Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
932 am HST Thursday Dec 18 2014
high pressure centered north of the islands will maintain locally
breezy trade winds through Friday. Mostly cloudy skies with frequent
showers will continue over most windward and mountain areas through
the rest of the morning hours...becoming scattered in the afternoon.
Isolated showers are possible over leeward areas today. The trades
are expected to become light over the upcoming weekend as the high
leaves the area and a front approaches the islands from the
northwest. The front will move down the island chain between Sunday
night and Tuesday night bringing with it some showery and windy
satellite and radar imagery from earlier this morning show a
persisting active shower pattern across windward areas of all the
main Hawaiian islands...with additional showers upstream heading
towards the Aloha state. Therefore...probability of precipitation have been increased for
all windward areas for the rest of the morning hours. Probability of precipitation over
windward Maui County have also been increased to scattered this
afternoon as the trade wind shower pattern should remain active
Differences remains between the two global models...the European model (ecmwf)
/ec/ and GFS...regarding the upper level short wave trough and its
support of the surface front early next week.
The most notable difference is in the timing of the upper level
trough where the ec has the trough just west of Kauai Monday evening
while the GFS has it over Kauai and Oahu.
The ec remains the stronger of the two with a cold 500 mb temperature of
minus 17 c. The GFS has a minus 14 c. Not much of a difference here
where both temperatures are cold enough to support at least a slight chance
of a thunderstorm and almost certain...spotty heavy showers.
The models differ thereafter. The ec keeps the upper trough over
the islands Tuesday...thus maintaining a chance of enhanced showers
and a slight chance of a thunderstorms. On the other hand...the GFS
has the upper trough far NE of the islands...suggesting more light
to moderate showers with the front. The two models do agree on the
location of the front...over Maui Tuesday afternoon. Should we
pursue the ec solution...improvement in the weather will come
Tuesday night as the upper trough finally lifts out.
This is still a good few days away so we will wait and see if the
models maintains a showery outlook for Monday and Monday night.
The weather pattern may turn slightly convective with local daytime
sea breezes as the trades eases to light to locally moderate during
the weekend. A strong and stable air mass will put a lid on these
convective clouds. There will be enough of an easterly wind flow to
carry a few shower bearing clouds to the windward areas of all the
clouds and showers carrying in with the trade winds are causing MVFR
ceilings and tempo mountain obscure this morning. Therefore...airmet Sierra has
been issued for mountain obscure on the windward sides of Oahu...Molokai and
Maui through 22z. The clouds and showers are expected to decrease by
afternoon as the cloud tops warm and mixing increases.
Elsewhere...VFR conditions will prevail.
Breezy trade winds are also resulting in leeward mechanical
turbulence over all islands. Airmet Tango is in effect for this...
and will likely carry on until Friday.
a Small Craft Advisory /sca/ remains in effect for the windward
waters of Kauai and Oahu...due mainly to a large northwest swell...and for
Maalaea Bay...the waters south of The Big Island...and the pailolo
and alenuihaha channels due to strong gusty winds. The Small Craft Advisory is valid
through 6 PM HST Friday.
We do look forward to the Small Craft Advisory being lower late Friday afternoon.
This will be short live however. Small Craft Advisory conditions are again expected
Saturday night for the anticipated large northwest swell.
Surf heights are expected to rise to advisory levels for the north
and west shores of most of the smaller islands tonight. So...a surf
advisory will be in effect from 4 PM HST this afternoon through
Friday. This swell will peak tonight and gradually lower Friday.
An upcoming large northwest swell on Sunday will likely need a high surf
warning involving both north and west facing shores of most islands.
The source of this swell is from a rather broad storm low currently
located off the East Coast of Japan. Another storm low is predicted
by the models to form north of the islands the middle of next week
which may keep surf elevated in at least the advisory range through
Small Craft Advisory until 600 PM HST Friday for windward waters of
Kauai and Oahu...Maalaea Bay...the waters south of The Big Island...
and the pailolo and alenuihaha channels.
High surf advisory from 4 PM this afternoon to 6 PM HST Friday
for Niihau-Kauai windward-Kauai leeward-Waianae coast-Oahu North
Shore-Oahu koolau-Molokai-Maui windward west-windward Haleakala.