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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 250654
Area Forecast Discussion
National Weather Service Honolulu HI
854 PM HST Mon Feb 24 2020
A mostly dry and pleasant trade wind pattern is expected through
midweek. Clouds and showers will favor windward areas, especially
through the overnight and early morning periods each day. Rainfall
chances are expected to trend up Thursday through the weekend as
an upper disturbance moves over the area. This combined with
increasing trades will translate to a wet and windy pattern this
weekend, which may continue through early next week.
The latest surface analysis showed high pressure building north
of the area behind a dissipating frontal boundary stalled to the
north. A return of moderate trade winds has been the result today
across the state. The leading edge of a band of moisture near the
stalled boundary combined with the trades are supporting
scattered, mostly light, showers over the adjacent windward waters
of the smaller islands this evening. Expect this general trend to
hold overnight into Tuesday morning with clouds and showers
favoring windward areas.
The short-term guidance remains in decent agreement and depicts a
mostly dry and stable trade wind pattern continuing through midweek
as high pressure builds to the north and upper heights rise. Trades
will steadily increase and become breezy, especially by Wednesday.
Clouds and showers will favor windward areas, especially through the
overnight and early morning periods. Rainfall accumulations should
remain light with precipitable water (PW) values forecast to remain
around or below an inch (dry).
The extended guidance shows a pattern shift later in the week
through the upcoming weekend as an upper low closes off across the
state and 1040 mb high pressure sets up to the north. Increasing
instability associated with lowering upper heights combined with a
modest increase in moisture being drawn into the area will
support increasing rainfall chances. As the pressure gradient
tightens in response to the 1040 mb high, local winds will
strengthen and become windy. Guidance suggests this wet and windy
pattern holding into next week, which may result in localized
flooding concerns where the heaviest rainfall becomes focused.
Although most of the showers will focus over windward areas, some
will spread into leeward sections given the strong winds expected.
Tonight a 1031 mb high to the north of the island chain will
allow for moderate east northeast trade flow to persist. Thus,
isolated to scattered showers will press up against the windward
coasts and slopes. Periods of MVFR conditions may develop due to
lower ceilings from the passing shower activity. Otherwise,
mainly dry weather and VFR for leeward locations.
No AIRMETs are in effect at this time.
High pressure will strengthen far northeast of the islands over
the next couple days, resulting in increasing trade winds across
the island chain. This in combination with an incoming large long-
period northwest swell will bring Small Craft Advisory (SCA)
conditions to most marine zones by late Tuesday or Tuesday night.
SCA conditions will continue through the weekend and into early
next week for most areas. In fact, the trades will become even
stronger late in the week and over the weekend as a new and 1040+
mb high builds north of the state, with near-Gale to Gale force
winds likely to develop across much of the marine area.
The current small short period northwest swell will linger
through tonight. A larger long-period northwest swell will fill
in Tuesday morning, peak late Tuesday and Tuesday night, then
gradually lower Wednesday and Wednesday night. Current guidance
continues to show surf reaching high end advisory levels with
this swell, but if the swell comes in a bit higher than expected,
the advisory may need to be upgraded to a warning. We will
continue to closely monitor the NOAA buoys northwest of the state
through the night as the swell begins to arrive.
Another long period, but slightly smaller northwest swell is
expected to build Wednesday night and Thursday, peak Thursday
night, then lower gradually Friday through Sunday. Advisory level
surf is expected with this swell.
Short period choppy surf along east facing shores will gradually
rise through the week and become rather large by the weekend due
to strengthening trade winds. Advisory level surf is likely by the
weekend, with warning level surf possible Sunday through early
next week. There will also be a series of small background south
swells through the week and on through the weekend.
High Surf Advisory from noon Tuesday to 6 AM HST Wednesday for
Waianae Coast-Oahu North Shore-Oahu Koolau-Molokai-Maui Windward
West-Maui Central Valley-Windward Haleakala.
High Surf Advisory from 6 AM Tuesday to 6 AM HST Wednesday for
Niihau-Kauai Windward-Kauai Leeward.
Small Craft Advisory until 6 PM HST Wednesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.
Small Craft Advisory from 6 AM Tuesday to 6 PM HST Wednesday for
Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward
Waters-Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-
Kaiwi Channel-Maui County Windward Waters.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office