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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 311945
Area Forecast Discussion
National Weather Service Honolulu HI
945 AM HST TUE MAY 31 2016
A somewhat wet trade wind regime will continue for another day or
so in wake of a dissipating front near Maui. Moisture left behind
by the front will enhance showers, especially across windward
sections. Moderate trades will strengthen behind the front, and
have already filled in over the western half of state. Trades will
then turn light and variable by Saturday as a surface trough
approaches the islands from the northeast.
Water vapor imagery shows a weak trough or col over the state,
with a well defined low far to the northeast. At the surface, a
nearly stationary front is located near Maui, with a cloud band
extending back across Oahu. This front is dissipating near the
islands, but extends to a low far to the northeast. 12z raobs
from Hilo and Lihue show inversions around 9kft and precipitable
waters of 1.4-1.5 inches, which is around one standard deviation
above normal for this time of year. Early morning MIMIC total
precipitable water imagery shows a band of higher moisture
centered over Oahu/Maui County, associated with the surface front.
There has been little movement to the moisture, and only slowly
movement southward is expected in the next day or so.
Greatest rainfall is occurring across the central islands. Almost
all windward gages on Oahu received some rain, with a number of
them near or greater than an inch in the past 12 hours. Given the
active cloud band associated with the front and rainfall expected
to continue, have updated the forecast to carry categorical pops
for windward sections of Oahu/Maui County. High resolution
guidance shows the band shifting southward tonight, to affect
mainly Maui County and the Big Island.
Trade winds will fill in behind the front as a new high builds
north of the state. Winds have already picked up across the
smaller islands, and will continue for the next few days. Forecast
soundings/cross sections show inversion heights briefly lower
Wednesday as mid-level heights rebuild slightly. Trade showers
will be most active near the Big Island where the old front
moisture could hang up, but the smaller islands may briefly return
to a more typical trade wind weather pattern.
The 12z GFS and ECMWF are somewhat similar with the evolving
pattern late in the week, with the upper low to our northeast
shifting eastward and the surface low mentioned above weakening to
a trough. This trough gets picked up in the low-level
east-northeasterly steering flow and shifts west-southwestward
toward the state. By this point the surface high to our north is
already weakening due to another north Pacific system encroaching
on it, and the surface trough will further weaken winds across the
state late in the week. We should see another light
wind/convective pattern again this weekend, with onshore sea
breezes during the day leading to interior showers in the
Differences emerge between the extended models with the pattern
after this point, with the ECMWF bringing an upper low over the
islands from the east early next week and the GFS keeping an
amplifying trough east of the state. As it stands, there is little
reflection of this low at the surface in the ECMWF, with both
models showing a new high building northwest of the area. Trade
winds should return with the high early next week, but it remains
to be seen how unstable we will be and how much in the way of
trade showers we will receive.
A dissipating frontal cloud band continues to effect the area from
Maui to the Kauai Channel at mid morning. The cloud band is
producing MVFR and isolated IFR conditions mainly for Molokai and
Oahu attm. An AIRMET for mtn obsc is in effect for both of those
islands. Kauai and Maui may be affected later in the day so we
will wait and see. The cloud band stretches east from Oahu and
Molokai to 22N150W then continues off to the east northeast. The
near the islands was moving south near 10 mph. Trade winds have
returned to most areas so any sea and land breezes will be at the
more typical areas such as Kona and perhaps, Lanai airport, if the
clouds do not alter the daytime heating effect.
A weak slow-moving front currently over Maui will drift southward
over the next couple of days, causing fresh northeast winds to the
north of the front to spread over the entire island chain. High
resolution models have been showing winds along the frontal
boundary touching small craft advisory strength at times, though
coastal wind observations have not been supporting winds this
strong. We will closely monitor these border line conditions
during the next couple of days. Expect a decrease in winds on
Friday and the weekend as high pressure weakens to the north of
the state and a surface trough develops to the northeast.
Surf will remain below advisory levels on all shores this week.
Peak energy associated with a south-southwest swell has shifted
from the 14 to 16 second band down to the 12 to 14 second band.
As a result, surf along south facing shores will be on a slow
decline today, with a small reinforcing swell possible tomorrow.
Small surf will prevail on north and east facing shores.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office