Wildfire in Hawaii, February 5 to 7, 2019

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
537 AM HST Tue Feb 5 2019

.SYNOPSIS...
High pressure north of the state will keep light to moderate trade
winds in place through tonight, with clouds and showers favoring
windward and mauka areas. A surface low will develop north of the
islands Wednesday through Thursday, disrupting the trades and
allowing for daytime sea breezes and overnight land breezes in
most areas. Clouds and showers will favor interior and mauka areas
during the afternoon and evening hours during the mid week
period. A cold front will then sweep through the islands Thursday
night and Friday, ushering in brisk northerly winds and showery
conditions late week through the weekend.

&&

.DISCUSSION...
Currently at the surface, a weak trough of low pressure is located
around 150 miles east of Hilo, while a 1031 mb high is located
around 1600 miles northeast of Honolulu. The gradient across the
island chain early this morning is producing light to moderate
trade winds, with some more sheltered areas experiencing land
breezes. Infrared satellite imagery shows partly cloudy skies in
most areas, with some areas of more extensive cloud cover noted
moving into windward areas, over leeward sections of the Big
Island, and upstream of the smaller islands. Radar imagery shows
scattered showers moving into and upstream of windward areas, and
a few showers grazing the leeward coast of the Big Island.
Meanwhile, leeward areas of the smaller islands are mainly rain
free at the moment. Main short term concerns revolve around
transitioning out of the trade wind pattern and the potential for
some locally heavy showers Wednesday through Thursday.

Tonight through Thursday night,
In the short term, model solutions are in good agreement, showing
the weak surface trough east of the state tracking steadily
westward just to the north of the islands through tonight. On
Wednesday, an approaching upper level trough will lower upper
level heights over the western end of the state, halting the
westward progress of the surface trough, and inducing
cyclogenesis a couple hundred miles north of Oahu. The developing
area of low pressure will remain nearly stationary north of Oahu
Wednesday night, then lift northeastward and away from the state
Wednesday night through Thursday night.

The current light to moderate trade winds should begin to
diminish later today and tonight as the surface trough moves
closer to the island chain. The gradient will be rather light
over the islands Wednesday through Thursday as the low develops
north of the state and lifts northeastward, with daytime sea
breezes and overnight land breezes expected in many areas. Later
in the day Thursday however, light to moderate northerly winds may
take hold across the western half of the state as a new area of
high pressure builds to the north of the islands and pushes a
surface cold front southward. This cold front will then push
southward into the western islands Thursday night, allowing
northerly winds to develop over the entire island chain, with
moderate to locally strong northerlies over the western end of the
state.

Based on the latest satellite and radar loops, it appears that the
GFS is too far north with the area of deeper moisture when
compared to the ECMWF which appears to be handling the situation
well. As a result, we have made some adjustments to the forecast
today into tonight to account for more cloud cover and showers
across the smaller islands. The clouds and showers upstream of
the islands associated with the surface trough are expected to
continue tracking westward into the smaller islands today into
tonight. The shower coverage and frequency will be greatest in
windward areas, but some showers will spill leeward as well,
particularly this afternoon into this evening. We should see
shower activity then begin to diminish after midnight as the
surface trough and associated deeper moisture begin to lift
northward and away from the area. The deepest moisture should
remain to the north of the Big Island today and tonight, so expect
more limited shower activity here. On Wednesday, expect a fairly
dry morning statewide with land breezes prevailing in most areas.
The shower activity should then favor interior and mauka areas
Wednesday afternoon as sea breeze development becomes widespread
statewide. This activity should linger into the evening hours,
with some locally heavy showers possible as upper level forcing
moving over the region erodes the low level inversion. In fact, a
rumble or two of thunder can't entirely be ruled out, but the
chances appear too low to mention in the forecast at this time.
Some drier mid-level air will begin to filter into western
islands on Thursday, and this could reduce the interior/mauka
shower activity here. The pattern will begin to shift toward a
more stable north flow shower regime Thursday night, with clouds
and showers favoring north facing slopes and coasts, with some
leeward spill over possible.

Friday through next Monday,
The extended forecast remains highly uncertain given the highly
unusual weather pattern depicted by the models for the local area.
Despite the anomalous nature of this pattern, model solutions are
in decent agreement given the amplitude of the upper level
pattern over the central Pacific. Both the ECMWF and the GFS show
a sharp shortwave trough digging southeastward well north of the
state Friday and Friday night, then close off a deep nearly
stationary upper level low just north of the islands over the
weekend through early next week. Both the models also show an
unusually deep surface low developing well north of the area, then
dropping southward toward the islands over the weekend into early
next week, although there remains differences in the the timing
and track of the low. Overall, this has the potential to be a
fairly high impact wind event across the area, given the strength
of the surface low in proximity to the islands along with the non-
typical northerly to northwesterly wind direction. As always,
this will be closely monitored as the event timing draws closer.

