Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
824 PM HST sun Mar 29 2015
a fairly wet trade wind weather pattern will prevail through most
of the upcoming week...although winds will be rather light on
Tuesday. While the trade winds will focus showers over windward
slopes and coasts...leeward areas will see a few showers as well.
scattered showers continue to stream across mainly windward
sections of the state this evening. Rainfall amounts have been
generally light...with most windward gauges recording less than a
quarter inch during the past six hours. Surface analysis depicts
a 1032 mb high about 1000 miles northwest of phnl...with another weaker
high about 1200 miles NE of phnl. A weak front between these two
highs is passing well north of the state. Moderate to fresh trades
prevail over the islands. Aloft...a Flat Ridge axis is being
gradually suppressed south of the state...as a shortwave trough
pushes eastward from the dateline along 30n. The 00z soundings
show well-defined inversions based around 7500 feet...with
somewhat below normal precipitable waters near one inch. Mimic-tpw satellite
imagery indicates little variation in the airmass around the
islands...with estimated precipitable waters mostly between 1.0 and 1.1 inches.
Expect little change overnight...with showers and clouds
continuing to focus windward...and some showers spilling over
leeward on the smaller islands. The existing forecast is on
track...and no changes are planned for this evening.
Moderate to fresh trades will continue across the state on
Monday...as the surface high passes by to our North. Heights will
begin to fall aloft...as a middle/upper level trough pushes eastward
from the dateline over the northwest Hawaiian islands. This will allow
for the inversion to rise and weaken somewhat...likely keeping
trade wind showers fairly active even during the afternoon hours.
Trades are expected to weaken Monday night and Tuesday...as
the middle/upper trough passes north of the state...and a weak
surface trough develops in response near and north of the islands.
Models indicate that the inversion will weaken and rise a bit
further during this time...with some modest increase of low level
moisture as well. The trades may weaken enough to allow seabreezes
to develop across many areas on Tuesday afternoon. Increased
moisture/instability could generate a few heavier downpours Monday
night and Tuesday...potentially also enhanced by seabreeze
convergence Tuesday afternoon.
Trade winds should increase over the islands once again on
Wednesday as the middle level disturbance moves away to our NE and
the nearby low level trough washes out. A shortwave middle level
ridge passing overhead should strengthen and lower the inversion
once again as well...thus would expect typical trade wind weather
with mostly light showers focusing windward and Mauka.
Forecast uncertainty increases from Thursday into next weekend...
due to model variations in the evolution of a complex deep layered
trough developing over the central Pacific. Most of the models veer
and weaken trade wind flow locally to some extent...with the
smaller islands possibly falling within the rain shadow of The
Big Island. As noted previously...a 10-20 degree difference in
wind direction would have significant implications for sensible
weather over the islands during this time. Forecast models have
been inconsistent with their solutions for early this week...
therefore confidence in the later period forecasts is not
particularly high at this time.
clouds and showers will remain focused over windward sections of
the state through tonight...as moderate to fresh trade winds
prevail. Airmet Sierra for mountain obscuration remains in effect
for windward sections of Kauai...Oahu and Molokai.
VFR conditions will prevail elsewhere...with just isolated MVFR
ceilings possible at times mainly in -shra.
Airmet Tango for moderate low level turbulence has been posted S
and west of the mountains due to increasing trade winds.
northwest swell appears to have peaked around midday Sunday at buoy 51101...
and is probably peaking now at the nearshore cdip buoys. Resulting
surf will be near advisory levels along north/west facing shores
of Kauai...and north facing shores of Oahu/Molokai/Maui overnight.
This swell and surf should begin to subside late tonight...and
continue dropping Monday through Tuesday.
Otherwise...small south swell will continue producing small surf
on south facing shores through middle week. A small west-northwest swell may
arrive into the islands next weekend.
The Sunday morning ascat pass found winds up to 25 knots in the
Alenuihaha Channel...and a Small Craft Advisory /sca/ has been
posted there as well as south of The Big Island. Recent mesonet
data from Maalaea Bay and Lanai indicate that winds may be near 25
knots in Maalaea and the Pailolo Channel as well. Will consider
expanding the Small Craft Advisory into those waters with the evening forecast
high surf advisory until 6 am HST Monday for north and west facing
shores of Kauai and Niihau and for north facing shores of Oahu
Molokai and Maui.
Small Craft Advisory until 6 PM HST Monday for Alenuihaha
Channel-Big Island leeward waters-Big Island southeast waters.