Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
930 PM PST sun Jan 25 2015
Synopsis...an upper ridge over the region with surface high
pressure east of the Cascades will give another dry mild day on
Monday. A weak front will dissipate over western Washington Tuesday
through early Wednesday. Another upper level ridge will build over
the region later in the week.
Short term...an upper level ridge is centered over Washington state
while a surface ridge extends from southeastern Idaho into eastern
Washington. Subsidence and drying in the lower levels gave a very
nice day to the area today with some record highs. Cirrus is moving
over the ridge but otherwise fair skies tonight with high dewpoints
means that areas of fog will develop. The fog will lift midday on
Monday...perhaps later in the afternoon in the south interior where
the valleys are blocked from the light offshore flow. Afternoon
sunshine and a continued warm air mass means another very nice and
mild day on Monday with highs again in the 50s to around 60 degrees.
The record high temperature at SeaTac Airport on Monday is again 58
degrees and should be broken by a degree or two.
The ridge shifts eastward into western Montana Monday night
as a frontal system and short wave trough offshore approaches the
area. However...the upper ridge appears to be strong enough to cause
the incoming front and upper level shortwave trough to mainly ride
by to the north of the area with little in the way of precipitation
in the interior of western Washington as the moves in and dies Monday
night through early Wednesday. Probability of precipitation were cut back a bit from the previous
forecasts for the Monday night through Wednesday time frame. Skies will be
cloudy and temperatures will be mild with lows in the 40s and highs
in the lower 50s Tuesday and Wednesday. Albrecht/19
Long term...a very weak short wave trough will move onshore Wednesday
night or Thursday. The 18z GFS is now dry with the Wednesday night and Thursday
system while the European model (ecmwf) brings some very light precipitation to the
coast. Will limit rain chances on Thursday to the coast. Model
consistency and continuity starts to suffer somewhat starting Sat
night with the faster 12z and 18z GFS bringing a front into the area
Sat night and sun and the European model (ecmwf) is about 18 hours slower...but still
faster than previous solutions. Will introduce clouds and a chance
of rain into the forecast starting Sat night and continuing into
Monday. With mean upper ridging along the West Coast...it will remain a
few degrees warmer than normal through early next week. Albrecht
Hydrology...flooding is not expected for the next 7 days.
Aviation...upper level ridge over western Washington through
Monday. Ridge axis east of the area with southwesterly flow aloft.
Ridge weakening Monday night as system moves into British Columbia.
Air mass stable and moist below 1000 feet.
Fog has already formed in some locations this evening. With the
moist air near the surface and fairly light gradients expect fog
coverage to expand overnight with many locations having ceilings
below 500 feet and visibilities a mile or less by 12z. The layer
will be shallow with tops around 1000 feet so a slow lift by late
morning followed by a midday burn off in the forecast.
Ksea...fog has already formed to the south of the terminal in Tacoma
and to the north in Everett. Still a four degree temperature dew point
spread at 05z ( 49/45 ) along with a bit of a north wind so there is
still a few hours to go before the fog forms at the terminal. Will
go for a gradual deterioration of conditions beginning around 10z
with ceilings and visibilities below 200 feet and a half mile around
12z. With the layer fairly shallow expect a late morning/midday
Marine...weak offshore gradients will persist through tonight. On
Monday afternoon...a 999 mb low will move NE to haida gwaii...
increasing the southerly winds over the coastal waters and a few
locations over the inland waters. The front will dissipate before it
moves inland Monday night into Tuesday morning. Low level flow will
turn onshore Tuesday. A return of light offshore flow is forecast on
Small craft advisories for hazardous seas over the coastal waters
and West Entrance to the Strait will come to an end with the next
forecast package as the swell subsidies below 10 feet. Felton
Pz...Small Craft Advisory for hazardous seas coast and West
Entrance until late tonight.
You can see an illustrated version of this forecast discussion at