Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
1000 am HST sun Nov 23 2014
lighter winds will allow sea breezes to develop today...bringing
clouds and a few showers to interior areas this afternoon and early
evening. A weakening front is expected to move across the state late
tonight through Monday...bringing a brief round of showers. Cooler
and drier air and stronger northerly winds will follow the front...
with winds turning northeast on Tuesday. Breezy trades will then
continue the week...bringing brief windward and Mauka showers.
water vapor imagery shows a weak upper level ridge over the state...
with a sharp trough over 700 miles to the northwest. This trough is
moving to the east-southeast at 25 miles per hour. At the surface...high
pressure is located far northeast of the state. A ridge extends from
the high to about a hundred miles north of Kauai. The approaching
cold front is located about 400 miles northwest of Kauai. 12z
soundings from Hilo and Lihue show inversions of 6-7kft.
Precipitable waters are around 1.1 inches...which is slightly drier
than normal for this time of year. Early morning mimic total
precipitable water imagery shows slightly higher moisture
approaching The Big Island from the east...with higher values to
the northwest of the state pooled along and ahead of the cold front.
As the ridge is forced southward toward the state...the weakening
gradient allowed widespread land breezes to develop overnight. Some
clouds linger over east Maui and windward Big Island...but most of
the state started the morning with clear skies. The light wind
pattern will allow onshore sea breezes to develop today...with
clouds already starting to fill in across island interiors. With the
low-level gradient turning more southerly/southwesterly ahead of the
approaching front...low-level convergence along sea breezes will be
greatest across north and northeast sections of the islands. These
areas will see the greatest cloud cover and showers. The atmosphere
remains stable...and the 6-7kft inversion will keep the afternoon
convection similar to that of typical trade wind showers.
The 12z European model (ecmwf) is a little faster than the GFS/Canadian/navgem
bringing the front down the island chain. The GFS appears to be
developing a prefrontal shower band near Maui County tonight as it
has the front reaching Kauai. This looks quite reasonable given the
persistent mass of clouds in The Big Island plume that is currently
developing northward toward Oahu. Given this developing pattern...
which the European model (ecmwf) does not show...will stick with the GFS and
consensus timing for the front. This area of clouds may actually
affect the South Shore of Oahu this afternoon...before spreading to
Maui County tonight.
The upper level trough northwest of the state will skirt by to our
north. The stronger dynamics and unstable air associated with it
will brush the far northern offshore waters...but the atmosphere
over the islands is expected to remain capped. Forecast soundings
and time sections show moisture depth rising to near 10kft briefly
with the front...then dropping lower than current levels behind it.
While we remain capped...there is a fair amount of moisture focused
at low-levels with this front...which should be enough to bring a
period of showers to the area...at least across the western half of
the state. The front should continue to weaken as it moves farther
southeast away from the upper level support. Strengthening
southwesterly/westerly winds ahead of the front will turn northerly
behind it on Monday...bringing gusty conditions and a much drier
airmass. MOS guidance shows dewpoints dropping into the upper 50s
A new high building north of the state by middle week will keep a deep
belt of moderate to locally fresh trade winds across the area.
Low-level moisture will gradually increase through the week with the
state returning to a more typical trade wind pattern of windward and
Mauka showers. Conditions look to remain stable with an upper level
ridge over the islands through the week. Remnant moisture from a
dissipated front may enhance trade showers late in the week...but it
is too far out to pin down any details.
VFR conditions will prevail today. Weak background flow ahead of an
approaching cold front will allow widespread sea breezes to develop
this afternoon...with isolated showers and MVFR conditions possible
across the islands.
the combination of increasing winds...seas and the arrival of a
long period swell may bring Small Craft Advisory conditions Monday
through Tuesday over most marine zones...especially the typically
windier areas near Maui County and The Big Island.
The incoming north-northwest swell has been building at buoy 51101
overnight...and long period energy from this swell is just starting
to increase at the Waimea buoy. Wavewatch iii guidance shows this
swell peaking just below advisory levels...but buoy 51101 shows the
swell may be slightly bigger than wavewatch. A reinforcement to the
swell is expected from Monday into Tuesday...which is more likely
to require an advisory. Breezy trade winds later in the week will
produce choppy surf along east facing shores...but it is expected to
remain below advisory levels.