631 
FXHW60 PHFO 291340
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
340 AM HST Mon Apr 29 2019

.SYNOPSIS... 
A trough of low pressure will remain nearly stationary over the
state today, keeping a land and sea breeze pattern in place, with
the potential for locally heavy rainfall and thunderstorms during
the afternoon and evening. Moderate trade winds will return 
tonight and continue through Thursday, with showers favoring 
windward and mauka areas, with a stray shower reaching leeward 
sections from time to time. A cold front will approach from the 
northwest on Friday, and move into the island chain over the 
weekend, bringing an unsettled and potentially wet weather pattern
to the state.

&&

.DISCUSSION...
Currently at the surface, a south-southwest to north-northeast
oriented trough of low pressure runs through the islands in the
vicinity of Oahu. Meanwhile, a 1023 mb high is centered around 
1050 miles northwest of Kauai, and a 1034 mb high is centered 
around 2500 miles north-northeast of Hilo. This is keeping a 
light wind regime in place across the island chain, with 
overnight land breezes dominant. Infrared satellite imagery shows
variably cloudy skies across the state, with a bit more cloud 
cover over the southern tip of the Big Island where jet stream 
cirrus remains rather thick. Radar imagery shows scattered 
showers mainly over the coastal waters, although some showers are
managing to move into land areas near the coast. Main short term 
concerns revolve around the potential for heavy rainfall and 
thunderstorms through this evening.

Model solutions remain in good agreement with the large scale
synoptic pattern through the work week, with some significant
differences noted next weekend. The trough of low pressure will
remain nearly stationary in the vicinity of Oahu today, before 
dissipating this evening as high pressure builds northwest of the
state. High pressure will then shift eastward well to the north 
of the islands through the remainder of the work week. A cold 
front will then approach from the northwest on Friday, then move 
into the islands next weekend. 

As for sensible weather details, a land breeze and sea breeze
pattern will continue into this evening, with showers favoring 
interior and mountain areas during the afternoon and evening, and 
the coastal waters and locations near the coast at night into the 
early morning. An upper level trough will approach from the west
today and move through the islands tonight, bringing a core of
cold air aloft with H5 temperatures dropping as low as -12 to -13C.
This cold air aloft combined with decently steep mid-level lapse
rates around 6.5 C/KM, 0-6 KM bulk shear values of 25 to 30 
knots, and surface based CAPE values of around 1500 J/KG may be 
enough to produce a strong thunderstorms or two this afternoon 
and evening, with some gusty winds and hail not out of the 
question.

A more typical trade wind pattern will return later tonight, with
moderate trades expected Tuesday through Thursday. Showers will 
favor windward and mauka areas, with a stray shower reaching 
leeward areas from time to time. The trades will then ease 
Thursday night and Friday, with a return to more of a convective 
pattern featuring interior and mauka shower activity expected for 
Friday afternoon/evening. Forecast confidence remains low over the 
weekend due to differences in the handling of a closed upper level
low diving southward toward Hawaii between the operational 
models. The GFS features a showery progressive frontal passage 
through the islands, while the ECMWF shows a slower frontal
progression and wetter more unstable pattern. Will hold off on 
making any significant changes to the forecast over the weekend 
given the low predictability of upper low positions this far out 
in the forecast period, and wait until the models converge and 
details become more clear.

&&

.AVIATION...
An upper level trough passing over the islands has destabilized
the local air mass, especially over the smaller islands. This, 
combined with ample moisture, will produce moderate to locally 
intense showers. Isolated thunderstorms are expected to develop 
across the water this morning and interior areas this afternoon. 

Weak flow at the surface is allowing land breezes to predominate 
during the late night and morning hours and sea breezes to 
prevail in the afternoon and evening hours. The most intense 
shower activity can be expected to occur in the afternoon and 
early evening hours and to be focused over island interiors. 
Mountain obscuration may become an issue once again this 
afternoon.

&&

.MARINE...
A weak surface trough in the vicinity of Oahu is maintaining 
light and variable winds across the coastal waters early this 
morning. This light background flow will likely persist through 
this afternoon. In addition, an upper-level trough moving across 
the area from west to east will destabilize the atmosphere. This 
will result in locally heavy downpours and a chance of 
thunderstorms through this evening.

The surface trough will eventually dissipate later today as a new
surface high pressure system builds far northwest of the area. 
This will allow moderate to locally breezy trade winds to develop
across the area from tonight through mid-week. The latest 
forecast indicates winds will likely reach the Small Craft 
Advisory criteria over the typically windier waters adjacent to 
the islands of Maui County and the Big Island from Tuesday through
Wednesday night. The background flow will begin to weaken again 
from Thursday through Friday as a new front approaches from the 
northwest.

The current moderate northwest swell is expected to peak today, 
then gradually subside from tonight through mid-week. A small 
reinforcing northwest swell arriving late Wednesday will persist 
into next weekend. 

A series of small south and southwest swells will maintain small 
to moderate surf along south facing shores through the weekend. 
Surf may briefly increase along east facing shores around mid- 
week as the trade winds strengthen. Otherwise, expect small surf 
along east facing shores into early next week.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Bedal
MARINE...Houston