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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 161349

Area Forecast Discussion
National Weather Service Honolulu HI
349 AM HST Fri Apr 16 2021

Clouds associated with a dissipated cold front will move in from
the northwest. Trade winds will decrease today as a small increase
in clouds and showers are forecast over the smaller islands
through Saturday. Trade winds will strengthen from Saturday to
Sunday, and then weaken to a light and variable pattern from
Monday into Wednesday as another cold front approaches the islands
from the northwest.


Cloud bands associated with a dissipated cold front are drifting
into the islands from the north this morning. A larger low
pressure area and stronger cold front remains far north of the
Hawaii region. A high pressure center also lingers far northwest
of the state with a weakening ridge axis north of the islands.

Trade will weaken today with localized sea breezes developing
over each island. Weak cloud bands drifting in from the north will
bring a small increase to cloud and shower activity through
Saturday. This slight increase in shower activity should diminish
by Sunday morning as these clouds diminish and drift westward
with returning trade winds.

Another change in our weather pattern will occur early next week
as another cold front approaches the Hawaiian Islands from the
northwest direction. This frontal system will break down the weak
ridge north of the island chain and produce a light and variable
wind pattern from Monday into Wednesday. Light large scale winds
will allow local scale daytime sea breezes to develop over all
islands. Converging sea breezes and island heating will build
clouds over island mountain and interior sections each day with a
few showers favoring the afternoon to early evening hours.

Long range forecast guidance shows the forward motion on the front
stalling out as it approaches the state. Differences between the
American (GFS) and the European (ECMWF) models in the eastward
frontal movement continue in the latest operational forecast
model runs. Both models are showing better agreement on the
frontal movement, however as expected some differences remain. The
GFS model continues to be more progressive and brings cloud and
showers ahead of the slow moving front into Kauai and Oahu on
Tuesday and Wednesday. While the latest ECMWF model stalls the
front over the island of Kauai. Shower coverage and trends with
the frontal cloud band will favor Kauai and Oahu. Precipitation
amounts remain modest in both global model solutions. We blended
both model GFS and ECMWF solutions into the precipitation
forecast for Tuesday and Wednesday in order to minimize forecast
errors from any one model solution.


A surface ridge far north of the islands will maintain light to
moderate low-level northeasterly trade winds today. The
background flow may be weak enough to allow local sea breezes to
form over leeward sections of some of the islands this afternoon.
The leading edge of remnant moisture from a dissipated front
appears to be just northwest of Kauai early this morning. As this
moisture continues to push down toward the islands, expect broken
low clouds and scattered showers to begin spreading across Kauai.
As a result, MVFR conditions due to lower ceilings and reduced
visibilities may become more widespread over north through east
facing sections of Kauai later this morning.

Elsewhere, the northeasterly flow will continue to transport
scattered low clouds and isolated showers into north through east
facing sections of the islands east of Kauai. This may result in
brief MVFR conditions along some north through east facing slopes
from Oahu to the Big Island today. Otherwise, expect VFR
conditions to prevail over most of the islands east of Kauai.

The remnant moisture from the former front may eventually spread
across portions of Oahu and the islands of Maui County later today
and tonight. Depending on the cloud cover and shower activity,
MVFR conditions may become more widespread over some north through
east facing sections of Oahu, Molokai and Maui, especially after
sunset this evening.

No AIRMETS are currently in effect, and none are anticipated at
this time. AIRMET Sierra for mountain obscuration may be needed
on Kauai later this morning if there is a significant increase in
cloud cover and shower activity.


Low pressure moving into the area north of the state will weaken
trade winds and swing them to northerlies today into Saturday.
Northeast winds are expected by Sunday as the low moves off to the
northeast and the high reestablishes itself north of the islands.
Light winds will dominate Monday and Tuesday as remnants of a
frontal boundary approach from the northwest.

Short period north swells will maintain well-below advisory level
surf along north shores into next week. The north swells will also
bring an uptick in surf to exposed east and west shores. Background
southerly swells will maintain small surf along south facing shores
with a slight bump possible over the weekend.






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