PW) values have lowered as a result
and should remain around and just under an inch across much of
the state (slightly higher windward Big Island). Winds will
remain light and variable with early morning land breezes and
afternoon sea breezes. Although a few showers associated with the
sea breeze can't be ruled this afternoon during peak heating,
coverage will be limited due to the dry air.
Increasing moisture and rain chances are anticipated by late
Wednesday with another period of unsettled weather setting up for
the second half of the week and into the upcoming weekend. Despite
some subtle differences between model solutions, guidance remains
in decent agreement and depicts deep tropical moisture being
drawn northward over the state as another upper low drops south
into the area Wednesday night into Thursday and holds into the
weekend. The upper low is forecast to stall over an area located
a few hundred miles northwest of Kauai. Strong southwest winds
aloft will setup over the western end of the state with 0-6 km
bulk shear magnitudes increasing into the 35 to 45 knot range by
Thursday afternoon. This combined with increasing lift and
instability will support heavy showers and thunderstorms
developing once again, especially over the western end of the
state where the better upper support will be. Model PWs will climb
into the 1.5-1.8" range, which is above normal for this time of
year. Winds are expected to increase and could become gusty for
some locations out of the east-southeast direction as a surface
trough shifts west of the state and high pressure builds to the
No significant changes are shown Sunday through early next week.
Guidance continues to advertise a wet pattern with a deep trough
positioned west of the state and moist east-southeast flow
continuing (PWs holding above average in the 1.4-1.7" range).
VFR conditions will prevail across the state today. The upper
trough axis has moved northeast of the islands, taking the
associated thunderstorms with it beyond the windward coastal
waters. Dry and stable conditions are expected over the state
through the day, with just some afternoon clouds and showers
along with isolated MVFR conditions possible over the island
interiors as seabreezes develop.
Model guidance suggests that the potential for moderate
turbulence aloft over the islands has decreased, and with no
recent aircraft turb reports we have dropped AIRMET Tango. No
other AIRMETs are anticipated today.
SIGMET Romeo remains posted for thunderstorms affecting the
primary West Coast to Hawaii air routes near the upper trough.
Small to moderate northwest swells will continue through the day,
with surf remaining below advisory level along north and west
facing shores. However, a longer period northwest swell is
expected to spread down the island chain beginning tonight and
peaking Wednesday. This may result in advisory level surf along
some north and west facing shores, especially Kauai and Niihau,
during the peak of the event. This swell will gradually subside
Thursday, but another west-northwest swell is expected Thursday
night into Friday, before fading into the weekend.
A long fetch due to the flow around a surface high over the
northeastern Pacific will send a short-period east swell into the
local waters during the next several days. This swell is expected to
increase today, with resulting surf likely reaching the advisory
threshold of 8 feet along most east facing shores. A High Surf
Advisory has been issued for these areas. A further increase in
this swell is expected Thursday, remaining elevated into the
weekend, with high surf likely continuing along east facing shores
through that period.
Combined seas may increase to around 10 feet over most windward
waters starting late Wednesday due to the combination of west-
northwest and east swells, and local wind waves. A Small Craft
Advisory may be needed for the latter portion of the work week.
High Surf Advisory until 6 AM HST Wednesday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-Big Island North and East.
High Wind Warning until 6 PM HST this evening for Big Island
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office