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FXHW60 PHFO 202017

Area Forecast Discussion
National Weather Service Honolulu HI
1017 AM HST Tue Mar 20 2018

Local trade winds will continue as a trough remains to the west of
the state. Abundant cloud cover with passing windward and mountain 
showers will be the rule through the work week. Wetter and
unsettled weather is possible this weekend. 


Watching strong west to northwest winds over the Big Island 
summits, as they remain just below the summit wind advisory 
criteria in the 40 mph range with a few stronger gusts. Model 
cross sections indicate winds are peaking this morning, with a 
slow decreasing trend expected through the afternoon and night.

Radar and satellite indicates layered thick clouds invading the
state from the west this morning. Outside of the typically wet
windward areas, radar is showing some shower activity from about 
the west and north shore of Kauai across Niihau and points to the
west. The older ECMWF is handling the current situation better 
than the new 12z GFS. The preferred model scenario in the ECMWF 
shows showery conditions around Kauai during the afternoon, but 
holds the deeper vigorous convection well to the west. Thus 
expecting increasing clouds statewide and some light or moderate 
showers to be around Kauai County through the afternoon. An update
to increase pops for Kauai and introduce a chance of light 
showers for Oahu this afternoon as the moisture aloft increases 
from west to east.

The outlook for the rest of the work week looks pretty static with
trades at the surface and plenty of upper level moisture resulting
in abundant cloud cover and occasional chance of light showers.
One spot to watch out for is Kauai County by late Wednesday night
and Thursday as the ECMWF indicates a threat of more significant
precipitation immediately to the west. Any adjustment further east
to this rainfall would impact Kauai with several inches of

Models have general agreement on an upper trough pushing down
across the state on Saturday which would pull the focus for
synoptic scale lift and moderate instability across the state, 
bringing widespread showers. A surface reflection of the trough,
should it develop, would cut off the trades as well. 

At this time, there is significant disagreement among the various
models and within the ensemble members on the strength, timing and
position of the upper trough with several solutions showing a
slower and more cutoff trough at upper levels further to the
northwest of the state, which would delay or weaken the showers
over our area.


A surface ridge NE of the area will maintain low-level ENE winds 
around the main Hawaiian Islands. The low level flow will continue
to push enough clouds and light showers over N and E sections of 
the islands to produce tempo MVFR ceilings. AIRMET SIERRA for 
mountain obscuration will likely remain up for these areas through
at least this afternoon.

Ridging aloft should keep the atmosphere relatively stable and
limit the chance of heavy showers today. However, a trough lingers
west of the area, sending middle and high clouds over islands today.


A former storm low 1000 miles northeast of the state has sent a 
large swell our way. The past few ASCAT passes have shown a good 
fetch of severe gale to storm force winds aimed at us from 
generally 020-040 degrees. Wavewatch guidance shows an 10-11 foot 
swell peak. Recent events have shown the guidance to be a foot or
two underdone, which also seems the case here when comparing 
overnight altimeter data with the model initialization. With the 
larger heights, we could see surf 20-25 feet along north and east 
facing shores, enough to prompt a warning for both. This swell 
will fill in after midnight and peak Wednesday/Wednesday night. 
Given the degree of potential impact to east-facing shores, will 
update to issue the high surf warning this morning, with the 
hazard starting Wednesday. Warning-level surf is possible along 
east-facing shores into Thursday before dropping off faster on 
Friday into Saturday.

High pressure far north of the area will move southeast and
tighten the pressure gradient over the islands. Moderate to fresh
trades will increase a notch this evening, reaching small craft
levels through the channels and other acceleration areas. Winds
will fluctuate near small craft levels through Thursday then
weaken from Friday into the weekend. 


High Surf Warning from 6 am HST Wednesday until 6 pm HST Thursday 
for north and east facing shores of Niihau, Kauai, Oahu, Molokai, 
Lanai, Maui Island and Big Island.


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