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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 251328
Area Forecast Discussion
National Weather Service Honolulu HI
328 AM HST Mon May 25 2020
Lighter winds and mostly dry weather are expected through Tuesday,
with a few showers possible, mainly during the afternoon and
evenings. Trade winds are expected to strengthen Wednesday, and
remain in place into next weekend, delivering a few brief showers to
Currently, light to moderate E-SE winds prevail on the large scale,
lighter near Kauai as compared to the Big Island, due to a surface
ridge located a couple hundred miles N of Kauai. These light winds
have allowed offshore-flowing drainage winds to develop over many
areas, helping to support mostly clear skies over land, despite
increased low cloud coverage over near shore waters. The Big Island
is a notable exception, where low clouds have persisted through the
night, both windward and leeward, while high clouds have moved E.
Early morning soundings show the island atmosphere is stable, with a
nearby mid- and upper-level ridge supporting a strong subsidence
Clear and mostly dry conditions will prevail through the morning,
except for the ongoing low cloudiness over the Big Island. A subtle
decrease in wind speed is anticipated through tonight as the ridge
is pushed a little closer to Kauai by a rapidly strengthening low
passing far N of the area. We may see a few more afternoon leeward
clouds today and Tuesday than we did Sunday, but significant
rainfall is not expected. Clearing of low clouds is expected over
land again tonight and early Tuesday, although the light background
E flow may deliver a brief windward shower.
A new high building N of the area will bring increasing trade winds
from late Tuesday into Wednesday that will persist into next
weekend, delivering what is expected to be only brief windward
showers. Leeward Big Island will see afternoon clouds and showers
through the period, sometimes lingering into the early morning
High clouds near the Big Island are associated with a slow-moving
closed low aloft centered S of the area. As the low moves W later
tonight, high clouds are expected to increase over all islands,
potentially persisting for several days.
A strong ridge of high pressure to the north of the islands will
maintain light east to southeasterly flow across the state. Shower
activity over southeastern exposures of the Big Island and Maui will
increase towards daybreak. Mountain obscuration could become an
issue for the eastern slopes of the Big Island.
Light sea breezes will predominate in the afternoon and early
evening hours then give way to weak land breezes after sunset.
Shower activity in the afternoon and evening hours will be limited
but should be focused over island interiors. The air mass over the
state remains mostly dry and mostly stable.
Isolated MVFR CIG and VIS are possible in brief showers. Otherwise,
VFR conditions will prevail all areas.
A weak east-northeast to west-southwest oriented surface ridge
located about 260 nm north of Kauai will continue to drift southward
today, and then will stall later tonight and Tuesday. This will
maintain moderate easterly trade winds around the Big Island and
Maui, with weaker east-southeast background flow from Molokai to
Kauai. Surface high pressure is forecast to build far north of the
state Wednesday, which will cause the easterly trade winds to
strengthen from mid-week through Friday. There are no Small Craft
Advisory (SCA) conditions in the current forecast this week.
However, the model guidance suggests that winds may briefly approach
the SCA criteria over some of the typically windier waters adjacent
to Maui County and the Big Island Wednesday night or Thursday.
South facing shores will see periods of surf around the May average
through much of the week. A longer period south-southwest swell will
likely combine with a small south-southeast swell to produce
moderate sized surf near or just below the May average this morning,
followed by a slight drop from later today through Wednesday.
Another pulse of south-southwest swell energy is expected from late
Thursday through Saturday.
Small north-northwest swells are expected to produce surf near the
May average along most north facing shores of the islands through
mid-week, followed by the swells shifting out of the north Friday
Surf along east facing shores will likely remain below the May
average this week due to the weaker than normal trade winds in the
vicinity of, as well as upstream of the state.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office