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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 211948
Area Forecast Discussion
National Weather Service Honolulu HI
948 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
The Winter Weather Advisory for the Big Island Summits is being
cancelled this morning. The webcams on the summits show snow has
stopped, and satellite shows few clouds reaching the summits.
Temperatures remain below freezing, and there remains the
possibility for and isolated thunderstorm or two this afternoon.
Any precipitation that does reach the summits will be in the form
of snow, however confidence is low for any doing so. Have lowered
the PoPs as a result.
The overnight surface analysis had a low pressure system about 850
miles north of Oahu, moving to the northeast. An associated
trough lay about 175 miles northwest of Kauai. A high pressure
ridge about 425 miles north of Maui County is maintaining a trade
wind pattern near the islands.
High clouds are streaming over the smaller islands from the west
southwest in the upper level wind pattern. At the lower levels,
clouds and showers continue to carried in on the trades to all
islands. The heaviest concentration of clouds and showers is over
Kauai and Oahu. The lingering cloud band from an old front is
slipping south and east of the Big Island, but is still being
carried in to the east side of the Big Island this morning.
There is good model agreement with an upper level trough digging
in from the north tomorrow, and lingering over the western islands
over through much of the work week. The surface ridge is expected
to shift to the east, turning our winds to a more southeasterly
background flow by midweek. As the background flow becomes
southeast and weakens, some areas will see local land and sea
breezes. The upper level feature will help to enhance shower
activity through the week.
Moderate trade flow will continue through today as a ridge of
high pressure persists to our north. Clouds and showers will favor
windward areas, with showers also reaching leeward areas from
time to time. VFR should dominate at all the terminals.
AIRMET Sierra is in effect for mountain obscuration across
windward sections of Kauai, Oahu, Molokai, Maui and the Big
Island through this morning. However, we anticipate dropping this
AIRMET for all islands at noon.
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.
Fresh trade winds will continue over the coastal waters with a
surface high located far east-northeast of the state. Another high
is forecast to build in from the NW, continuing similar wind
speeds into the middle of this week. The fetch of breezy trade
winds upstream of the state the last few days has directed a swell
towards east facing shores, which is expected to gradually lower
Monday and Tuesday. This swell is causing advisory level surf
heights along east facing shores as well as the windward coastal
zones to reach Small Craft Advisory (SCA) criteria today for seas.
Trade winds over the islands are adding additional zones to the
SCA for winds. In short, a High Surf Advisory is in effect for the
rest of the day for exposed east facing shores, and SCA is in
effect some coastal waters for both winds and seas. See CFWHFO
(Coastal Hazard Message) and CWFHFO (Coastal Waters Forecast)
products on our website for more information about the affected
The current small northwest swell will be reinforced by a long
period, west northwest swell tonight. This swell will peak Monday
night and gradually decline through Wednesday, and bring advisory
level surf heights to exposed north and west facing shores of the
smaller islands by Monday morning. A High Surf Advisory will
likely be issued for this event this afternoon, while the current
advisory for east facing shores expires. The west northwest swell
will also bring SCA seas to most coastal water zones, and we will
be reconfiguring the affected areas this afternoon. A smaller
northwest 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.
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