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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 160650
Area Forecast Discussion
National Weather Service Honolulu HI
850 PM HST Sun May 15 2022
Warm and humid conditions with increasing rainfall chances are
expected through the week as an out of season upper disturbance
and front move into the area Tuesday through midweek. Winds will
diminish and shift out of the south to southeast direction through
the first half of the week. This will result in spotty afternoon
showers forming over our dry leeward areas where sea breezes
form. More widespread rainfall with embedded locally heavy showers
and a few storms are possible over the western end of the state
Tuesday night through midweek. Conditions should begin to improve
by the weekend.
Guidance remains in good agreement and depicts a significant
pattern change evolving across the region this week as a strong
pulse aloft dives southward into the region carving out an upper
trough near the western end of the state. Deep southerly flow
associated with this feature and its attendant front moving in
from the northwest will draw plenty of deep tropical moisture
northward into the area -- with dewpoints surging into the lower
70s. This combined with light south to southeast low-level flow
will translate to warm and humid conditions with rainfall chances
trending up over southeast facing slopes and our dry leeward
areas where sea breezes form. Peak heating due to the high sun
angle for this time of year may support a few heavy afternoon
showers through Tuesday.
The southerly winds are expected to increase to moderate levels by
midweek as the front draws closer and upper heights fall. This
combined with the deep moisture axis setting up near/over the
western end of the state will lead to the best chance for showers
to become more widespread. If this axis shifts farther east,
localized flooding can't be ruled out Tuesday night through
Wednesday night. Some showers will be heavy and a few
thunderstorms can't be ruled out through this midweek period.
Guidance shows the upper support lifting northeastward and away
from the area Thursday through Friday. This combined with the
ridge lifting northward should lead to improving conditions by the
Moderate easterly trades will ease and transition over to a more
southeasterly flow across the islands tonight through Monday. This
will allow for overnight land breezes and daytime sea breezes to
become increasingly dominant across the state. Showers will favor
mainly windward slopes and coasts overnight, although a few
showers will likely affect southeast facing sections of Kauai and
also the leeward coast of the Big Island. These showers will
fizzle out quickly Monday morning, with more convective shower
development expected over the island interiors Monday afternoon.
Mainly VFR conditions should prevail at the TAF sites, although
periods of MVFR cigs will affect windward areas as well as the
PHTO and PHLI terminals at times.
AIRMET Sierra is in effect for mountain obscuration across
northeast through southeast sections of the Big Island. Conditions
may improve here later tonight.
The winds have become much weaker early this evening compared
with recent days. The background flow is from the east-southeast
to southeast, which means most of the coastal waters near the
smaller islands are in the shadow of the Big Island. These
conditions are expected to remain in place into Tuesday. A front
moving toward the region from the northwest will likely cause the
winds to strengthen from a more southerly direction by mid-week,
with the strongest winds around Kauai and Niihau on Wednesday.
Expect light to moderate southeast background flow for most
Hawaiian waters on Thursday and Friday.
The current choppy, moderate surf along east facing shores is
expected to gradually lower through Monday due to the local winds
weakening and becoming southeasterly. An upstream fetch of east
winds will likely keep small background surf along exposed east
facing shores for most of this week.
The current small, medium-period south swell may provide a slight
bump in surf along most south facing shores through Monday. A
larger south swell with a similar wave period will likely arrive
on Tuesday, and peak on Wednesday. This will produce a noticeable
increase in surf heights along south facing shores by mid-week,
Note that this swell coincides with the strengthening southerly
winds, so surf may become choppy, especially along south facing
shores of the western islands on Wednesday. This south swell will
gradually lower from late Thursday through Saturday.
Surf will remain nearly flat along most north and west facing
shores through Tuesday, with only small, short-period northwest
swell energy available. The wave model guidance still indicates a
small northwest swell may arrive late Wednesday night, and peak
on Thursday. Assuming this swell does show up, it will gradually
lower from Thursday night through Friday.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office