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Hawaiian Islands Synoptic Discussion and Guidance

699
FXHW60 PHFO 241946
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
946 AM HST Fri Feb 24 2017

.SYNOPSIS...
Dry and breezy trade winds will slowly ease today and drop to
moderate levels by tonight. Clouds and limited showers will favor
windward and mountain areas through Saturday. Showers will
increase over the state from east to west starting Sunday with the
Big Island seeing the initial increasing rainfall over windward
and southeast areas. Enhanced moisture will push northwest over
the smaller islands as winds veer from the east southeast Monday
and Tuesday. Locally heavy showers may be possible with afternoon
showers. Scattered showers and light winds are expected for the
second half of next week.

&&

.DISCUSSION...
The general forecast philosophy remains the same this morning with
no change to the forecast at this time. High pressure remains
northwest of the islands with little change expected for our
weather. A low pressure system to the north will push the ridge
closer to the islands tonight, which will help to weaken the
winds. For today and into tomorrow morning, still expecting drier
than normal conditions with breezy trades on the decrease.

The overnight soundings from Lihue and Hilo continue to show the
below average dry airmass with precipitable water (PW) values of
0.72 and 0.87 respectively. Upper level temperatures are also on
the cold side with -13.3 C and -12.1 C at 500 mb. At 700 mb, Hilo
is around 0 C, well below normal for February, while Lihue is at
4.2 C which is colder than normal. Despite the drier airmass over
the islands, the colder temperatures at the upper level could
help to produce more showers than we would normally expect.

As the above mentioned ridge moves closer and over the islands,
expect winds to continue to decrease and to turn to the east
southeast. The models show the moisture increasing in this flow from
the south beginning Saturday night and continuing through the
first half of the week. Meanwhile colder temperatures aloft will
linger. Certainly as the moisture increases we will see more
shower activity, but the role of the upper level temperatures
creates a level of uncertainty.

During the first half of the new week, a series of short-wave
troughs are expected to pass through the area adding to the
instability. There remains a potential for heavier afternoon
convective showers over the Big Island during this time. One
factor for the spread to the smaller islands will be how fast, and
how much of the moisture, spreads to the north.

Light southerly winds look to take over during the second half of
the week. The upper level trough is expected to weaken for the
second half of the week which should allow for upper level
temperatures to warm. This combination should limit afternoon
convective showers during the second half of the week. Both the
GFS and ECMWF show a low forming in the vicinity of the islands
midweek, but both models quickly speed the low to the northeast
away from the islands, leaving the state under a light southerly
flow with passing scattered showers.

&&

.AVIATION...
Brief MVFR conditions in lower ceilings and showers will
concentrate over windward and mauka sections through the day,
though VFR will predominate overall. Trade winds will gradually
decrease as low pressure areas more than 900 miles north of the
isles disrupt the flow from high pressure centers located around
2000 miles to the north.

An AIRMET for low level turbulence to the south through west of
the mountains, all islands, remains in effect, though will likely
be dropped later in the day as trades continue their downward
trend.


&&

.MARINE...
With the area of high pressure eroding north of the state, trade
winds will diminish today and tonight, then veer to a
southeasterly direction over the weekend. Thus, the current Small
Craft Advisory is still on track to be dropped by this evening and
should remain below the wind threshold for the remainder of the
forecast period.

Buoys 51001 and 51101 have started to show 17 sec forerunners but
they appear to be about 6 hours later than earlier model guidance
indicated and a bit smaller as well. This arriving swell should
peak late tonight or early Saturday then slowly decline Sunday
through Monday. Based on current trends and the most recent
guidance, the arriving swell may not reach High Surf Advisory
criterion. Buoy observations over the next several hours will be
monitored to see how much of a swell will arrive tonight.

Elsewhere, a low to the north of the state should provide a small
north swell today and Saturday. This swell should turn more out of
the northeast Sunday through Tuesday. There's also a small long
period south-southwest swell forecast to fill in today and tonight
before gradually subsiding through the weekend, which could
provide a small bump to the relatively quiet surf along the south
shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Southeast Waters.


&&

$$

DISCUSSION...M Ballard
AVIATION...Kinel
MARINE...Kodama

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



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