Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
830 PM HST Monday may 4 2015
moderate trade winds will prevail through the forecast period.
Clouds and showers will be most active over windward slopes...though
a few may drift leeward at times.
surface observations around the main Hawaiian islands reported
solidly moderate trade winds. A scatterometer pass from this
afternoon showed winds over the northwest coastal waters approaching
the criterion for a Small Craft Advisory in places...so there was
little doubt that speeds over usually windy areas around Maui and
The Big Island did reach that limit. Cloud motions upwind of the
state also suggested that the winds extended through most of the
mixed layer below the inversion.
The inversion was a little on the high side...and not particularly
strong...leaving ample headroom for showers to develop. Although
sounding measurements of precipitable water over the state suggested
a relatively dry air mass...satellite estimates showed slightly
moister air immediately upwind. A loose band of stratocumuli and
cumuli visible in satellite images as it moved in from the east
probably explained the discrepancy...such as it was. As usual...
individual small showers also continued to enhance over windward
slopes...and likely will continue to do so through the week.
Aloft...broad zonal ridging across most of the central Pacific Ocean
kept the main part of the storm track well to the north of the
islands...but shallow extensions of passing troughs and ridges did
reach the state. These waves...and their influence on the
subtropical ridge at the surface...will bring most of the changes to
local weather through the week.
Recent model runs continued to show differences in phase among the
passing impulses over the more fundamental ridge. On the whole...it
seems that middle-level ridging should increase by late in the week...
possibly bringing increased trade winds over the weekend. Otherwise
climatology may prove difficult to outpredict. Similarly...it may be
tempting to forecast trends in trade-wind showers simply from model
consensus precipitable water...but in practice the correlation tends
to be less than ideal. Additional runs should be instructive.
a Small Craft Advisory remained in effect for typically windier
areas around Maui and The Big Island through much of the week.
The trade winds also will maintain elevated short-period surf along
east-facing shores through the week...though heights should remain
below the advisory threshold. A long-period south-southwest swell
will build Tuesday...and then peak Wednesday into Thursday...likely
bringing advisory-level surf to south-facing shores. A tiny north
swell is expected through Wednesday...and a slightly larger
north-northwest swell from Thursday into Saturday.
Small Craft Advisory until 6 PM HST Friday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island leeward waters-Big
Island southeast waters