Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
300 PM PDT sun Mar 29 2015
Synopsis...a stalled warm front will bring rain at times to northern
parts of western Washington through Monday. The associated cold front
will bring widespread rain to the area Monday night. An upper level
trough will settle over the Pacific northwest Tuesday and Wednesday
providing cool and showery weather. Showers will decrease
Thursday...then rain develops Friday as the next trough moves over the
region. Unsettled weather will continue into the weekend.
Short term...warm advection rain continues late this afternoon from
the north coast/olympic peninsula across the north interior. Over a half inch
of rain fell the past 12 hours on the north coast and around a quarter inch
at a few other spots toward the Canadian border. Radar and observation show
light rain briefly extending S into Hoquiam but is not making much
progress inland. Mesoscale models are mostly keeping the interior dry from
around Everett southward tonight. However...the latest hrrr does show some
spotty light quantitative precipitation forecast just north of Seattle so the forecast was tweaked a bit to
throw in some low probability of precipitation for the Everett/Bremerton/and east Puget Sound lowland
zones for this evening. Sprinkles are possible further S this evening
but nothing measurable is expected.
Models deepen the associated surface low offshore to about 1000 mb as
it tracks into S/central b.C. Monday afternoon. Once the low moves
inland...the trailing front and upper trough will move into the region.
The GFS/ECMWF/and mesoscale models all show widespread rain with the front
Monday night. A cooler and unstable air mass will follow the front with
rapidly lowering snow levels later Monday night into Tuesday. Snow
levels will start off close to 7000 feet but will fall to roughly 3000
feet by Tuesday morning. A few inches of snow are possible through
Tuesday morning...mainly at the higher passes and ski resorts. Quantitative precipitation forecast off
the NAM-12 and GFS imply mostly sub advisory amounts of snow. Locally
higher amounts of snow could fall in the Cascades of Snohomish/King
County if a Puget Sound sound convergence zone develops. This would mainly
affect Stevens Pass if the latest mesoscale models are correct. Will need to
keep a watch on this feature Tuesday.
Showers linger into Wednesday as additional upper level energy drives
southeast through the region. The GFS shows a -34c cold pool at 500 mb
settling over the area and lifted indice's go negative both Tuesday and Wednesday
afternoon. This looks like the typical Spring pattern with showers and
isolated afternoon thunderstorms. High temperatures will be much
cooler...only in the low to middle 50s.
Long term...the GFS/European model (ecmwf) are coming into better agreement showing a
short wave ridge on Thursday...the another trough arriving Friday.
There could still be some weak diurnal shower activity Thursday and the
GFS does keep some spotty light quantitative precipitation forecast around. Left a chance of showers in
the forecast for now but probability of precipitation might need to be pulled if models
continue with the ridge solution.
A front brings rain into western Washington on Friday...then a trough
moves into the region over the weekend. It will be cool and unsettled
with showers across the area. Snow levels will be low at around 3000
feet but the showery pattern is probably not conducive for much snow
Hydrology...flooding is unlikely over the next 7 days.
Aviation...a broad flat upper level ridge over the Pacific northwest
will maintain fairly moist westerly flow aloft over West Washington through Monday
morning. A nearly stationary front is draped across southern b.C. And
extends SW from Vancouver Island across the offshore waters. The front
will continue to bring a little light rain at times across the far north
part of West Washington...from the north coast to kclm to kbli.
VFR conditions are expected to continue over the interior with ceilings
generally bkn040-060 with higher overcast layers. Ceilings will probably be a
little lower over the north interior. MVFR ceilings are expected on the
coast with areas of IFR conditions over the north coast closer to the
A cold front will approach the area Monday...crossing the coast late
Monday afternoon and the interior Monday evening.
Ksea...VFR conditions are expected to continue through tonight with
cloud layers generally scattered-bkn050-060 bkn-ovc080-100. Surface winds
will remain southerly 5-10 knots. Kam
Marine...a small weak surface ridge will remain over western
Washington through Monday morning. Lower pressure will continue over
British Columbia and the waters offshore where a baroclinic zone
extends southwestward from Vancouver Island. This pattern will
result in moderate S flow across the coastal waters through tonight.
S flow over the inland waters this afternoon is taking its Sweet
time weakening. Observed winds are trending weaker...and the 20z
hrrr drops the winds below Small Craft Advisory at 3 PM...but the winds are still
trying hard not to cooperate. I will extend the inland waters Small Craft Advisory to
5 PM to cover any non-cooperative stragglers. Otherwise the inland
waters winds should remain below advisory criteria tonight.
An approaching cold front will increase winds back to advisory
levels Monday morning. The cold front is expected to cross the coast
late Monday afternoon and the inland waters Monday evening. Moderate
to strong westerly onshore flow will develop over West Washington Monday night
behind the cold front. Gale force winds are likely in the central
and east Strait of Juan Delaware fuca Monday night and Tuesday...with
advisory winds over the waters adjacent to the Strait.
There will be a westerly swell of 10 to 12 feet over the coastal
waters through Tuesday and over the outer waters into Wednesday. Bar
conditions will be rough as well.
Weaker onshore flow will continue through mid-week...but will be
just strong enough to rise to advisory levels in the central and
east Strait Tuesday night and Wednesday night. Kam
Pz...Small Craft Advisory all waters except the central Strait.
Small Craft Advisory Grays Harbor for rough bar.
Gale watch for Monday night and Tuesday central Strait and
East Entrance Strait.
You can see an illustrated version of this forecast discussion at