FXHW60 PHFO 220112 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
312 PM HST Wed May 21 2025

.SYNOPSIS...
High pressure will meander around far northwest of Hawaii through
Saturday, keeping breezy trades blowing. A low aloft approaching 
the islands from the north will enhance incoming trade wind 
showers starting tonight, and continuing through Friday. A few 
downpours are possible, and a rumble or two of thunder can't be 
ruled out, mainly for Kauai and Oahu which will be closest to the
upper low. The holiday weekend will see us trend to much drier 
and more settled tradewind weather as the upper low moves away.

&&

.DISCUSSION...
Breezy trade winds over the islands are being driven by a
relatively strong and large high, shown on scatterometer data
roughly 1200 mi NNW of Kauai. Models show this high will meander
around the same general area through Sat. Meanwhile, a mid- to 
upper- level low located about 350 miles NNW of Kauai is pushing 
SSW. Global models are in good agreement that this low will settle
in near or over the main Hawaiian Islands on Thu. 

The low will be accompanied by fairly impressive cold air aloft, 
with 500 mb temperatures over Kauai expected to drop to about 
2-sigma below normal Thu morning. Guidance thus shows about 
1000-1500 J/kg of 500 m MLCAPE, along with a small pocket of upper
level divergence over the northern main islands. However, at the 
same time, models insist that trades will continue in the boundary
layer with bands of low-level moisture riding through from time 
to time, with one of those noteworthy bands arriving Fri.

This setup will help to enhance incoming bands of clouds and 
showers riding in on the trades, and might even lead to a couple 
of brief thundershowers as well (a pattern sometimes colloquially
known as "thundertrades"). The latest NBM thunder guidance maxes 
out below 10 percent near Niihau Thu night, then fades quickly. 
However, our locally developed hekili guidance shows probabilities
from 10-20 percent over and around Kauai and Oahu from both GFS 
and ECMWF, lingering into Friday before dropping as the upper low 
starts to move away to the NE. Usually in patterns like this, the 
most favorable areas for thunder are in the ribbons of low-level 
convergence downwind of the islands, but it's not impossible to 
get hefty downpours and a rumble or two windward and mauka too as
trade wind shower areas move in from the ENE. Based on a blend of
this data and pattern recognition, will keep a mention of 
t-showers in the forecast for portions of Kauai and Oahu tonight 
through Fri. Interior Big Island will also continue to have a
small chance for a couple of upslope thundershowers Thu 
afternoon, just like we saw today over the south flank of Mauna 
Loa where NLDN detected a single lightning strike there.

The southern end of the chain should not see as much shower  
enhancement from the rather compact upper low, but trades will 
still be somewhat wet from time to time due to the proximity of 
the upper low, as any more organized pockets of moisture ride in.

The airmass stabilizes by the holiday weekend as the upper low 
moves quickly away. Wet trades will become much drier, with fewer
clouds and showers, and more sunshine. Trade wind speeds will 
eventually ease some as the high to the N weakens to a much weaker
subtropical ridge by the middle of next week.

&&

.AVIATION...

Moderate to locally breezy trade winds will strengthen slightly
heading into tomorrow. Low clouds and showers will favor windward
and mauka areas, particularly overnight and during the morning 
hours, with showers reaching leeward communities at times. MVFR
conditions will be possible in showers. Otherwise, VFR conditions
will prevail.

No AIRMETs are in effect this afternoon. AIRMET Sierra might be 
needed for mountain obscuration across windward areas overnight 
into tomorrow morning as shower coverage is expected to increase. 

&&

.MARINE...
High pressure to the distant north- northwest will maintain 
moderate to locally strong trade winds through the weekend. An 
upper level low to the north of the waters may bring periods of 
instability and isolated thunderstorms over Kauai and portions of
Oahu waters tonight through Thursday. A Small Craft Advisory is 
in effect for the windier waters and channels around Maui County 
and the Big Island through Thursday and may need to be extended 
through the weekend. 

Surf along south facing shores will remain small to moderate as 
a long period south-southwest swell is expected to fill in late 
tonight into Thursday. A second pulse from the same direction will
fill in over the weekend and keep small to moderate surf through 
the weekend before declining early next week. 

East shore surf will remain near seasonal average during the next
couple of days as trade winds persist. Tiny surf will prevail 
along north facing shores this week and into the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Thursday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big 
Island Southeast Waters.

&&

$$

DISCUSSION...R Ballard
AVIATION...Farris
MARINE...Shigesato