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Hawaiian Islands Synoptic Discussion and Guidance

193
FXHW60 PHFO 020632
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
832 PM HST Wed Apr 1 2020

.SYNOPSIS...
Deep low pressure moving in from far northwest of the state on
Thursday and Friday will cause trade winds to veer out of the
southeast, leading to a somewhat unstable land and sea breeze
regime that will produce spotty showers, briefly heavy at times,
over each island in the afternoon to early evening hours. Winds
will turn more southerly on Friday and the weekend, likely drawing
up deeper moisture over the islands and fueling more shower
activity. A convergent cloud band with the low will develop over
the state starting Friday night keeping unsettled weather in the
long range forecast through Tuesday.

&&

.DISCUSSION...
The satellite picture this evening shows an upper level low east
of the main Hawaiian Islands and a low level trough moving
westward through Kauai producing a slight enhancement for showers
over Kauai County. On the radar imagery conditions appear fairly
stable with passing isolated showers outside of the showers near
Kauai. A strong low pressure system roughly 1300 miles north-
northwest of Kauai will approach the islands over the next few
days, potentially delivering additional wet weather impacts to
the state on Saturday and Sunday.

A light east to southeast wind regime will continue for the next
several days as a low pressure area north of the island chain
continues to displace the high pressure ridge that normally
lingers north of the region. Expect land and sea breezes to
continue over all islands through Friday. Sea breeze convergence
over each island will cause clouds and showers to develop during
the day favoring afternoon to early evening hours. Overnight
divergent land breezes will bring improving weather conditions
over each island will diminishing cloud cover. Short range
forecast guidance through Friday shows the subsidence inversion
heights around the 7000 to 8000 foot level, enough for scattered
shower activity, however the southeasterly background wind flow
may limit shower production along eastern slopes as clouds tend to
move more parallel to the island chain.

The weather pattern changes this weekend as a large low pressure
system drifting in from the north becomes cut off from the mid
latitude steering flow. This low will slow down and become
stationary roughly 900 miles northwest of Kauai on Friday.
Initially a cold front out ahead of the low will approach the
islands before losing forward motion as it weakens. A low level
convergent cloud band will form over the islands, between the
southerly and southeasterly winds, out ahead of the diminishing
frontal boundary. Winds over the western half of the state will
become light southerlies bringing up deep and unstable tropical
moisture from the south by Friday afternoon onward. Shower
activity will trend higher from Friday into the weekend especially
over the western half of the state and along the southeastern
slopes of the Big Island from Kau to Puna. Additional moister and
instability within this convergent band may lead to heavy showers
at times with isolated thunderstorms. Medium range model guidance
between the American (GFS) and European (ECMWF) is in good
agreement through Sunday.

Longer range model guidance diverges a bit from Monday onward with
the GFS showing a drying trend from Monday into Wednesday as the
southern tail of this convergence band drifts east, while the
ECMWF model keeping the convergent band with enhanced showers
lingering over the islands through the middle of next week.
Predicting where the tail of this cloud boundary will end up this
far out remains challenging for the long range precipitation
forecast.

&&

.AVIATION...
Mostly a nighttime land breeze, daytime sea breeze scenario into
Thursday. Instability and light trades will keep the chance for
isolated showers tonight, especially over Kauai and on windward
coasts and slopes. Mountain obscuration should be brief and
passing but will issue AIRMET Sierra for mountain obscuration if
necessary.

Thursday afternoon inland areas again could see showers,
especially on the Big Island with sea breeze activity, some of
which could be heavy.

No AIRMETS are in effect at this time.

&&

.MARINE...
A surface high far northeast of the area is maintaining easterly trade
winds across Hawaiian waters, but a trough over the Kauai Channel will
keep the winds gentle to locally fresh though Thursday.

A low south of the Aleutians is forecast to move south and approach
the area, causing winds to veer out of the southeast tomorrow. Winds
will weaken over the western part of the coastal and offshore waters
but winds may strengthen over eastern waters, especially where the
southeast flow accelerates around the Big Island and Maui. Winds could
even reach Small Craft Advisory speeds over the Big Island windward
waters and Big Island leeward waters off South Cape.

A convergence band associated with the low will likely move over the
northwest offshore waters Saturday. Winds will veer out of the
south and a chance of thunderstorms will develop.

The current small northwest swell will continue subsiding through Friday
night. A new swell building Saturday will produce moderate surf along
north and west shores over the weekend, but is expected to remain well
below the advisory threshold. Swells from the southeast Pacific are
partly blocked by the islands of French Polynesia, but the current
swell is coming through a narrow swell window. This swell will maintain
elevated surf on the south shores through tomorrow, but surf will remain
below the advisory threshold. Surf along east shores will remain up
through midweek due to an upstream fetch, despite the trades relaxing
locally, then subside by the end of the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Bohlin
AVIATION...Chevalier
MARINE...Donaldson

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