National Weather Service Raw Text Product

335 
FXHW60 PHFO 080731
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
931 PM HST Thu Feb 7 2019

.SYNOPSIS...
A cold front will continue to move down the island chain tonight 
with strengthening northerly winds expected behind the front. 
Windy conditions will develop through Friday, with very windy and 
unsettled weather expected over the weekend as a low strengthens 
and moves south toward the islands. Unsettled weather may persist
into the middle of next week as the low lingers in the island 
vicinity.

&&

.DISCUSSION...
Currently at the surface, a 1007 mb low is located around 450 
miles north-northeast of Honolulu, with a cold front extending 
south-southwestward through Maui County. Meanwhile, a strong 1034 
mb high is centered around 1450 miles northwest of Kauai. Behind
the front moderate to locally breezy north to northwest winds 
have filled in, with moderate westerlies in advance of the front.
Infrared satellite imagery shows clear to partly cloudy skies
across Kauai and Oahu, variably cloudy conditions across Maui
County, and mostly cloudy skies over the Big Island. Radar imagery
shows scattered showers affecting Maui County and the Big Island,
with a few showers also affecting mainly north facing slopes and
coasts of Kauai and Oahu. Main short term concerns of many,
revolve around the dangerously large surf and potentially 
damaging winds expected over the weekend. 

Tonight and Friday,
The area of low pressure north of the state will track steadily
eastward over the next 24 hours or so, dragging a cold front 
eastward through the Big Island overnight then southeastward and 
further away from the Big Island on Friday. Meanwhile, the high 
northwest of the islands will strengthen to around 1039 mb by 
Friday afternoon. The gradient across the state will tighten as a 
result of these two features, with moderate to breezy north-
northwest winds overspreading the entire island chain by daybreak
Friday. The winds will increase further during the day on Friday,
and could potentially reach advisory levels across the smaller
islands by mid to late afternoon. Will evaluate the latest
guidance and make a decision on when to hoist a Wind Advisory for
portions of the state with the morning forecast package. 

As for sensible weather details, scattered showers will affect
Maui County and the Big Island tonight as the cold front sweeps
through. Behind the front light showers will be possible across 
north facing slopes and coasts in the northerly flow regime, with 
mostly rain free conditions aside from a spill over light shower 
or some sprinkles in leeward areas. The post frontal weather
regime will continue statewide through the day on Friday.

For the Big Island summits, strong winds are expected to continue
through the remainder of the night, and a Wind Advisory remains 
in effect through 6 AM Friday. The advisory may need to be 
extended into the day on Friday. Additionally, with mid and high
cloud streaming overhead tonight, some patchy freezing drizzle and
light snow will be possible at the summits, but amounts are
expected to remain light and below Winter Weather Advisory
thresholds. 

Friday night through Sunday night,
Model solutions remain in good agreement, showing a new area of low
pressure developing well north-northeast of the islands Friday
night, then deepening as it moves rapidly southward toward the 
islands Saturday and Saturday night. The low is then forecast to 
slow down and stall out a couple hundred miles north of the state 
Sunday and Sunday night. A surge of strong northerly winds is 
expected Friday night into early Saturday, and a Wind Advisory 
will likely be needed for all or a large portion of the island 
chain. The winds may trend briefly below advisory levels across 
portions of the state on Saturday, but the winds should ramp back 
up out of the northwest Saturday evening, then becoming very windy
out of the west-northwest after midnight Saturday through at 
least Sunday evening. High Wind Warnings will likely be needed for
at least portions if not the entire island chain during this 
period of time. Given the unusual northwest thru west direction 
during the expected peak of the wind event, downed trees and 
power outages will be a distinct possibility statewide, with the 
areas where the winds get funneled most effectively in these wind 
directions being the most susceptible. 

As for sensible weather details, periods of showery weather will
impact the islands with the showers favoring north and northwest
facing slopes and coasts. Given the strength of the winds and
elevated inversion heights, some of the showers will spread over
the terrain into the typically drier leeward areas as well. The
airmass will be plenty unstable given the cold temperatures aloft,
but the moisture depth is forecast to remain below 20 kft, so at
this time the probability of thunder although non-zero, appears
low. That said, with the strong winds aloft in place, as bands of
showers move through the islands squally weather can be expected,
and damaging winds within these squalls can't be ruled out.

For the Big Island summits, winds will increase Friday night with
High Wind Warning level winds likely Saturday through the 
remainder of the weekend. In addition, some wintry weather could
occur mainly Sunday and/or Sunday night as some deeper moisture
rotates through the area. In fact, temperatures will likely be 
cool enough that some wintry weather could even occur at the 
highest elevations of Haleakala as well. 

Overall, this low is shaping up to be a high-impact event, 
bringing multiple weather hazards, and significant threats to life
and property. As such, we have attempted to highlight the most 
significant weather impacts in an ongoing Special Weather 
Statement (SPSHFO) - until such time that Watches and Warnings 
become appropriate.

