Area Forecast Discussion

FXHW60 PHFO 180721 AAA
Area Forecast Discussion...UPDATED
National Weather Service Honolulu HI
921 PM HST Sat Mar 17 2018
Just updated the zones for Kailua Kona coast, where a few showers
have popped up there. They will be dissipating by midnight.
Trade winds are back, and may even strengthen to locally strong by 
Tuesday. But will be here for most of the new work week. A band of
moisture will be moving through the island chain tonight through 
Sunday, bringing a uptick in showers to especially the windward 
and mountain areas. A front will approach the islands from the 
west and stall temporarily west of Kauai on Thursday. This will 
cause the trades to turn southeast by Friday. The front finally 
moves down the island chain over the upcoming week end, 
accompanied by a bout of wet weather.
Some cosmetic changes were made to the afternoon forecast package,
such as lowering the POPs and keeping the skies fair over 
windward Big Island overnight.
It is a nice evening out there for most islands. It is even nicer
now that the trades are back. We expect the trades to pick up a 
notch to moderate speed after a band of moisture moves through the
islands between tonight and Sunday. The leading edge to this band
is just north of Kauai's north shore this hour. However, there 
are already fragments of this band floating ahead of the main band
now affect the Garden Isle, which is the only fly-in- the-ointment
area wide. 
Next on the list is Oahu, and satellite imagery shows the leading
edge some 35 miles north of Kahuku. The band is pressing south at
10 to 15 mph. So, we expect increasing showers for windward Oahu 
mainly after midnight. The core of the weather is slated to reach 
Oahu around sunrise Sunday. The band will be working down to Maui
county Sunday afternoon, although fragments of this band riding 
ahead of it will reach windward Molokai and Maui by mid morning 
Sunday. The depth of this band of moisture is shallow, no higher 
that 7k feet. This mean, the showers will be confined and trapped 
to the windward and mountain areas. Lee areas of the smaller 
islands will be mostly dry, such as the Waianae coast, or even the
Oahu's south shores.
The moisture band will be slow to clear windward Big Island once 
it gets there Sunday evening. Models have the moisture lingering 
through Monday evening. Both the new GFS run and the old ECMWF 
solution, points to moderate trades through Tuesday, with a 
typical trade wind rainfall pattern, of scattered showers mainly 
nights and mornings. We will have the support of an upper level 
ridge over the island leading up to Wednesday that will limit the
amount and frequency of the trade showers. 
As for the strength of the trades, there is the potential of it 
reaching locally strong over the sensitive coastal waters of the 
island chain as early as Monday afternoon. The collapse of the 
strong trades is expected Thursday afternoon, as per GFS, with
east- southeast wind taking over area by Friday. This comes
about as the front slows down to a crawl some 350 miles west of 
Kauai on Thursday. The front finally pushes through the islands
during the upcoming weekend. The ECMWF has a similar picture as 
the GFS, although it brings in the softer east- southeast winds a 
day earlier on Thursday. With the lighter winds, we expect a rise
in shower activity area wide. The GFS is not showing this thus
far, not until the weekend when the front make its move.
Surface high pressure building far northwest and north of the 
state will produce a moderate low-level trade wind flow through 
Sunday. A band of low clouds and showers near Kauai will continue
to spread southward over the smaller island through early Sunday
morning. This band is expected to reach the Big Island on Sunday.
Expect an increase in low clouds and showers, with periods of 
MVFR ceilings / visibilities, particularly along most north and 
east facing slopes as this band continues to move south.
AIRMET Sierra has been issued for mountain obscuration on Kauai. 
This AIRMET may need to be extended to additional smaller islands
later tonight or early Sunday as the band of low clouds and
showers continues to shift south across the state.
No marine warnings or advisories are likely to be in effect for 
the next couple of days. Light winds will give way to moderate 
trades tonight, which will persist through Tuesday. Winds may 
approach Small Craft Advisory (SCA) levels in the windy zones 
around Maui County and the Big Island around Tuesday or Wednesday.
Winds will begin to shift out of the east southeast late Tuesday 
through the second half of the upcoming week as a front approaches
from the west and stalls west of Kauai. The front may bring a 
period of wet weather during the second half of the week and next 
The current northwest swell will fade, with a more northerly  
short period swell filling in Sunday. Small long period south 
swells should provide above null surf for the south facing shores 
over the next few days. Surf heights will remain below advisory 
levels on all shores through the early part of the new week.
A broad low is forecast to develop and move into the northeast
Pacific late this weekend and direct a large swath of gale force 
winds towards the islands. This should translate to a building 
north-northeast swell late Tuesday, and could bring advisory 
level surf to north and east facing shores through at least
Thursday, possibly longer for east facing shores.
H Lau