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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 251341

Area Forecast Discussion
National Weather Service Honolulu HI
341 AM HST Tue Feb 25 2020

Moderate to breezy trade winds are expected through midweek.
Clouds and showers will favor windward areas, especially through
the overnight and early morning periods. Rainfall chances are
expected to trend up Thursday through the weekend as an upper
disturbance moves over the area. This combined with increasing
trade winds will translate to a wet and windy pattern this
weekend, which may continue through early next week.


The latest surface analysis showed a ridge of high pressure north
of the state extending from east to west. Satellite-derived
precipitable water (PW) imagery showed values hovering around an
inch, which is around or just below normal for this time of year.
Moderate trade winds combined with this low-level moisture are
supporting isolated to scattered showers, mostly favoring windward
areas early this morning. Overnight rainfall totals have been
light with peak measurements ranging from a quarter to half of an

The short-term guidance remains in decent agreement and depicts a
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 beginning this afternoon.
Clouds and showers will favor windward areas, especially through
the overnight and early morning periods. Rainfall accumulations
should remain light with PW values forecast to remain around or
below an inch.

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 and
mountain areas, some will spread into leeward sections given the
strong winds expected.


A 1030 mb high to the north of the island chain will allow for
moderate east northeast trade flow to persist today. 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 with partly cloudy skies and VFR for leeward

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 coastal waters. This in combination with an incoming large
long-period northwest swell will bring Small Craft Advisory (SCA)
conditions to most marine zones by late today or tonight. 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 1040+ mb high
builds north of the state, with near-Gale to Gale force winds
likely to develop across much of the marine area.

A large long-period northwest swell has just arrived at the NOAA
buoys northwest of Hawaii as of 3 AM HST, and will fill in from
northwest to southeast across the island chain today. The swell is
expected to peak late this afternoon through this evening, then
gradually lower late tonight through Wednesday night. The latest
buoy observations show the swell timing and predicted height are
nearly spot on with the WW3 forecast guidance. This is expected
to result in high end advisory level surf, and could be close to
warning levels at the peak of the event. We will continue to
closely monitor the NOAA buoys northwest of the state for a need
to upgrade to a warning.

Another long period, but slightly smaller northwest swell is
expected to build Wednesday night and Thursday, peak Thursday
night, then gradually lower Friday through Sunday. Advisory level
surf is expected with this swell along north and west facing

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 during the next 7 days.


High Surf Advisory from noon today 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 until 6 AM HST Wednesday for Niihau-Kauai
Windward-Kauai Leeward.

Small Craft Advisory until 6 PM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha
Channel-Big Island Leeward Waters-Big Island Southeast Waters.

Small Craft Advisory from 6 PM this evening to 6 PM HST Friday
for Maui County Leeward Waters-Big Island Windward Waters.




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
Send comments to: Ryan Lyman