Weather Service Report

FXHW60 PHFO 200306 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
506 PM HST Mon Mar 19 2018

Trade winds will increase to locally breezy levels by midweek, 
then ease toward the weekend as high pressure strengthens to our 
north before getting edged out by a front approaching from the 
west. Trade winds will push clouds and showers across the 
islands, focusing rainfall mainly across windward and mauka 
areas, with only light amounts expected. Showers will increase 
late in the work week and over the upcoming weekend as the front 
passes eastward through the islands, with locally heavy rainfall 


Currently at the surface, a 1040 mb high is located around 1850 
miles north of Honolulu, with the ridge axis extending southward
from the high, and running roughly northwest to southeast around 
250 miles northeast of the state. The resulting gradient is 
driving moderate trades across the island chain this afternoon. 
Satellite imagery shows mostly cloudy skies in place across the 
state, with radar imagery and rain gauge observations indicating 
shower coverage is on the decline over windward areas of Oahu and
Kauai, with numerous to widespread showers continuing to roll into
windward sections of Maui and the Big Island. Additionally,
scattered thunderstorms have developed over the Big Island this
afternoon. Main short term concerns revolve around thunderstorm
potential through this evening and rainfall trends as drier air
works its way into the state from the north.

A pair of strong highs to the north of the area will merge into a
1045 mb high just south of the Aleutian islands by Tuesday 
morning. This will shift the band of showery low clouds affecting 
mainly windward areas of Maui and the Big Island, with improving
conditions expected later this evening across Maui, and by 
Tuesday morning across the Big Island. A shortwave impulse will 
pass over the Big Island this evening, so will continue to mention
a chance of thunderstorms in the forecast. After midnight things
should stabilize sufficiently that thunder will no longer be a 
threat. Moderate trade winds are expected to prevail through the 
overnight hours. 

Tuesday through Thursday night, 
Model solutions are in good agreement showing mid and upper level
ridging holding in place during the mid week time frame. The 
source airmass through this period is over the far northeast 
Pacific and will have advected southwestward over the islands due 
to the large high north of the state and the persistent low to the
northeast. As a result, we expect a stable weather regime over 
the island chain through the period, with dewpoints generally in 
the upper 50s and lower 60s. This will greatly limit shower 
activity across the state, with spotty light showers primarily 
confined to windward areas. Moderate trade winds will strengthen 
on Tuesday, and reach locally breezy levels Wednesday through 

Friday through next Monday,
The 12Z runs of the GFS and ECMWF are in reasonably good 
agreement showing shortwave energy digging to the west of the 
state Friday and Friday night, with a surface low developing near
or over the islands along a lingering surface trough over the
weekend. The surface low is then expected to lift northeast and
away from the state early next week. 

As for sensible weather details, deep tropical moisture is
expected to begin advecting over the western islands on Friday, 
then shift slowly eastward Friday night and over the weekend
resulting in a wet conditions statewide. Conditions should begin
to improve early next week as the surface low pulls northeast and
away from the state, dragging the deepest moisture east of the
island chain. There is the potential that heavy rainfall and flash
flooding could affect all or portions of the state over the
weekend, but it remains too early to tell how much rainfall will 
occur or where flooding, if any, would threaten. This will 
continue to be monitored closely as the week progresses. The wind
forecast will be a bit problematic as it will depend greatly on
the location, track and intensity of the surface low late week
through the weekend. Will take a blended approach to the winds for
the time being, although there will likely be some significant
changes as details become more clear.


A surface high far N of the area will maintain NE surface flow 
over the main Hawaiian Islands. These NE winds will continue to 
push showery low clouds from the trailing end of a front over the 
islands. MVFR ceilings will likely be extensive enough to require 
AIRMET SIERRA for MTN OBSC to remain up throughout the night for 
NE sections of most islands.

A sharp short-wave trough aloft is enhancing shower activity over
the islands today. Isolated thunderstorms have already developed
over the Big Island interior and waters west of the Big Island 
and south of Maui. Thunderstorm activity will likely continue 
into the evening hours but taper off after midnight. 


The morning ASCAT pass showed a wide swath of 40-50 knots 
associated with the strengthening low northeast of the state. 
Wavewatch guidance shows the swell associated with this low 
exceeding 10 feet from the northeast, peaking Wednesday/Wednesday 
night. Surf will likely reach warning levels along east-facing 
shores (with advisory along north-facing shores) during this 
period, then gradually diminishing through the week with the 
potential for advisory surf through Friday. 

Fresh to strong trades associated with a ridge to the north will 
drop a bit early on Tuesday, then ramp up again late 
Tuesday/Wednesday as the high far to our north starts to move 
southeastward and tighten the gradient. A surface trough 
associated with a low northwest of the state will start to edge 
eastward late in the week, gradually weakening the winds and 
turning them more southeasterly. Portions of the small craft
advisory that drop below the wind threshold of 25 knots on 
Tuesday will see seas building above 10 feet Tuesday night.


Small Craft Advisory until 6 AM HST Tuesday for Kauai Northwest 
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui 
County Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha 




Return to Home page

Western Regional Climate Center,