Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
846 PM HST Tuesday Sep 16 2014
weak surface high pressure north of the islands will maintain
gentle trade winds across the state through Wednesday...with
localized afternoon sea breezes developing. The ridge will break
down from Thursday into the weekend...allowing for widespread sea
breezes along with afternoon clouds and showers. Dry and stable
air aloft will keep most rainfall amounts light. A tropical wave
passing south of the area may bring an increase in rainfall...and
boost wind speeds a bit during the weekend and into early next
widely scattered light showers dot the Hawaiian coastal waters
on this Tuesday evening. Rain gauge data shows just isolated
light amounts during the past several hours...with most land
locations remaining dry. Surface analysis depicts low pressure
centered far northeast of the state...with a dissipating
front/trough trailing SW to east and south of The Big Island. A
1022 mb high is centered about 1000 miles northwest of Kauai and
is building slowly eastward...providing gentle trade wind flow
across the state. Aloft...a weak upper level low is centered near
Kauai and moving slowly northward...with a large middle/upper level
ridge sprawling northwest of our region from west of the dateline
to near 160w. Mimic-tpw satellite imagery shows a stripe of higher
moisture along the dissipating surface trough extending across
the southern tip of The Big Island and south of the smaller
islands. The airmass over the state as of 00z contained near
normal moisture...with the Lihue/Hilo soundings measuring
precipitable water near 1.4 inches and inversions based around
7000 feet. Another area of somewhat higher moisture is approaching
from the north...and appears poised to move across the western
half of the state late tonight into Wednesday.
Overnight...expect shower activity to increase across windward
waters and slopes over the western sections...due to the typical
nocturnal rainfall maximum plus the aforementioned area of higher
moisture moving in. A considerably drier airmass lies upstream
from The Big Island...and would expect less shower activity there
even during the late night period. May perform some minor updates
to the overnight gridded/text forecasts to reflect the above
thinking and short term trends...but no major changes are planned.
Surface high pressure will build slowly eastward to the north of
the state through Wednesday night...maintaining a gentle background
trade wind flow. Trades will remain light enough to allow
localized sea breezes to develop once again Wednesday afternoon.
Expect a hybrid pattern similar to that observed today...with
showers and clouds focusing on windward slopes during the night
and early morning...and across some of the leeward and interior
areas during the afternoon. The area of increased moisture
crossing the state on Wednesday may allow for a bit more leeward
shower activity than was observed today...with a few locally
heavier downpours possible.
From Thursday into the weekend...another cold front is forecast
to drop southward across the north central Pacific. This will
push the subtropical ridge south and east...disrupting the
pressure gradient across the state. Local trade wind flow should
weaken further or disappear entirely during this time...allowing
for a period of more widespread daytime sea breezes and nighttime
land breezes. This pattern will bring mostly clear nights and
mornings with spotty showers over interior areas during the
afternoons and evenings. The light winds will allow volcanic haze
to build up around The Big Island...but it does not appear as if
there will be enough southerly component in the flow to carry the
haze over the smaller islands.
Forecast uncertainty increases during the weekend and into next
week...when the models generally bring a tropical wave and
associated surge of deep moisture northwestward toward the state.
The 00z guidance suite continues to show some variations
in the timing and extent of this moisture. It appears that the
increased pressure gradient between lower pressure to the south...
and a narrow high pressure ridge between the islands and the
stalled frontal boundary to our north should induce some increase
in easterly flow over the state for Sunday and Monday. Rainfall
amounts may increase with locally heavy showers becoming possible
as well. Thereafter...a drier airmass should spread back into the
state from the east...with the low level flow possibly veering
toward the east-southeast as another cold front drops into the
north central Pacific. This would bring the potential for volcanic
haze to spread west northwestward over some of the smaller islands
at that time.
VFR conditions will prevail for the next couple of days with only
localized MVFR ceilings over the slopes. Clouds and showers will be
most prevalent over windward slopes during the nights and mornings
and over the leeward slopes during the afternoons and evenings.
evening observations from the Barbers Point buoy show the south
swell continuing to slowly decline...with peak energy values
gradually shifting into the shorter wave period bands. This swell
will slowly fade away through Friday. Surf along south facing
shores will remain elevated through the next couple of days...
but should stay below advisory levels. Otherwise...a small
northwest swell has arrived during the last several hours...as
indicated by Waimea Bay buoy data. Another small northwest swell
will arrive Wednesday...but surf along north facing shores will
remain well below advisory levels.
Winds and seas are expected to remain below Small Craft Advisory
levels through at least early next week.