Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
400 PM HST Tuesday Sep 2 2014
light to moderate trade winds will continue for much of the upcoming
week. The trades will focus showers mainly across windward areas...
especially during nights and mornings...with some afternoon showers
across leeward sections as well.
water vapor imagery shows an upper level high over/just south of the
state...with a shortwave trough far to the northwest. At the
surface...high pressure is located far north-northeast of the
state...with a ridge passing several hundred miles to our north. Low
pressure...former tc Marie...is far northeast of the state and
moving generally westward. The 00z sounding from Lihue shows a
slightly lower and stronger inversion near 6kft...with a
precipitable water near 1.4 inches. Early afternoon mimic total
precipitable water imagery shows an area of higher moisture
extending up to 200 miles northeast and east of the state...with
drier air farther upstream.
The pressure gradient across the islands remains on the light side
due to our separation from the high and influence from former Marie.
However...trades have been stronger and more widespread today
compared to yesterday. The gradient between buoys 51100 northeast of
the state and 51004 southeast of the state has risen by up to a
millibar over the past 24 hours. The ascat imagery from this morning
showed winds have increased further since last night...with 10-15
knots of wind over the western half of the state and 15-20 knots
over the eastern half. The high resolution WRF from last night...as
well as the 12z/18z global models...show the stronger winds
spreading westward across the state through tonight as well.
An area of low clouds is currently located northeast of Maui and The
Big Island...and is moving westward at about 10 miles per hour. These clouds
are an indicator of the higher moisture just upstream of the
state...and will help to enhance trade wind showers mainly across
windward sections tonight into Wednesday. We have received a couple
rain gage reports of rates over 1 inch per hour on The Big Island
from showers that look relatively innocuous. As this area of
moisture moves through the state tonight...we will remain on the
lookout for potential for localized heavy rain if showers become
GFS forecast soundings/moisture plots show drier and more stable
conditions spreading across the islands during the day on
Wednesday...with lower rainfall chances. There should remain enough
of a stronger gradient for trade winds to dominate...with localized
sea breezes confined to sheltered leeward areas. Even with more
persistent trades...we will likely see increased cloud buildups
across interior and leeward sections during the afternoon and early
By the end of the week...both GFS and European model (ecmwf) continue to show a
tongue of moisture trailing from former Marie reaching the islands.
The GFS is faster/more aggressive than the European model (ecmwf)...but both show the
area of clouds/moisture currently extending down to 25n between
140w/150w reaching the islands late Thursday/Friday. Forecast
soundings/cross sections show increasing moisture depth back to near
10kft. During this timeframe...winds may also weaken a bit. The
remnant of Marie will move westward and weaken as it encroaches upon
the ridge. The gradient across the islands may weaken
Thursday/Friday...before strengthening further Saturday into Sunday
as the low dissipates and we feel more of a direct impact from the
light to moderate trades will continue over the next few days. Winds
should remain light enough to limit the need for a low level turb
airmet. Showers will favor windward and Mauka areas...especially
overnight and early morning. MVFR conditions are likely within these
showers. Otherwise...VFR will prevail.
elevated surf will continue along S facing shores this week.
Forerunners of a very long period S swell /20-25 seconds/ that
arrived early today are expected to build further tonight...although
at low heights. The largest part of the swell will miss US to the
east...as seen by the greater long-period energy that continues to
increase at buoy 51004 southeast of the state versus the much lower
energy at buoy 51003 south of the state. Locally...the latest
observations from the cdip buoy at Barbers Point show initial energy
from this swell is dominated by the fading shorter period south
swell. As this new swell fills in...it will produce surf near
advisory levels along S facing shores from Wednesday into Thursday.
With the bulk of the energy passing to the east...the current
expectation is that surf will remain just below advisory levels for
south facing shores. Therefore we will hold off on an advisory for
now...but will keep an eye on conditions as they build through the
Forecast models show a fairly large gale low developing northwest of the
state this week...generating a northwest swell that arrives late
Monday/Tuesday. It is too early for specifics as the low has yet to
develop...but resultant surf looks to remain below advisory level
heights along north and west facing shores.
Winds and seas will remain below Small Craft Advisory thresholds for
at least the next several days.