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Hawaiian Islands Synoptic Discussion and Guidance

170
FXHW60 PHFO 222004
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1004 AM HST Mon Apr 22 2019

.SYNOPSIS...
Moderate to fresh trade winds will continue through the work week,
bringing periods of clouds and showers to windward areas, while
leeward areas remain mostly dry. The trades will begin to decrease
over the weekend allowing for the development of sea and land
breezes. This will favor showers developing over interior and
mountain areas during the day, and in windward areas near the
coast at night.

&&

.DISCUSSION...
A 1031 mb surface high is centered about 1500 miles northeast of
Hawaii with a ridge axis extending west southwestward and passing
450 miles north of Kauai. The high is driving fresh trade winds
across the state this morning. The atmosphere is dry and stable
with weak upper level ridging over the area. 12Z soundings show
strong inversions between 6 to 7 kft and precipitable water values
of 1.0 to 1.2 inches. Visible satellite shows scattered to broken
low clouds focused over windward sections of the islands and
waters. A few showers are associated with the low clouds. Leeward
areas are mostly sunny this morning, and aside from the typical
cloud builds ups along the Kona coast on the Big Island, the
current weather should continue through the rest of the day.

Fresh trades will continue through Wednesday as the high pressure
system to the northeast will maintain and then slowly weaken as a
low pressure system moves in it's place. Winds will drop slightly
to moderate to fresh by Thursday and Friday. Meanwhile, ridging
aloft will continue keeping the atmosphere stable, with only a
few passing showers embedded in the trade wind flow move across
the islands during the next few days.

Model solutions continue to exhibit some differences over the
weekend, and this is where forecast confidence decreases. Both
the GFS and ECWMF do however show a deep upper level trough
developing to the northwest of the state and digging southeastward
towards islands during the weekend. This results in a further
erosion of the ridge to the north of the state as a surface cold
front edges closer to the area from the northwest, with surface
troughing developing in advance of the front and in the vicinity
of the island chain. This is expected to shift the background
boundary layer flow around to the southeast and south over the
weekend, with surface winds becoming light and variable allowing
sea and land breezes to develop in many areas. Additionally, the
low level inversion will erode as the upper level trough moves
into the area, and a modest increase in precipitable water values
to above normal levels is expected as the boundary layer winds
shift southerly. Overall, we should see a more connectively
driven weather pattern over the weekend, with showers favoring
interior and mountain areas during the afternoon and early evening
hours and windward and coastal areas favored under a background
east- southeasterly to southeasterly boundary layer flow. There is
the potential that some of the shower activity could be locally
heavy and a thunderstorm can't be ruled out at this time. Given
the differences in the models and being so far out in the forecast
period, will hold off on adding any mention of thunder to the
forecast until details become more clear.

&&

.AVIATION...
Surface high pressure to the far northeast of the islands will
allow for moderate easterly trades to persist today. As is
typical with this type of pattern, expect a slight chance of
showers along the windward and mauka slopes with mainly dry
conditions for leeward locations. Highest chance for precipitation
will be tonight over the windward coastal slopes and waters.

AIRMET Tango remains in effect south through west of all islands
below 6000 ft due to moderate turbulence.


&&

.MARINE...
A ridge north of the islands will persist through Thursday, with a
front stalling north of the islands Friday and into the weekend.
The ridge will maintain the current fresh to locally strong trade
winds will continue through Thursday, with winds decreasing at
the end of the week. The current Small Craft Advisory (SCA) for
the windier areas near Maui and the Big Island continues through
Wednesday afternoon, but may need to be extended into Wednesday
night or Thursday.

Surf is expected to remain below advisory levels through the week
for all shores. A few small west-northwest and north-northwest
swells will provide small surf along north and west facing shores
throughout the week. A series of small south swells will provide
small surf starting today and will likely continue through most of
the week and into the weekend. The local PacIOOS/CDIP Barbers
Point buoy has shown some energy in the 16 to 18 second bands this
morning, and this swell will fill in today, peaking tonight and
slowly dropping over the next couple of days. Several reinforcing
south swells are expected towards the latter half of the week.
Surf along east facing shores will hold steady in the small to
moderate range through Thursday due to the breezy to locally
strong trades.


&&

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


&&

$$


DISCUSSION...Foster
AVIATION...Thomas
MARINE...M Ballard

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



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