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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 010636

Area Forecast Discussion
National Weather Service Honolulu HI
836 PM HST Thu Nov 30 2023

A kona low west of the islands will bring a continued threat for
heavy rain and flooding through Friday. Isolated thunderstorms are
also possible. Conditions will gradually improve over the weekend
with stable trade wind weather expected for the first half of
next week.


Currently at the surface, a kona low is located around 400 miles
west-northwest of Kauai, while a 1030 mb high is centered around
1400 miles northeast of Honolulu. A general southeast to south
flow prevails across the islands this evening. Infrared satellite
imagery shows mostly cloudy conditions across the state, with a
bit less cloud cover over Kauai. Radar imagery shows widespread
light rain across the Big Island, a few spotty showers across the
smaller islands, with numerous showers and thunderstorms
developing off to the south of Kauai. Main short term focus
revolves around heavy rain potential and thunderstorms.

The kona low will gradually break down during the next 24 to 36
hours, weakening into a surface trough by daybreak Saturday.
Before it breaks down however, south to southeast boundary layer
flow will continue to pump deeper moisture northward into the
islands with the upper low and jet forcing continuing to provide
forcing for ascent. This will keep the threat for heavy rain and
thunderstorms in the forecast, particularly over the western end
of the state.

The most immediate concern for tonight revolves around the
potential for some strong thunderstorms with plenty of deep layer
shear in place and respectable surface based instability over the
western end of the state.

The unstable airmass will linger over the islands through Saturday
with thunderstorms remaining possible across the entire state.
More stable weather will then build into the islands on Sunday as
trade winds begin to return. The trades will strengthen early next
week, with stable trade wind weather prevailing through the middle
of the week. The trades may ease for the second half of next week
as a new front approaches from the west.


Deep tropical moisture pooling northward over the state due to the
kona low to our west will keep the threat in place for periods of
heavy rainfall and thunderstorms into Friday. Mesoscale
convergence bands south of Kauai and east of the Big Island will
likely be the focus for our overnight activity. The band evolving
south of Kauai drifting northward has strong to severe
thunderstorms embedded within it. Whether or not this band
persists and impacts Kauai or Oahu or remains out over the open
waters will be the main question/concern overnight.

Aside from turbulence and icing associated with any TS that
develops, AIRMET for mid-level turb will continue into Friday, as
plenty of PIREPS indicating up to MDT TURB due to wind shear in
the 100-FL200 layer. Light icing in layered clouds has also been
reported between 120-FL220. AIRMET Sierra for mountain
obscuration will likely include Kauai later this evening depending
on how the band to the south evolves. If conditions continue to
improve farther east, some islands may drop out of the AIRMET
Sierra overnight.


A kona low west of the Hawaiian Islands will continue to produce
southeast to south winds on Friday with bands of showers, heavy
at times, and thunderstorms developing across the region. The kona
low will weaken this weekend as the ridge builds in north of the
islands. Expect gentle to fresh trade winds spreading across the
region from Saturday to Sunday, strengthening to fresh to strong
trade winds from Monday onward. A Small Craft Advisory (SCA)
remains in effect through Friday morning for most coastal waters
surrounding Maui County and the Big Island. This SCA will likely
be cancelled on Friday as wind speeds decrease below advisory

The recent small to moderate, medium period northwest (320
degree) swell has peaked and will gradually decline through
Saturday. The next moderate size, longer period northwest (320
degree) swell will arrive and begin to fill in Saturday afternoon
and evening. This swell will peak surf to near High Surf Advisory
(HSA) heights by Sunday, then slowly subside Monday. A
reinforcing north northwest (330-350 degree) swell arriving early
Tuesday may lift surf well above HSA thresholds Tuesday and
Wednesday. Expect the short period wind wave chop to persist along
south facing shores. Strengthening trades through next week will
build surf heights along east facing shores.


Flood Watch through Friday afternoon for all Hawaii islands-

Winter Weather Advisory until 6 AM HST Friday for Big Island

Small Craft Advisory until 6 AM HST Friday for Maui County
Windward Waters-Maui County Leeward Waters-Alenuihaha Channel-
Big Island Windward Waters-Big Island Leeward Waters.




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