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Area forecast discussion...corrected
National Weather Service Honolulu HI
436 PM HST Fri Feb 24 2017

Corrected synopsis

Synopsis...
dry trade winds will continue to ease overnight. Clouds and
showers will favor windward and Mauka areas through Saturday.
Expect showers to start to increase Sunday with The Big Island
seeing the initial increase. 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 unchanged, with relatively
dry conditions expected to persist for another 24 hours. Winds
have started to subside, and are expected to continue this trend
through the weekend.

Afternoon soundings at Lihue and Hilo continued to show below
normal precipitable water (pw) values of 0.73 and 0.60
respectively. Satellite derived precipitable water shows the dry atmosphere
extending well to the northeast of the islands. However
increasingly moist air to the south of the islands will be the
focus as we head into the second half of the weekend.

Saturday night we expect to see precipitable water values increasing near The Big
Island, with values continuing to climb into Wednesday as the
moisture south of the islands moves to the north. The wind flow
near the islands is expected to veer to the east southeast to
start the new work week, and the moisture will spread the smaller
islands.

Meanwhile, an upper level trough over the northern end of the
state is already bring below normal temperatures to the upper
levels. A series of short waves traveling through the upper
trough during the first half of the week will help to enhance
showers, particularly afternoon showers Monday and Tuesday. While
not yet in the forecast, there is the potential for some of the
afternoon showers to be heavy. The latest model runs indicate more
instability during this time than the previous runs, but will be
monitoring to see if this upward trend continues.

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 European model (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 evening
and overnight periods, though VFR will predominate overall. Trade
winds have been gradually decreasing 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. However, it will
be dropped with the evening issuance at 1800 (6 pm) HST.



&&

Marine...
the expected northwest swell appears to have arrived at buoys
51001 and 51101, albeit a little later than forecasted by the
previous wave model runs. This swell is expected to peak later
tonight or Saturday before slowly declining Sunday through Monday.
It was earlier thought that surf may not reach high surf advisory
criterion but a jump in the swell heights to nearly 8 ft suggests
that surf may reach low end advisory levels. Thus, a high surf
advisory has been issued for the north and west facing shores from
Kauai County to Maui County.

With respect to the winds, the area of high pressure north of the
state has been eroding due to the approach of a developing low
pressure system to the northwest. As a result, trade winds are
weakening and will veer to an east-southeastly or southeasterly
direction over the weekend. This means the Small Craft Advisory
for the windier coastal waters of Maui County and The Big Island
will end this afternoon. However, the winds will not drop fast
enough before the new swell reaches the western coastal waters so
a Small Craft Advisory has been issued for the areas north of
Kauai County and Oahu.

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. A reinforcing swell from the
northeast may arrive on Monday and produce advisory level surf for
the east-facing shores. There's also a small long period south-
southwest swell forecast to fill in through tonight before
gradually subsiding over the weekend. This will provide a small
bump to the relatively quiet surf along the south shores.



&&

Hfo watches/warnings/advisories...
high surf advisory until 6 am HST Sunday for Niihau-Kauai
windward-Kauai leeward-Waianae coast-Oahu North Shore-Oahu
koolau-Molokai-Maui windward west-Maui Central Valley-windward
Haleakala.

Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island southeast waters.

Small Craft Advisory until 6 am HST Saturday for Kauai northwest
waters-Kauai windward waters-Oahu windward waters.



&&

$$

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