Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
1035 am HST Monday Mar 2 2015
deep moisture near The Big Island will continue to keep a threat
for heavy showers and thundershowers there the next few days. More
limited moisture and an upper level trough over the smaller
islands could lead to enhanced showers at times. Drier air will
begin to overspread the islands from the north later this week.
Trade winds will strengthen over the state through midweek.
very complex weather pattern over the state today with lots of
features contributing their part to the weather puzzle. A sharp
moisture gradient continues to split the main Hawaiian islands...
with dry air and precipitable water /pw/ values below 1 inch near
Kauai...and moist air with precipitable water values over 1.75 inches around The
Big Island. A broad middle-to-upper level trough over and to the northwest
of the islands is making the airmass more unstable over the
state...but at the moment most of the islands appear to be in a
zone of subsidence and dry air aloft behind an upper low about 370
miles to the NE of Oahu.
One exception is a small area of active showers riding in on the
trade winds about 70 miles to the NE of Oahu and Molokai. This
area has been moving SW at about 15 to 20 miles per hour...and is fighting
what is our climatologically least active time of day with cold
tops and even a lightning strike or two. Should be moving over the
coastal waters and reach our shores during the late-morning and
into the afternoon...and have adjusted pop and weather grids to
better capture this area. Another area where things will get
active this afternoon will be over the puna district of The Big
Island...as well as The Big Island slopes where heavy thundershowers
should blow up thanks to moist sea breezes and upslope flow.
Other than that...we may see a period of relatively quiet weather
for most places today and tonight because of the aforementioned
subsidence. The models are indicating that the deeper moisture
S of The Big Island will slosh back north as a shortwave moves through
the broad trough Tuesday into Wednesday...and an upper level jet streak
passes by to the north. This will put US under a zone of upper level
divergence in the right entrance region of the jet...helping to
reactivate heavy showers or thundershowers...mainly for windward
Big Island and possibly Maui County. It is difficult to say how
much enhancement Oahu will see...with the outlier GFS solution
showing more of a northward push to the deeper moisture than the
other models. Kauai looks too far removed from the deep moisture
to have many issues. There is enough uncertainty with timing and
placement of these somewhat subtle features that confidence is not
high enough for a Flash Flood Watch yet...but we will have to keep
an eye on things.
Deep moisture shifts S again starting Thursday...bringing a
progressively drier airmass over the smaller islands during the
latter part of the week into the weekend. However...it will remain
unstable with an upper trough just to the north of the islands and
helping to enhance any patches of trade wind cumulus that are
riding by. Typically these patterns yield lots of sunshine
especially for leeward sections...but when showers blow through
they can come down hard for a few minutes before the sun comes
back out again. The weather will probably remain a little more
cloudy and showery during this period for windward Big Island...
along with cloudy...showery afternoons over the Kona slopes.
Will be making some grid adjustments today to reflect these ideas
in the afternoon forecast package.
airmet Sierra for mountain obscuration remains posted over eastern
portions of The Big Island due to clouds and showers. This airmet
may need to be expanded depending on afternoon convective development
that is expected over The Big Island this afternoon.
Middle to upper level turbulence remains over The Big Island and Maui
due to the middle level trough over these islands. Anticipate these
conditions to continue through today...with some improvement tonight
Instability to the east and north of Maui...Molokai and Oahu
associated with the middle level trough could spark a few thunderstorms
over the waters in that area. Daytime heating over The Big Island
could also spark isolated thunderstorms. Elsewhere...predominantly
VFR conditions will prevail at the terminals...with isolated IFR
conditions found over windward and mountain areas with passing
clouds and showers.
the pressure gradient between a strong high far to the northwest of the
islands and a weak surface trough near The Big Island will keep
winds near Small Craft Advisory /sca/ levels in some areas around
Kauai today. The high will slowly push eastward which will lead to
stronger trades for all areas through midweek...strongest near
Kauai. The winds today in combination with a quick pulse of northwest
swell will result in Small Craft Advisory conditions for most waters. The Small Craft Advisory will
likely need to be adjusted and extended as the winds increase.
high surf advisory until midnight HST tonight for Niihau-Kauai
windward-Kauai leeward-Waianae coast-Oahu North Shore-Oahu
koolau-Olomana-Molokai-Maui windward west-Maui Central Valley-
Winter Weather Advisory until 6 PM HST this evening for Big
Small Craft Advisory until midnight HST tonight for Kauai
northwest waters-Kauai windward waters-Kauai leeward waters-
Kauai Channel-Oahu windward waters-Oahu leeward waters-Kaiwi
Channel-Maui County windward waters-Maui County leeward waters-
Pailolo Channel-Alenuihaha Channel-Big Island windward waters.