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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 211427
Area Forecast Discussion
National Weather Service Honolulu HI
427 AM HST Sun Jan 21 2018
Moderate trade winds will deliver a steady stream of clouds and
showers the next couple of days as moisture associated with an old
frontal boundary combines with a weak disturbance aloft. Showers
will favor windward and mauka areas with periods of isolated to
scattered showers drifting leeward. Winds may diminish Thursday
through Saturday with a land and sea breeze weather regime
A few isolated thunderstorms developed over the Big Island last
night in the afternoon and evening hours with snow showers falling
on the summits of Maunakea and Maunaloa last night. We issued a
Winter Weather Advisory for 2 to 4 inches of snowfall above 12000
feet elevation valid until 6pm tonight. Roads to the summit of
Maunakea have been closed due to dangerously slick and icy road
conditions. Periods of additional snowfall are possible over the summit
areas through the upcoming work week.
Satellite imagery from this morning shows a band of cumulus and
high level cirrus clouds northwest of Kauai associated with an
approaching cold front and the upper level jet stream. A long
band of unstable clouds, extending roughly 2000 miles east of
Hawaii, has shifted slightly south of the Big Island overnight.
Short term forecast guidance shows this band will move northward
streaming back into the islands later this afternoon and evening.
The large scale analysis continues to show a high pressure system
centered over the East Pacific with ridge north of the islands,
and a cold frontal system moving through the Central Pacific.
Moderate easterly trade winds are now expected to blow across the
island chain through Wednesday. Unsettled and wet weather
conditions are forecast to continue through the middle of the week
as the high pressure ridge axis remains near 30 degrees north
latitude with troughing in the upper atmosphere. The position of
this ridge and the upper trough will help lift trade wind
inversion heights above normal levels, allowing a deeper boundary
layer to form with more active showers. The highest coverage for
clouds and showers will occur along the windward slopes of each
island with some showers drifting over leeward sections.
A second cold front with a much deeper low center will pass
into the Central Pacific on Wednesday. This cold frontal trough
will weaken the ridge north of the islands from Thursday through
Saturday. Winds will veer towards the southeast and become light
with surface heating and cooling over each island driving daytime
sea breezes and nighttime land breezes. Expect typical sea breeze
cloud ceilings over island interior sections with isolated to
scattered showers each afternoon. Mostly clear skies and isolated
showers are forecast each night as the land breeze develops.
The main updates to the forecast this morning include adding
isolated thunderstorms to the entire Big Island for the afternoon
through evening hours, and weather adjustments for the developing
land and sea breeze regime over the islands from Thursday to
A ridge of high pressure north of the state will keep a moderate
trade wind flow in place through through tonight. Clouds and
showers will favor windward areas through the period, with
showers also spilling over into leeward locales from time to time.
AIRMET Sierra is in effect for mountain obscuration across
windward sections of Kauai, Oahu, Molokai, Maui and the Big
Island. Some improvement is expected over the smaller islands
later this morning, with chances for improvement not as high over
the Big Island.
AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of the terrain of all islands. The AIRMET is
expected to remain in place through the day today.
Moderate to fresh trade winds will continue over the coastal
waters with a surface high located east-northeast of the state.
Another high is forecast to build in from the NW, continuing the
breezy trade wind pattern. A trade wind swell generated east of
the state will keep seas above 10 feet through Monday, so the
Small Craft Advisory remains in effect for most zones. The High
Surf Advisory (HSA) for east facing shores continues through today
The current small northwest swell will be reinforced by a long
period, WNW swell tonight. This swell will peak Monday night and
gradually decline through Wednesday. This second swell may warrant
an HSA for exposed west and north facing shores Monday and
Tuesday. A smaller NW swell is possible late in the week.
High Surf Advisory until 6 PM HST this evening for Kauai
Windward-Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-
Windward Haleakala-South Big Island-Big Island North and East.
Winter Weather Advisory until 6 PM HST this evening for Big
Small Craft Advisory until 6 PM HST this evening for Kauai
Windward Waters-Kauai Channel-Oahu Windward Waters-Kaiwi Channel-
Maui County Windward Waters-Pailolo Channel-Alenuihaha Channel-
Big Island Windward Waters-Big Island Leeward Waters-Big Island
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office