Current Conditions
Temp-3.7 C
RH16 %
WindSSW 3 mph
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 130634

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM HST Mon Apr 12 2021

High pressure northwest of the state and a building ridge aloft
will produce stable northeasterly trade winds on Tuesday through
Thursday, with showers favoring typical windward slopes. An
increase in showers and a decrease in winds is possible Friday and


The shortwave trough that brought locally heavy showers and a few
thunderstorms to the state during the last 48 hours is now shifting
to the east resulting in an increase in stability over the forecast
area. Local satellite and radar imagery indicates most of the
existing trade showers topping out below 10kft as mid-level
subsidence increases, although a few isolated heavier showers will
remain possible through tonight. Moderate to locally breezy trades
will trend increasingly dry during the next several days as the
upstream mid-level ridge gradually migrates eastward. Forecast
soundings from the GFS suggest inversion heights potentially falling
below 6kft as early as Tuesday night. PWATs will follow suit as they
hover near the low end of normal for April (around 0.75 to 1") for
the next several days which further increases confidence in a benign
dry trade wind type pattern. Dewpoints will also remain
pleasantly low, in the upper 50s to low 60s, during this time.
This will favor a cooler feel in the afternoons and crisp, cool

Global models are in general agreement that longwave amplification
over the north Pac during the latter portion of the week will force
the front presently established near 35N southward toward the
Hawaiian Islands by this weekend. This will result in weakening
trades areawide by Friday and Saturday. While typical uncertainty
exists with respect to forward progress of the boundary through
the state, consensus exists that wet trades will be in store for
at least Kauai and Oahu by the end of the week.


This evening, light to moderate northeasterly trade wind flow is
returning to the island chain. Showers and low clouds that were
observed earlier in the day across portions of the Big Island are
diminishing as more stable air settles into place. Therefore,
AIRMET Sierra for mountain obscuration across the southeast
section of the Big Island has been cancelled this evening with no
other AIRMETs in effect at this time.

For the rest of tonight and tomorrow, the trough aloft over the
islands will slowly move off to the east as the associated
surface trough weakens. Therefore, showers and cloud coverage
will favor the typical north through east sections of the islands
as northeast trade winds increase a bit through mid-week.
However, VFR conditions are expected to prevail throughout the
forecast period.


.MARINE...Northeast trade winds will strengthen overnight as a
high far northwest of the area moves east. Wind speeds are
expected to exceed the Small Craft Advisory threshold tomorrow
through Wednesday night. Winds will subside later in the week as a
cold front approaches from the north.

The current northwest swell is forecast to peak overnight, then
slowly subside through Thursday. A larger, but shorter-period
north swell arriving Thursday night will persist through the
weekend, then slowly subside next week. Surf from these swells
along north and west facing shores will remain well below the
advisory thresholds.

Surf will remain small along east facing shores through mid-week.
The north swell will wrap in to some east facing shores later in
the week. Swells from distant southern hemisphere storms will
produce small to moderate surf along south facing shores.


Small Craft Advisory from 6 AM Tuesday to 6 AM HST Thursday for
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Leeward Waters-Big Island Southeast Waters.




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