Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
1000 am HST Friday Sep 19 2014
light winds through the weekend will allow land and sea breezes to
drive island weather. In this regime...clouds and showers will
develop over interior and Mauka areas during the afternoons...
followed by gradual overnight clearing. Trade winds will strengthen
early next week... and remain light to moderate most of the
week...delivering brief passing showers to windward areas. The trade
winds may become wetter by Thursday...as moisture associated with a
weakening front becomes embedded in the trade wind flow.
currently...a weak pressure gradient is over the area...with light
large-scale winds containing an easterly component. Surface highs are
far northwest and NE of the islands...while a southeast-Ward digging front is
noted about 750 miles north of Kauai. Latest surface analysis contains a
weak north-S trough axis near Oahu...with weak NE winds west of the trough
and weak southeast winds east of the trough axis. A pair of tropical
disturbances passing S and southeast of The Big Island contain deep
moisture...with The Big Island on the northern periphery of this
moisture. For today...forecasts have been updated to account for the
abundant sunshine of this morning /aside from a small clump of
showers that moved ashore S Oahu/. Sunny skies will give way to
clouds and showers over interior and Mauka sections by this
afternoon as converging sea breezes develop in response to heating
of the islands. Overnight soundings show a moist and relatively
unstable air mass in place...and thus today/S convection is expected
to extend to the summits of Maui and The Big Island...and there is a
decent chance of locally heavy but isolated downpours statewide.
Through Sunday...the pressure gradient over the islands will remain
sufficiently loose for land and sea breezes to be the dominant
factor driving island weather...with generally clear skies at night
and in the morning...with clouds and showers developing by late
morning that linger through the early evening. One potential
exception would be the southeast and windward portions of The Big Island...
where overnight showers may be a little more active due to the
passage of the tropical disturbances to the S. A weak middle-level
trough passing north of the islands will eventually give way to
middle-level ridging...with the ridge building on Sunday. Thus this
weekend/S afternoon convection could also feature some isolated
heavy downpours...but may be a little less likely on Sunday.
The front north of the islands today will move S through Sunday...but
will stall north of the islands over the weekend...contributing to the
continued light wind regime. Local water temperatures are quite
warm...contributing to the muggy conditions...with dew points in the
lower 70/S. A second front will dig toward the islands early next
week...with a moderately strong high passing north of the islands
Tuesday through Friday of next week. Latest guidance shows trade
winds increasing Monday in response to the high/S passage...with
light to moderate trade winds persisting for the remainder of the
week. Models shows the second front becoming embedded within the
trade flow as an east-west oriented band of clouds and showers that
potentially affects island weather around Thursday and Friday.
With trade winds blowing the bulk of this moisture will focus on
windward slopes and coasts...possibly lingering into the weekend.
Deep easterly flow will likely advect some of these showers over to
leeward areas on the smaller islands.
overnight land breezes helped to keep the islands clear this
morning...with the expectation of the South Shore of Oahu. The light
background flow will allow for sea breezes to form late this morning
into the afternoon. Interior and leeward areas are likely to see
tempo MVFR and possible isolate IFR ceilings and visible with tops rising above
15 kft this afternoon as clouds and showers are expected to form
along the sea breeze convergence. Windward areas may experience isolate
MVFR ceilings is passing showers.
winds and seas will remain below Small Craft Advisory criteria /sca/
into early next week...after which increasing trade winds may get
strong enough to warrant Small Craft Advisory issuance for the windier marine zones
around Maui and The Big Island.
Surf will remain below advisory levels along all shores over the
next week...but small swells will provide some surf. Small swells
from the northwest are expected over the weekend and again on Wednesday/
Thursday...with a north-northeast swell possible Thursday/Friday. Tiny
long-period swells from the S will maintain surf near or below
seasonal averages along S facing shores through the period.