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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 261402
Area Forecast Discussion
National Weather Service Honolulu HI
402 AM HST Tue Jan 26 2021
A strong high pressure system far north of the islands and a
disturbance passing south of Kauai this morning will keep
moderate to breezy easterly trade winds, wet weather with heavy
showers, thunderstorms, and the potential for flash flooding in
the forecast through this afternoon. The flooding threat will
likely decrease by tonight with more stable weather conditions
moving into the region. Shower trends statewide will decrease
through Wednesday. Breezy trade winds will focus most of the
shower activity over windward and mountain areas through the
weekend. An increase in shower activity may develop statewide
from Friday night into Saturday.
A strong high pressure system far north of the state will produce
breezy easterly trade winds across the state into the weekend. An
upper level low passing south of Kauai this morning will continue
to slowly drift northwestward, moving away from the islands on
Wednesday. Unstable air around this low will combine forces with
the breezy trade winds to produce numerous rain showers, heavy at
times, across the state. Local radar imagery this morning shows
continued moderate showers moving into the eastern half of the Big
Island. An elevated flash flood threat will continue across the
state today and a Flash Flood Watch remains in effect for all
Hawaiian Islands until 6 PM HST this evening.
Decreasing rainfall trends will return to the region starting tonight,
with diminishing threats for heavy showers through Wednesday as
the upper low continues to drift northwestward away from the
island chain, and a stable upper level ridge builds in over the
region. The strong high pressure system lingering far north of the
state will keep breezy easterly trade winds in the forecast
through the weekend. Passing trade wind showers will favor
windward and mountain areas this week with an increase in shower
activity possible from Friday night into Saturday.
Short range global model guidance remains in good agreement with
this decreasing rainfall trend as an upper level ridge keeps more
stable weather conditions in place through Thursday. Medium range
guidance for this weekend are trending towards more unsettled
showers mainly affecting windward and mountain areas.
Strong ESE flow combined with abundant moisture is bringing
visibility concerns due to clouds and SHRA. Areas of MVFR and IFR
will remain today with isolated LIFR in the heaviest pockets of
AIRMET Sierra remains in effect for mountain obscuration for most
AIRMET Tango remains in effect for low-level mechanical turb and
strong surface winds.
AIRMET Zulu remains in effect for light to moderate rime icing.
The early Tuesday morning weather map shows a new 1034 mb surface
high located far N of the islands moving southward. It will
dissipate a good distance north of the islands on Thursday. A new
surface high follows in over the weekend. This series of highs
will help maintain breezy conditions through the rest of the week.
Eventually, the breezy trades will trim back to the typical locally
breezy picture late in the weekend.
The current wind has a slight south of east component, making it
not your true trade winds. A pair of surface troughs, one 620 nm
NW of Kauai and the other, some 280 nm SW of Kauai, is responsible
for this wind situation. The one furthest NW will continue to
weaken, while the second one heads further W, away from the islands.
This action will cause the winds to become east today, which is
considered trade winds.
The strong and breezy winds generated locally and upwind of the
islands have resulted in very rough and elevated seas and high
surf along east facing shores. A Small Craft Advisory (SCA) is in
place for all near- shore marine zones through 6 pm HST Wednesday.
A High Surf Advisory (HSA) has been extended through Wednesday
night to 6 am HST Thursday for the elevated surf along the east
facing shores. The fetch area of this trade wind swell will be
back to normal heights later in the week, so the surf, although in
the advisory range, will be smaller.
The current NW swell will continue to trend down through
Wednesday. Then a set of WNW to NW swells arrives Thursday night,
peak Friday night and Saturday, raising the surf along the north
and west facing shores to near advisory levels. After a busy
first few weeks of January where we saw some very very large
northwest swells. these large swell producing storm systems will
be absent for a while. So for early next week, we expect small to
moderate size swells ranging from the NW to NNE directions.
A surface low is forecast to drop south along the Pacific Northwest
in the next 24 hours before dissipating off the California coast
on Wednesday and Wednesday night. This low has already generated a
N to NNE pulse toward the islands. This swell will give boost to
the surf along exposed north and some east facing shores. This
swell is slated to arrive Thursday, peak Friday/Friday night, then
gradually subside through the weekend.
Surf along south facing shores will trend up through midweek, as
an out-of-season Tasman source arrives. Surf should drop and
return to normal levels through the second half of the week.
Flash Flood Watch through this afternoon for all Hawaii islands.
High Surf Advisory until 6 AM HST Thursday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-Big Island North and East.
Winter Storm Warning until 6 PM HST this evening for Big Island
Small Craft Advisory until 6 PM HST Wednesday for all Hawaiian
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office