High Wind in Hawaii, January 27, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
350 PM HST Sun Jan 27 2019

.SYNOPSIS...
Locally gusty north to northeast winds will keep a cool air mass in
place through the upcoming week. Clouds and limited showers will
primarily affect windward areas, while leeward areas remain dry.
Lighter winds and increasing moisture are possible by the end of the
week.

&&

.DISCUSSION...
Currently, a cool, dry and locally breezy N to NE flow prevails in
the low-levels, due to the gradient between a surface low about 700
miles NE of the islands, and a surface high to the distant NW. PWAT
is below normal (near 0.8"), with a weakened subsidence inversion
based between 6000' and 8000'. The low-levels are dominated by
scattered stable stratocumulus that are primarily affecting windward
areas, with radar and rain gages indicating zero associated
rainfall. However, layered mid- and high-level clouds extend
eastward from Maui and the Big Island, developing in response to
very cold air aloft moving in from the NW, with radar detecting
precipitation from these clouds that is not reaching the ground.

The forecast anticipates a long wave mid- and upper-level trough
persisting just E of the islands through the upcoming week, with a
cut-off low aloft developing within the trough over the next 12-24
hours. Once this low develops, it is expected to loiter just E and
NE of the islands through Friday. This will support a surface low in
the same general area, while surface high pressure will prevail to
the NW of the islands. The associated gradient will keep cool and
locally breezy to strong N to NE winds over the island chain through
the week. With the (relatively weak) surface low forecast to drift
closer to the islands over the next 48 hours, another surge in N to
NE winds is possible as the gradient tightens, and a Wind Advisory
for lower elevations may be necessary, especially where winds are
most accelerated by island terrain. The low aloft will also bring
periods of strong winds over the summits and upper slopes of the Big
Island, with impacts strongly dependent on the position and movement
of the low. A Wind Advisory has been issued for the summits, and may
need to be upgraded to a High Wind Warning on Monday.

The low-levels are expected to remain on the dry side through
Tuesday, and shower coverage and intensity is expected to be
limited, with stable low clouds favoring N and E slopes and coasts.
However, very cold air aloft, as low as -19C at 500 mb, will be
brushing past the islands to the east (especially the Big Island)
tonight through Monday. This extreme (by island standards) cold air
aloft will destabilize the mid- and upper-levels of the atmosphere,
but low-level air parcels are not expected to be able to tap into
the instability aloft. Having said that, a heavy shower that
recently originated over the leeward Big Island slopes (it dropped
0.89" of rain at Honaunau) is currently soaking the South Point
area, but is expected to dissipate shortly.

The extreme cold air aloft is expected to remain E of the islands
thereafter, but spokes of vorticity/cold air aloft are expected to
periodically rotate around the slow-moving low aloft this week. This
could bring subtle increases in low-level moisture riding in on the
NNE flow, leading to increased windward showers. Latest GFS guidance
indicates a moisture increase around Tuesday night, which generally
remains in place through the end of the week, while the ECMWF
indicates less of a moisture increase. Otherwise, mostly dry and
cool conditions will continue - with dewpoints in the 50s, and
daytime highs struggling to hit 80.

Toward the end of the week, guidance indicates that the surface low
will move into a position N of the islands, potentially leading to
light and variable winds while also bringing another increase in low-
level moisture.

&&

.AVIATION...
Satellite imagery shows a a disturbance passing by north of the
state this afternoon, with a blanket of stratocumulus and layered
higher clouds covering the eastern half of the state. The moisture
layer with this disturbance is too thin to generate any significant
precip, but is enough to create light to moderate icing issues.
AIRMET Zulu is now in effect over/around Maui and the Big Island in
the 120-FL220 layer. AIRMET Sierra is not expected to be an issue
through tonight.

A NW to SE oriented jet stream currently sits just north of the main
Hawaiian Islands, and has been producing light to moderate
turbulence between FL200 and FL400 feet today. Models are indicating
little change through tonight as an upper level low forms well NNE
of Oahu. As the jet stream moves south over the islands through
Monday, the likelihood of turbulence aloft will remain. At the
surface, a low pressure system is forecast to form in a similar
location this evening, reinforcing the breezy northerly winds.
AIRMET Tango for low-level turbulence remains in effect south
through west of all mountain ranges below 8k feet and should
continue through at least Monday. Cool and breezy northeasterly
winds will persist over the islands for the next several days.

&&

.MARINE...
A nearly stationary area of low pressure east and northeast of the
state along with a series of reinforcing highs building northwest of
the island chain, will keep moderate to strong north-northeasterly
winds in place across the marine area over the next several days.
These winds will produce Small Craft Advisory (SCA) conditions for
most of the marine zones east of Oahu due to winds and seas. As
reinforcing highs build northwest of the islands and northwest
swells move through the area through the week, the SCA will need to
be adjusted at times to add or remove the appropriate marine zones.

Buoy data show that the current northwest swell continues to slowly
decrease. A series of new, moderate to large, northwest swells will
move through the islands during the week. The first will start
arriving late tonight, peak around the High Surf Advisory (HSA)
level across north and west facing shores Monday into Monday night,
then lower Tuesday. At present, the HSA only covers Kauai County but
will be monitored as the swell train reaches the buoys to see if the
HSA coverage needs to be expanded.
The second northwest swell is expected to build Tuesday night and
Wednesday, then peak close to warning levels late Wednesday night
and Thursday. This swell will then slowly lower through the end of
the work week. Another northwest swell may then bring advisory level
surf to north and west facing shores next weekend.

A north to north-northeast swell generated by a gale low northeast
of the state will produce rising surf along exposed shores over the
next couple of days. Surf is expected to peak above the east-facing
shores HSA threshold Monday night through Wednesday, then slowly
decline through the rest of the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from 6 AM Monday to 6 AM HST Tuesday for North
and West facing shores of Niihau and Kauai.

Wind Advisory until 6 AM HST Tuesday for Big Island Summits.

Small Craft Advisory until 6 AM HST Tuesday for Kaiwi Channel-Maui
County Leeward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-
Big Island Windward Waters-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory from midnight tonight to 6 AM HST Tuesday for
Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel.

&&

DISCUSSION...Birchard
AVIATION...TS
MARINE...Kodama