As for sensible weather details, a surface cold front is expected
to push southward through the central and eastern islands on
Friday, with moderate to strong northerly winds filling in behind
the front. Strong north-northwest winds will likely continue
through the weekend into early next week, as the strong surface
low rotates southward and makes a close approach to the island
chain. It's still a bit early to know how strong these winds could
be given timing and track differences between the models, but at
this point, wind advisory level winds look like a distinct
possibility across much of the state beginning as early as Friday
and continuing right through early next week.

The extended period looks rather showery as bands of low clouds
and showers rotate southward through the island chain. Some
locally heavy showers will likely affect the area given the
unstable pattern that will be in place, but widespread flooding
problems are not expected given the fast moving nature of the
showers in the strong north-northwesterly boundary layer flow.
Showers will favor north facing slopes and coasts through the
period, but showery conditions will likely affect leeward areas at
times as well, given the strong winds and unstable airmass in
place.

&&

.AVIATION...
NE trades have been easing into the light to moderate range as a
high far NE of the islands moves southeast and slightly weakens.
VFR will prevail overall, with MVFR conditions likely, from time
to time, mainly in windward and mauka sections with passing lower
clouds and showers.

No AIRMETs are currently in effect.

&&

.MARINE...
Moderate to fresh trades associated with high pressure to the
north will continue today, then weaken tonight through midweek as
an upper trough approaches and disrupts the trade flow. Land and
sea breeze conditions near the coasts are likely tonight through
Thursday morning.

Guidance remains persistent and continues to advertise low
pressure evolving north of the islands within 500 nautical miles
by Thursday in response to the amplifying upper pattern. As this
feature develops, northerly winds on its backside will begin to
fill in across the island chain and strengthen Thursday through
Friday. The GFS is the more aggressive solution and suggests this
feature will reach storm-force category on Saturday with a decent
area of 40-50 kt winds focused at the islands out of the
north-northwest within 500 nautical miles and seas climbing into
the 25-35 ft range. The ECMWF is similar, but slightly weaker than
the GFS.

Despite the small differences, the likelihood for low-end gale-
force northwest winds can't be ruled out across the waters this
weekend. The latest wave-model guidance responds and depicts seas
ranging from 15-25 ft locally. Changes will be likely in the
coming days as confidence steadily rises and the scenario
evolves.

Surf along north facing shores will remain up through midweek as
overlapping, moderate west-northwest (300 deg) swells continue. A
moderate west-northwest swell is expected to arrive by tonight,
hold through Wednesday, then lower into Thursday. Surf along north
and west facing shores will near advisory levels for Kauai as it
peaks. Slightly smaller surf is anticipated on the rest of the
island chain due to the west-northwest direction. The exception
could be along the leeward coast of the Big Island from Kona to
South Point that is exposed to this west-northwest swell
direction. The best chance for advisory-level surf along this
coast will be through the day Wednesday.

A more significant west-northwest (300-320 deg) swell is expected
Friday through Saturday that will drive surf well above warning
criteria for exposed north and west facing shores. The source for
this large swell is currently being analyzed as a hurricane-force
low centered over the far northwest Pacific. Monday's ASCAT pass
showed a large batch of storm- to near hurricane-force westerly
winds focused at the islands within the 300-320 deg directional
bands. An altimeter pass near the head of this fetch showed seas
as high as 43 ft.

A combination of this large long-period west-northwest swell,
strong- to near gale-force north-northwest winds locally and
large short-period seas will increase the likelihood for harbor
surges and coastal inundation from wave run-up Friday through the
weekend. The latest PacIOOS wave run-up guidance and surge index
guidance for the Haleiwa Harbor reflect this potential through the
weekend.

Surf along east facing shores will hold steady through Tuesday, then
potentially rise by Wednesday as the gradient increases upstream of
the state. A long-period northeast (030 deg) swell from a large batch
of strong- to gale-force northerly winds over the far eastern
Gulf of Alaska is possible through the second half of the week.
WAVEWATCH III guidance depicts this swell peaking Thursday near
advisory level.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...Jelsema
AVIATION...Kinel
MARINE...Gibbs