Current Conditions
Temp-1.0 C
RH99 %
WindS 35 mph
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 210705

Area Forecast Discussion
National Weather Service Honolulu HI
905 PM HST Tue Mar 20 2018

Cloudy conditions with breezy trades will continue into the second
half of the work week. Although most of the shower coverage will
focus over windward and mountain areas through Friday, a few passing
leeward showers, mostly light, will remain possible. Light winds
along with increasing rain chances are expected over the upcoming
weekend. Improving conditions will be possible Sunday into early
next week.


Cloudy and breezy conditions continue across the state this evening
due to strong high pressure north of the state and an upper trough
positioned several hundred miles west of Kauai near Midway. Water
vapor imagery showed plenty of middle- to upper-level moisture being
drawn northward east of this upper trough around a mid-level ridge
over the state, which supports the cloudy conditions in place.
Although most of the shower coverage expected will continue to focus
over the typical windward and mountain locations, radar showed light
showers, mostly in the form of sprinkles, being carried eastward
from the mid-level cloud coverage (some of which are not reaching
the ground). As a result, rainfall accumulations are expected to
remain on the light side, with the best chance for a few heavier
showers remaining over the western end of the state.

Guidance remains in decent agreement through the short-term period
(tonight through Thursday) and appears to be initializing well with
the current trends and patterns. Consensus supports breezy trades
holding as strong high pressure far north of the area shifts
southeastward across the northeast Pacific. The aforementioned upper
trough positioned to the west is forecast to hold in place near
Midway through Wednesday, then begin to dig southeast toward the
region Thursday. This upper pattern will continue to support cloudy
skies. Highest rainfall chances will remain over the western end of
the state where the best source of deeper moisture will reside.
Elsewhere, showers will remain focused mostly over windward and
mountain areas with a few passing showers leeward periodically.

Conditions will begin to evolve Friday through the upcoming weekend
as a shortwave trough digs southeastward and into the area bringing
a threat for heavy rainfall and even a few storms. Despite the model
spread between various solutions that is shown in each cycle, most
indicate weak low pressure developing along an approaching trough in
the vicinity of the islands Saturday with an abundance of deep
moisture being drawn northward over the islands (2" precipitable
water values). A combination of falling upper heights, instability
and lift, and deep tropical moisture will lead to a wet weekend
with a few thunderstorms, especially late Friday through

Sunday through early next weekend, improving conditions are expected
as drier air fills in and the upper trough and surface low lift
northeastward and away from the state. Guidance depicts light west
winds, potentially giving way to overnight land breezes and
afternoon sea breezes by early next week. Forecast confidence,
however, remains low through this extended period provided the
spread between solutions shown.


A surface ridge NE of the area will maintain low-level ENE winds
for Hawaii through tonight. Winds will increase through the
morning and afternoon hours tomorrow with an increase in the pressure
gradient. Slight instability in some areas will produce clouds
and light showers over N and E sections of the islands with tempo
MVFR ceilings and vis through tonight. An increase in showers is
not expected until the weekend.

AIRMET SIERRA for mountain obscuration for N and E sections of all

AIRMET Zulu for light icing from 120 to FL230 for entire area
through tonight.


An intense low pressure system from a day of days ago northeast
of the state has generate a large ocean swell in our direction. An
ASCAT passes earlier in the day showed a good fetch of severe
gale to localized storm force winds aimed at us from generally
020-040 degrees. Wavewatch guidance shows a max 10 foot swell
Wednesday and Wednesday night. Guidance has been a foot or two
underdone with recent events, and is possible with this episode as
well, especially when comparing overnight altimeter data with the
model initialization. With the larger heights, warning level surf
of 20-25 feet along north and east facing shores is expected.
The 7 pm HST reading from deep ocean buoy 51000, located 215 nm
NE of Maui, had a swell height as high as 9.5 feet with a 13
second period. Wave watch guidance has the swell topping out at 13
feet and 15 seconds beginning just before midnight tonight. We
will be monitoring the data from this particular buoy overnight.

As for other swells, there will be a series of small to medium
size NW swells through out the forecast period. Another NE swell,
but of a much smaller size of 3 feet is due in over the weekend.
It is about that time of the year to start looking out for swells
from the south. There is a small one of 2 feet, 16 second period,
from 210 degrees, slated to arrive on Friday.

The pressure gradient will be tightening across the islands
tonight into Wednesday as the high pressure system far north of
the islands moves southeast. Small craft advisory (SCA) level
winds will affect not only the typical sensitive areas such as the
Pailolo and Alenuihaha Channels, but also the near shore waters
NW of Kauai. Expect the trades to start weakening early Friday.

This combo moderate to strong trades and the expected large NE
swell, has result in the SCA covering all the near-shore coastal
marine zones.


High Surf Warning from 6 AM Wednesday to 6 PM HST Thursday for
north and east facing shores.

High Wind Warning until 6 AM HST Wednesday for Big Island

Small Craft Advisory until 6 PM HST Thursday for all Hawaiian




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