Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
316 am PST Friday Dec 13 2013
Synopsis...a moist air mass and Post-frontal onshore flow will keep
skies cloudy with a chance of showers through early Saturday. A weak
cold front will bring a chance of rain to the north part Saturday
afternoon and to the rest of western Washington Saturday night.
Another round of Post-frontal onshore flow will produce mostly
cloudy weather with a chance of showers Sunday. Northwest flow aloft
and weak high pressure will bring mostly dry conditions Monday. A
more active weather pattern may develop by middle next week.
Short term...the upper level trough crossing the area Thursday
evening has moved east and drier northwest flow aloft has developed. Radar
shows just about all shower activity has moved over the Cascades
with very little elsewhere. The Post-frontal air mass remains quite
moist at low levels...and onshore flow will keep it that way through
Saturday morning. The onshore flow shows up best at 925 mb with west
winds 10-15 knots today...shifting SW 10 knots tonight. Time height
sections and boundary layer relative humidity both suggest skies will remain cloudy
through Saturday morning. With all the moisture around a chance of
showers will continue...especially over the Cascades with the
upslope westerly flow.
Models remain in agreement with the next weak cold front expected to
reach the area later Saturday. This front will evolve out of the
disorganized baroclinic zone over the NE Pacific...extending SW from
the central b.C. Coast. Looks like there is a ton of moisture with
it on satellite imagery this morning...but the models have been very
consistent in reducing it almost to nothing as reaches the coast
Saturday afternoon...then moving inland Saturday evening. Low level
warm advection ahead of the front Saturday could warm maximum temperatures
almost up to 50 Saturday afternoon.
The front should have low impact to the area...bringing 2 to 5
tenths of an inch of rain to the coast and barely a tenth to inland
areas. On Sunday another round of Post-frontal onshore flow coupled
with westerly flow aloft will keep skies mostly cloudy...again with
the low chance of lowland showers and better chance of Cascade
Long term...moderate west flow aloft will shift northwest on Monday as a
broad low amplitude upper level ridge develops offshore along 140w.
This will allow a weak surface high just offshore to slide farther
NE over West Washington. The offshore upper level ridge amplifies just enough
to push the warm front from the next frontal system up over b.C.
Late Monday. The combo of the surface high moving over the area and
the warm front passing to the north should produce a dry Monday.
However...confidence is not quite high enough to remove the chance
of precipitation on Monday...but it might be dropped in a later forecast
Both the GFS and European model (ecmwf) drag the associated weak cold front over the
northwest part of West Washington on Tuesday. The 00z editions of both models bring a
fairly strong upper level trough southeast over the area around midweek.
However since recent model solutions have been pretty jumpy beyond
Monday it is a little early to invest much confidence in this
solution. Suffice that the pattern looks like it will be more
Hydrology...river flooding is not expected for the next 7 days.
Aviation...high pressure surface and aloft will build over the area
today and tonight. Air mass moist and stable. The low levels are
starting out very moist today. Low clouds and areas of fog will only
show marginal improvement later today as the low levels remain
moist. The low clouds should persist into tonight.
Ksea...low clouds and 3-5sm br will show only slight improvement
later today under light onshore flow. South wind 3-7 knots. Schneider
Marine...winds will be relatively light with weak high pressure
over the area today. Southerly winds will begin to increase tonight
with most areas rising to Small Craft Advisory levels Saturday or
Saturday night as a front moves inland. Onshore flow will prevail
Sunday and Monday behind the front. Another weak front will reach
the area on Tuesday. Schneider
Pz...Small Craft Advisory for rough Grays Harbor bar.
You can see an illustrated version of the forecast discussion at