Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 181411

Area Forecast Discussion
National Weather Service Honolulu HI
411 AM HST Sun Mar 18 2018

Trade winds are back, and will be here for a few days. They will
also turn a bit windy in some place by Tuesday. A band of clouds
and showers has reached Oahu. It will continue to move south and
east, getting to Maui by mid morning, then on to the Big Island
early this evening. With trades, expect the showers to focus on
the windward and mountain areas, especially the nights and
mornings. Some of these showers will extend into the leeward areas
of the smaller islands, but rainfall will be light. An eastward
moving front will be in range by Thursday night to cause the trades
to become lighter. Expect the front to move down the island chain
over the upcoming weekend, accompany by some scattered showers.


The band of clouds and showers has behaved nicely, reaching both
Kauai and Oahu in a timely manner. The core of this band should be
over Oahu around sunrise. Then, it is on to Maui by mid morning,
and the Big Island early this evening. The depth of this band is
shallow, no higher than 7k feet. Thus, the southern half of Kauai
is mostly dry. No so on the island north side, with rainfall totals
ranging from 3 hundredths of an inch at several locations to 1.22
inch at Kilohana. Wainiha picked up 49 hundredths, and Kapahi
with 22 hundredths. Mount Waialeale has only two hundredths of an
inch, telling us how shallow this moisture band is. Waialeale`s
elevation is 51 hundred feet. These rainfall totals are from the
3 am HST readings.

We expect the remnant moisture to get hung up temporarily over
the windward areas after the band has moved on. Conditions should
be drier with more sunshine for the smaller islands by Monday
afternoon. As for the Big Island, its size and terrain will slow
the clearing process. It is not until Tuesday morning where there
will be some significant improvements.

Up until Thursday, an upper level ridge over the islands will
help limit the trade showers.

The models have the trade winds strengthening to moderate speed
behind this band. We have noticed a glimpse of it when it roll
through Kauai earlier in the evening. By Tuesday, the trades will
kick it up another notch, to moderate to locally strong, as a
ridge of high pressure builds in from the north. The parent high
is a very strong 1046 mb cell centered over the Aleutian Islands,
some 2000 miles north of here. It is a very broad high, taking up
a good chunk of the central North Pacific Ocean. The trades will
remain at this range through Wednesday night. Then on Thursday,
the locally strong trades collapses as a front nears Kauai. Light
easterly winds fills in across the area on Friday as the front
nudges closer to the islands. The front is expected to ease down
the island chain over the upcoming weekend. It will have some
upper level support by way of an upper level trough approaching
the islands from west late Thursday night. So, there is the
potential for some rough weather during this time.


Surface high pressure building far north of the state will
maintain moderate trade winds through tonight. The leading edge
of a diffuse band of low clouds and showers is approaching the
central islands early this morning. The clouds and showers within
this band will continue to spread southward over the smaller
islands, before reaching the Big Island later today. These low
clouds and showers will produce periods of MVFR ceilings /
visibilities, particularly along north and east facing slopes of
most islands.

AIRMET Sierra remains in effect for mountain obscuration on Kauai
and Oahu. This AIRMET will likely need to be extended to
additional islands east of Oahu later today 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 have been giving way to
moderate trades, which will persist through Tuesday. Winds may
approach Small Craft Advisory (SCA) levels in the windy zones
around Maui County and the Big Island by Tuesday or Tuesday night.
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 today. 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.





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