Area Forecast Discussion
Issued by NWS Honolulu, HI
FXHW60 PHFO 300658
Area Forecast Discussion
National Weather Service Honolulu HI
858 PM HST THU SEP 29 2016
High pressure north of the state will keep moderate to breezy trade
winds in place through the weekend, with showers focused primarily
over windward and mauka areas. Showers are expected to increase
in coverage and intensity over the weekend through much of next
week, as a moist airmass associated with the remnants of Ulika
moves over the state.
Currently at the surface, a 1038 mb high is centered well north of
the state, while Tropical Storm Ulika is located around 900 miles
east-southeast of Hilo. Aloft, a broad upper level low resides
between Ulika and the islands, with upper ridging to the north and
west of the state. Infrared satellite imagery shows mostly cloudy
skies over most windward areas and partly cloudy conditions over
leeward locales. Some high level jet stream cirrus is also
increasing cloud cover over the Big Island. Meanwhile, radar
imagery shows scattered to numerous showers over windward areas,
with isolated showers spilling over into leeward locales. Main
short term concern revolves around rain chances.
Tonight and Friday,
High pressure well north of the state will keep a moderate to
breezy trade wind flow in place, with clouds and showers focused
mainly over windward and Mauka areas. MIMIC total precipitable
water values over and upstream of the islands are in the 1.0 to
1.3 inch range, so rainfall amounts should remain light. Low level
lapse rates are fairly steep however, on the order of around
9C/KM in the 0-1 KM layer, so expect fairly good coverage of
showers over windward/mauka areas, with isolated showers spilling
over into leeward locales mainly during the overnight and early
Friday night through Saturday night,
Moderate to breezy trade winds will continue as the gradient
remains tight due to a large surface high to the northeast of the
islands and the approach of Ulika from the east. Aloft, there are
some differences between the 00Z GFS and 00Z ECMWF, with the GFS
indicating more pronounced upper level troughing over the islands,
while the ECMWF is not as aggressive. Deep layer moisture will
increase from east to west across the islands Friday night
through Saturday morning, with PW values will rising into the 1.5
to 1.8 inch range for Saturday afternoon and Saturday night.
Overall, with the troughing aloft and increase in moisture, we
should see an increase in shower coverage and frequency across
the state, with showers favoring windward and mauka areas.
Sunday through next Thursday,
Both the 00z GFS and 00z ECMWF are in fairly good agreement,
showing high pressure remaining to the northeast of the state,
keeping a moderate trade wind flow in place through the period.
Aloft however, there are some significant differences and model
run to run continuity has been poor. The GFS and ECMWF both close
off a pair of upper level lows to the east and west of the state
Sunday through early next week, but the GFS solution shows these
features weaker with more residual troughing remaining in place
across the island chain. The ECMWF on the other hand shows
stronger upper level lows displaced further apart, with upper
level ridging over the state. Obviously, this has significant
impacts on the weather pattern across the islands. The GFS
solution suggests a wetter and more unstable pattern, while the
ECMWF suggests more of a wet trade wind set up.
Given that both models indicate plenty of deep layer moisture
remaining over the state through the period, along with
considerable uncertainty in the large scale synoptic pattern
aloft, we will stick with forecast continuity which appears to be
a good blend of the two solutions at this time. As a result, we
should see a wetter than normal pattern across the state through
the period, with the potential for some locally heavy rainfall at
Fairly typical, and breezy, trade wind weather expected. Most
areas will see VFR most of the time. However, isolated to TEMPO
MVFR conditions will likely affect windward and mauka areas as
showery low clouds move along on the low-level flow. Some of this
cloudiness may make its way over leeward sections of the smaller
isles and West Maui from time to time as well.
An AIRMET for moderate, intermittent low level turbulence to the
lee of the mountains, all islands, remains in effect.
High pressure far to the north of the state will keep moderate to
breezy trade winds in place through the weekend. A Small Craft
Advisory is in effect for the typically windier waters around
Maui and the Big Island through Friday, and will likely need to
be extended through the weekend.
Short period easterly swell resulting from the trade winds will
continue through the remainder of the week. A bit of a bump in
surf heights is expected for east facing shores over the weekend
as the remnants of Ulika approach from the east. Elsewhere, surf
heights will remain small through early next week.
Small Craft Advisory until 6 PM HST Friday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.