Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
400 am HST Wednesday Mar 12 2014
high pressure north of the islands is moving to the northeast. An
associated building ridge north of the islands will bring moderate
trade winds to the islands through Thursday...along with showers
over mainly windward and Mauka areas. A front approaching from the
northwest Friday night and Saturday will bring additional showers to
the islands. Strong and gusty trade winds are expected behind the
front through the weekend...diminishing back to moderate levels by
the middle of next week.
no major changes in forecast philosophy at this time...with just
some minor tweaks to the grids overnight.
Radar continues to show just a few showers over the islands
overnight...while satellite shows some showery low clouds upstream
of Maui and The Big Island. These clouds are shallow...and the
overnight sounding from Hilo shows precipitable water values of just
over an inch. Have knocked back the probability of precipitation for this morning across the
islands...but the chance for rain is just a little higher over Maui
and The Big Island this morning.
A 1026 mb high was analyzed northeast of the islands at 2 am...with
a ridge building north of the islands. Most reporting sites at the
lower elevations of Kauai and Oahu overnight had light winds...with
moderate trades being reporting elsewhere. The models are in good
agreement with the high slowly sinking to the south over the next
day. An ascat pass over the islands just before 10 PM shows 20 knots
winds over the Alenuihaha Channel...and over the Pailolo Channel. So
despite the light winds over the lower elevations of Kauai and
Oahu...we are seeing winds on the increase in response to the ridge
to the north. Expect winds to pick up to more consistent moderate
trades across the state today. These moderate trades are then
expected to continue through Thursday.
As we head into Friday...the models are in good agreement with a
front approaching the islands from the northwest. At this time none
of the models show a definitive wind shift...so believe the front
will weaken to more of a shearline by the time it reaches the
islands Friday night. Limited moisture is expected to arrive with
the front/shearline...but expect showers to expand from windward and
Mauka areas to include other areas of the islands Friday night and
Behind the front comes the main forecast challenge for the period.
The models are in good agreement with a 1037 mb high moving
northwest of the islands Friday night. The tightening of the
pressure gradient between the front/shearline and the high will
bring strong and gusty winds to the islands for the weekend. By
Saturday morning it appears the winds will have reached Kauai and
Oahu...and spread to Maui County and The Big Island by Saturday
night. The models have been consistent in bringing in this
event...and if it pans out...wind advisories will be possible for
some areas of the lower elevations.
The high pressure moves to the east...being north of the islands
Sunday morning...and then starts lifting to the northeast through
Monday night when it becomes nearly stationary. Monday as the high
moves to the northeast it also is expected to weaken...so expect
trades to back off to moderate levels.
the current moderate northwest swell is expected to peak today just
below advisory levels...and then gradually subside tonight through
Friday morning. A new slightly larger northwest swell is due in
Friday...peaking Friday night or Saturday at advisory levels.
With moderate trades expected today...and winds already at 20 knots
over the Alenuihaha Channel overnight...a Small Craft Advisory
/sca/ will begin at 9 am. As winds increase behind the front this
weekend...fully expect many other areas to meet Small Craft Advisory conditions...
with some areas possibly reaching gale force.
A short period trade wind swell is likely as the trades increase
this weekend that will likely produce advisory level surf for east
Small Craft Advisory from 9 am this morning to 6 PM Thursday for