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fxhw60 phfo 290625 
afdhfo

Area forecast discussion
National Weather Service Honolulu HI
825 PM HST Tue Mar 28 2017

Synopsis...
a trade wind regime will persist through the upcoming weekend and
into the early part of next week as a ridge of high pressure
remains north of the islands. Showers embedded in the trade wind
flow will favor the windward and mountain. The frequency of these
showers will higher during the nights and mornings. The trades
will turn windy over the weekend and lasting through early next
week.



&&

Discussion...
the morning clouds and showers that affected some of the smaller
islands this morning gave way to a brief period of clouds across
the Lee and interior sections early this afternoon. The trades
picked up enough by mid afternoon to disperse these clouds towards
evening. It was a rather wet morning in particularly the Hilo
area of windward Big Island. Most of these showers have moved on.
The Lee side of The Big Island is mainly cloudy but dry at sunset.
These clouds will gradually disperse during the evening hours.

During the overnight hours, the trade showers will be picking up
especially after midnight as a batch of low level moisture
approaches The Big Island from the east. Satellite imagery shows
the leading edge of this area 160 miles east of Cape Kumukahi at
sunset. Models have this shower area impacting windward Big
Islands in the next couple of hours before moving to as far as
Molokai before dispersing by mid morning Wednesday.

On Wednesday, we will see a repeat of today as a tinge of
weakness in the trades will allow for a brief period of afternoon
clouds and isolated showers to form over the Lee and interior
areas of the smaller islands. The models maintain this pattern
through Thursday as the last of this area of moisture finally
leaves the islands. This air mass will be followed by a pocket
of slightly drier air mass Friday and Friday night.

On Friday night, the surface high feeding the islands with the
gentle trades will be reinforced by another surface high. This
will take place some 1200 miles north-northeast of the islands resulting in a
strong 1034 mb high. This translate in a boost in the trades to
moderate to strong speeds. At this time, there is a chance of a
Wind Advisory for the typically windy areas of places like the
Waimea saddle on The Big Island and or Lanai over the weekend.
This boost in the trades will be accompanied by an uptick in the
frequency of trade showers.

The weather pattern takes a change north of the islands Sunday
into Monday where the high is shoved off far to the east. This
will lead to a weaker trade wind flow by Tuesday. And with the
weaker trades, there will be fewer trade showers.




&&

Aviation...
with a broad surface high far northeast of the state, trade winds
will be light to moderate through Wednesday. Low clouds, with any
accompanying showers moving along on the trades, will mainly
affect windward and Mauka sections on the individual isles, but
conditions overall will remain VFR.

There are no airmets currently in effect, and none are anticipated
through the nighttime hours.



&&

Marine...
just over 1500 nm northwest of Kauai near the date line (around 40n)
and a ridge axis extending westward from a 1030 mb high centered off
the coast of California to just north of the islands. A very strong
pressure gradient was depicted south of this low northwest of the
state, that was supported by an ascat pass earlier today where a
large area of strong west-northwest gales (focused within the 290 to
310 directional band relative to the islands) were shown out to
around 500 nm south-southwest from the low (leading edge of the
fetch was around 1300 nm northwest of the islands). According to the
latest altimeter passes, seas associated with this system have
dropped slightly from yesterday, but still upwards within the 28 to
33 ft range over The Heart of the fetch region focused toward the
islands. Despite some small differences between solutions, the
latest model guidance is remaining in decent agreement with these
latest satellite trends and shows this system continuing on an
east-northeast track crossing the date line Tuesday night into
Wednesday around 40n.

A large west-northwest swell (290-310 deg) associated with this
system discussed above will result across the region, that should
reach the islands Thursday night, peak through the day Friday and
Friday night, then slowly ease over the weekend. Warning-level
surf and advisory-level seas will be expected Thursday night into
the weekend before trending down over exposed waters and beaches.
For timing, the European model (ecmwf)-wave, GFS-wavewatch iii and ensemble mean
solutions all remain in close agreement and depict seas ramping up
quickly around midnight Thursday night to Small Craft Advisory
levels, peaking within the 10 to 14 ft range Friday, then slowly
easing through the weekend. A few of the ensemble solutions are
coming in slightly higher (by a couple of feet) at the northwest
buoys and converge at a peak near 15 ft Thursday night. If the
swell ends up coming in larger than predicted or later, warning-
level surf could continue into Saturday before dropping to
advisory levels along exposed north and west facing shores. Surf
should dip below advisory levels Sunday.

Local winds and seas will likely remain below advisory levels
through Friday as the ridge of high pressure north of the state
slightly weakens in response to a cold front that is forecast to
approach the region later in the week, then stall and weaken north
and northwest of the area into the upcoming weekend. Advisory-
level trade winds will return across portions of the waters over
the weekend as high pressure builds north of the state behind this
front.

Prior to the arrival of the large swell expected Thursday night
through the weekend, surf along north and west facing shores will
hold at levels below the advisory criteria Wednesday through
Thursday due to a lingering northwest swell from earlier this week.
Will continue to monitor buoy observations overnight for an
expected small reinforcement out of the northwest (320 deg).

Surf along east facing shores will remain small and choppy through
the week due to moderate to fresh onshore winds persisting. Surf
will begin to build over the weekend as the winds strengthen locally
and upstream of the islands and may near or reach advisory levels
along east facing shores by the end of the weekend and into early
next week.

A slight increase in surf along south facing shores will be possible
Thursday through Saturday due to a long-period swell (190 deg)
associated with recent activity across the southern Pacific.
In addition to this southerly swell, wrap from the previously
discussed large west-northwest swell will add to the mix by Friday
at select spots along exposed southern facing beaches.

&&

Hfo watches/warnings/advisories...
none.

&&

$$

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