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Hawaiian Islands Synoptic Discussion and Guidance

722
FXHW60 PHFO 231415
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
415 AM HST Fri Feb 23 2018

.SYNOPSIS...
A low pressure system west of the islands will continue to draw
deep and unstable tropical moisture northward across the islands
into the weekend. Unsettled weather conditions are expected to
continue with good chances for rain along with locally heavy rain
and thunderstorms possible. Expect a drying trend next week as
the low center drifts slowly westward and high pressure builds in
from the east.

&&

.DISCUSSION...
Widespread rain showers with isolated thunderstorms are expected
to develop across the state today in an unsettled weather pattern
forecast to last into the weekend. Expect an elevated threat for flash
flooding to continue across the state with a Flash Flood Watch in
effect for all islands through this afternoon. The Flash Flood
Watch may need to be extended beyond the afternoon time period as
this dynamic weather situation evolves. A Winter Storm Warning
also remains in effect for the Big Island Summits above the 12000
foot elevation level.

A slow moving low pressure system west of the islands and a
strong 1040 mb high pressure center roughly 1800 miles northeast
of the state will dominate the weather pattern for Hawaii over the
next few days. These two systems will continue to advect deep and
unstable tropical moisture northward producing widespread rain
showers across the state with unsettled weather lingering into the
weekend. An upper level jet stream over the islands complicates
the pattern by lifting the atmosphere and assisting convection
due to upper level divergence forcing.

The challenge with this pattern lies in predicting where the smaller
mesoscale surface convergence and upper level divergence features
will interact with the islands to produce deeper convection with locally
heavy showers and thunderstorms. The latest forecast models
suggest the best chances for showers today will develop along the
eastern slopes of Maui and the Big Island. However due to deep
moisture amounts, strong instability and forcing aloft all
islands remain at an elevated risk for flash flooding today.

In the short term, the American (GFS) and European (ECMWF) models
are in fair agreement with an overall unstable weather pattern
producing widespread shower activity with locally heavy showers
and thunderstorms possible into the weekend. As the low slowly
drifts westward today and tomorrow, the deep tropical low level
moisture and upper level jet stream forcing will remain locked in
place.

On Sunday, a negatively tilted upper level trough rotating around
the western low passes over Kauai. This trough may bring a second
round of heavy shower and thunderstorm potential to the western
islands of Kauai and Oahu by Sunday afternoon. Model solutions on
the intensity of this trough remain less certain at this point.
We expect Sundays weather impact details to come into focus as the
time period gets closer.

The long range forecast for next week shows the high pressure
system moving westward with higher pressure building in over the
state. Downward moving subsident air under this ridge will
stabilize the air mass and produce a drying trend. However, east
to southeast wind speeds will also increase across all islands
from Monday night into Tuesday morning. These elevated wind speeds
are forecast to continue through the rest of the week.

&&

.AVIATION...
High pressure northeast of the state and a trough of low pressure
to the west, will keep an east to southeast flow in place across
the island chain through 12Z Saturday. A moist and unstable
airmass will remain over the island chain, with the most
unsettled weather expected across the eastern islands,
particularly the Big Island, where widespread MVFR and isolated
IFR conditions are expected over the next 24 hours. Across the
smaller islands daytime heating will likely lead to some shower
development over interior and leeward areas, along with some
brief MVFR cigs and MVFR and IFR vsbys. Conditions should improve
across most of the smaller islands tonight, with some more showery
weather possibly lingering over windward sections of Kauai.

AIRMET SIERRA remains in place for mountain obscuration across
interior terrain and NE to SE facing slopes of Oahu Molokai Maui
and the Big Island. Little change is expected through the early
morning hours, with some improvement possible later this morning,
particularly over Oahu and Molokai. Light icing will be possible
in the thick mid and high clouds blanketing the state through
the early morning hours. This is mentioned in the AIRMET
bulletin, but conditions are expected to remain below the AIRMET
threshold.

&&

.MARINE...
An upper level disturbance west of the islands will maintain an
unstable atmosphere over the region today, with heavy showers and
some thunderstorms possible. Although the threat is for all
waters, the greatest threat will be over the waters south and west
of all islands, as well as the waters near Kauai due to the
proximity to the upper level low. Expect reduced visibility,
gusty winds and rough seas with the heavier showers and
thunderstorms. The threat for heavy showers and thunderstorms will
gradually shift north and west into Saturday.

Persistent strong high pressure northeast of the state will
continue to produce an area of strong winds directed towards the
area. A lingering surface trough, just west of the offshore
waters will help to force a more southeast wind pattern for the
state. The east-southeast/southeast winds have continue to
increase over the last 12 hours, and some additional increase is
likely today, with winds remaining elevated through most of next
week. Model guidance suggests winds will increase further by the
middle of next week, possibly reaching gale force, with the
strongest winds around the Big Island and windward waters of the
smaller islands, with the southeast flow being maintained through
much of the event. The swell generated from these winds will
continue to produce elevated surf along east-facing shores.

A High Surf Advisory for east facing shores today will continue
through the weekend, and will likely continue into the middle of
the week when surf could potentially reach warning levels. A
Small Craft Advisory is posted across windward waters and around
the Big Island for the combination of the elevated seas and
increasing winds. As mentioned above, gale force winds are
possible with the increasing winds next week.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this afternoon for Niihau-Kauai-Oahu-
Molokai-Lanai-Kahoolawe-Maui-Kona-South Big Island-Big Island
North and East-Kohala-Big Island Interior.

Winter Storm Warning until 6 PM HST this evening for Big Island
Summits.

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

Small Craft Advisory until 6 AM HST Monday for Kauai Northwest
Waters-Kauai Windward Waters-Oahu Windward Waters-Kaiwi Channel-
Maui County Windward Waters-Alenuihaha Channel-Big Island
Windward Waters-Big Island Leeward Waters-Big Island Southeast
Waters.

&&

$$
DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...M Ballard

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