926 
FXHW60 PHFO 020146
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
346 PM HST Wed May 1 2019

.SYNOPSIS...
Moderate trade winds will focus clouds and mainly modest showers 
along windward slopes through tonight, with higher rainfall 
chances on Kauai. Trades will steadily weaken on Thursday, 
allowing clouds and spotty showers to develop over leeward and 
interior sections in the afternoon. An unseasonably strong front 
will approach the state on Friday, leading to an unstable 
southerly flow that will produce increasing showers over most 
islands. The front will move down the island chain late Friday 
night through Sunday, likely bringing gusty westerly winds, heavy 
showers, and possibly thunderstorms. Cooler northwesterly winds 
are expected behind the front into early next week.

&&

.DISCUSSION...
A stable trade wind flow will remain in place through tonight. 
The trades are being driven by a weakening surface high centered 
nearly 900 miles north of the state, while a ridge aloft is 
maintaining stable conditions with an inversion holding between 
5,500 and 8,000 ft. A band of low level moisture that has been 
draped over windward Kauai will maintain somewhat wet conditions 
on the Garden Isle through tonight, while mode rainfall pattern 
continues across the rest of the island chain.

On Thursday, trades will steadily decline, and the mid to upper
level ridge overhead will erode. The band of low level moisture
should remain over Kauai and near Oahu, keeping shower chances 
higher over windward Kauai and Oahu. With trades dropping off, 
afternoon sea breezes could produce spotty showers over leeward 
and interior sections, especially over the western half of the 
state. 

An unstable southerly flow will develop on Thursday night and 
Friday as an unseasonably strong front approaches the state from 
the northwest. The upper level trough supporting the front will 
produce cooling aloft that will wipe out the inversion. The GFS 
and ECMWF are showing a developing prefrontal trough focusing 
moisture just west of Kauai, leaving the state under an unstable 
land and sea breeze regime with near normal precipitable water. 
Though there could be an isolated downpour at any time, showers 
over land will favor the afternoon hours.

The front will reach Kauai as early as late Friday night then 
bring unsettled weather down the island chain through the weekend.
Future fine tuning will be needed, but the front will likely be
near Kauai in the predawn hours on Saturday, reach Oahu midday,
move to Maui County Saturday afternoon and evening, then
advance to the Big Island by Sunday morning. During this time, a 
potent upper level low will drop southward into the upper level 
trough and pass just north of the state. A jet stream along the 
southern flank of the upper level low should move over the islands
and produce forcing that will generate heavy showers along the 
front. At this time, the greatest amount of forcing appears to be 
to the northeast of the state, where a surface low could form. 
However, we will be looking to add thunderstorms to the weekend 
forecast, and either way, a line of heavy showers or thunderstorms
along the front could be capable of producing strong, or even 
damaging, winds. Another item of note is the potential for gusty 
downsloping southwest to west winds along and just behind the 
front. At higher elevations, the high summits on the Big Island 
and Maui will almost certainly experience warning level winds. 

Cooler northwesterly winds will build over the state behind the 
front. Though this flow will generally be drier, the close 
proximity of the rather strong, late-season upper level low could 
maintain chances for showers into early next week.

&&

.AVIATION...
As a high pressure center passes by north of the islands, trade winds
will be in the light to moderate range, with locally breezy periods,
through tonight. An extensive area of clouds and showers remains over
and upstream of Kauai, so AIRMET Sierra for mountain continues there,
but has been dropped for Oahu due to improved conditions. VFR will
predominate over the next 24 hours, though isolated areas of MVFR
conditions in lower ceilings and passing showers are possible and
will be mainly confined to windward and mauka sections state-wide.

&&

.MARINE...
A significant change in the island weather pattern is expected to
evolve into the weekend, as the trade wind-supporting high to the
N will be replaced by a deep-layer low that will push a front 
across the island chain.

Moderate trade winds will be diminishing through Thursday as high
pressure to the N moves NE and weakens. Thus, the Small Craft 
Advisory (SCA) for windier areas around Maui and the Big Island 
has been cancelled. A surface ridge will shift SE to near the Big
Island Friday and Friday night as a developing low (and associated
surface front) move toward the islands from the NW, resulting in 
light SE to S winds.

The front is expected to move from NW to SE across the island chain
over the weekend, with S to SW winds shifting to the NW after the
front passes. Significant uncertainty remains regarding the vigor
and placement of the associated convection, the strength of the 
wind, and (to a lesser degree) the timing of the frontal passage. 
Potential marine hazards include heavy downpours reducing visibility,
and erratic gusty winds, while sustained W to NW winds may 
briefly reach 25 kt over the weekend, requiring a SCA for some 
zones. Eventual impacts are highly dependent on the track and 
intensity of the low. Longer range outlook into next week 
anticipates weak low pressure centered just N of the islands, with
light to moderate W to NW winds prevailing.

The developing low will send a short-period swell toward the 
islands from the NW on Sunday/Monday, with surf expected to 
remain below advisory levels as the swell peaks Sunday night. 
Model guidance has not been consistent in the depiction of the 
surface winds associated with the approaching low, leading to 
reduced confidence in the surf forecast. In the meantime, distant 
lows in the NW Pacific have generated small NW swells that will 
arrive tonight and Thursday, and persist through Saturday.
Storminess deep in the S Pacific will send small S and SW swells
our way, with an uptick in swell energy expected over the weekend,
and maybe again early next week. The just-updated Oahu Surf 
Discussion (SRDHFO) contains details on the sources of the 
expected swells. 

&&

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

&&

$$

DISCUSSION...Wroe
AVIATION...TS
MARINE...Birchard