Monday through next Thursday,
Uncertainty in the forecast increases following the weekend 
storm, with the GFS and ECMWF continuing to display some
differences in the synoptic pattern evolution. Despite the
differences in the movement and of the low to the north of the
islands, they both indicate that unsettled weather will hold in
place through much of the upcoming week. As a result, will keep
showers in the forecast even for the typically drier areas 
through the period. Trade winds don't appear like they'll return
anytime soon, with the models showing a general west-northwest
wind direction continuing through much of the new week at anywhere
from moderate to locally strong speeds.

&&

.AVIATION...
A low pressure system north of the islands will produce isolated
to scattered showers across the region tonight and tomorrow. A 
high pressure ridge will build in over the islands on Friday with
increasing north to northeasterly winds forecast for all TAF 
sites. VFR conditions will prevail for most areas over the next 
24 hours.

AIRMET Sierra for mountain obscuration remain in effect for the 
Big Island this evening with low cloud ceilings improving after 
10Z.

AIRMET Tango in effect for moderate clear air turbulence from an 
upper level jet stream is producing over the Hawaiian Islands 
with pilot reports confirming this forecast. Expect this upper 
level turbulence to diminish after 10Z. 

&&

.MARINE...
A low pressure system is currently centered near 27N 154.5W, or 
slightly less than 400 nm north-northeast of Honolulu. This
feature, which is moving slowly eastward, is expected to gradually
intensify through Friday. Meanwhile, a much stronger low pressure
system is forecast to develop far north of the state on Saturday,
and then race toward the south through early next week. This 
second feature, is expected to bring unprecedented impacts, with 
very hazardous, and potentially life-threatening, conditions 
spreading across most Hawaiian Waters from this weekend into 
early next week. If experienced mariners are considering 
venturing away from protected waters this weekend, please prepare 
not only for very strong sustained winds and treacherous seas, but
for the likelihood of extreme squally weather conditions. If in 
doubt, DO NOT GO OUT!

Currently, increasing northwest to north winds are spreading down
the island chain from west to east. Gale force winds are likely
starting Friday evening over the western waters, and a Gale Watch
remains in effect for the waters adjacent to Kauai. Across the 
northern Hawaiian Offshore Waters beyond 40 nm from the Hawaiian 
Islands, winds will likely reach gale force starting Friday 
night, and then possibly increase to storm force (at least 50
knots!) by Saturday night. A Small Craft Advisory (SCA) has been 
issued for most Hawaiian Waters starting early Friday morning for 
a combination of increasing seas and winds. Expect the 
deteriorating conditions to eventually require either a SCA or a 
Gale Warning for all coastal waters from this weekend into early 
next week.

A High Surf Advisory (HSA) remains in effect for east facing 
shores of the Big Island, Maui, Molokai, Oahu and Kauai due to a 
long-period northeast swell. This HSA is currently in effect 
through early Friday morning.

Forerunners from a new large west-northwest swell are evident at 
buoys 51101 and 51001 early this evening. This swell will cause 
surf to build late tonight along most north and west facing shores
of the smaller islands by Friday afternoon. Therefore, a High 
Surf Warning has been issued for increasing surf along north and 
west facing shores of Kauai, Niihau, Oahu, and Molokai, and for 
north facing shores of Maui. Increasing surf may also require a 
HSA for west facing shores of the Big Island starting Friday or 
Friday night.

By late Saturday, the model guidance continues to insist that the
increasing winds and captured fetch associated with storm-force
low pressure system will cause extremely large and disorganized 
waves to spread across the coastal waters. The combined seas will 
become extremely high due to a combination of the longer period 
swell, and the huge wind waves generated by near gale to gale- 
force winds over the coastal waters. 

There is also a strong potential for extreme harbor surges in
north and west facing harbors, as well as significant coastal 
inundation and beach erosion along north and west facing shores of
most islands this weekend. This event may bring unprecedented 
coastal flooding to north and west facing shores, so anyone with 
vulnerable ocean front property needs to complete their 
preparations for this event as soon as possible.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning from 6 AM Friday to 6 AM HST Monday for Niihau-
Kauai Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu 
Koolau-Olomana-Molokai-Maui Windward West-Windward Haleakala.

High Surf Advisory until 6 AM HST Friday for Kauai Windward-Oahu 
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward 
Haleakala-Big Island North and East.

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

Small Craft Advisory from 2 AM to 6 PM HST Friday for Kauai 
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-Kaiwi 
Channel-Maui County Windward Waters-Maui County Leeward Waters-
Alenuihaha Channel-Big Island Windward Waters.

Gale Watch from Friday evening through Saturday afternoon for 
Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward 
Waters.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Houston