Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
920 am PST sun Jan 25 2015
Synopsis...an upper ridge over the region with surface high pressure
east of the Cascades will give dry and mild weather to the area
through 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 between 2000 and 10000 feet will dry the
lower and middle levels out later today and temperatures around
2000-3000 feet will warm to around 17c. Some moisture moving over
the top of the ridge will bring some middle level clouds at times
Fog and stratus is showing signs of thinning out this morning over
portions of the interior. This trend is expected to continue with
only the area from around SeaTac-Olympia-Hoquiam valley locations
southward holding onto the low level moisture into the middle afternoon
hours. Skies this afternoon will be partly to mostly sunny most
places with only middle level clouds adding some character to the sky.
The air mass is quite warm and moist with most places already in the
lower 50s and dewpoints around 50. The record high of 58 degrees at
SeaTac will likely be broken with 60 in the forecast. It will really
be a nice day to enjoy the outdoors today.
Radiational cooling with high dewpoints and a lowering inversion to
around 1000 feet late tonight will mean that areas of fog will
redevelop over the interior valleys and through the Chehalis gap.
With less cloud cover this evening...expect temperatures to drop 4-8
degrees cooler than they did this morning.
Fog will lift midday on Monday...except maybe later in the afternoon
in the south interior. 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.
The incoming 12z GFS shows the ridge shifting eastward into the
Idaho Panhandle Monday night as a frontal system and short wave
trough offshore approaches the area. The upper ridge is a tad
stronger in the latest solution...so rain chances for Tuesday into
Tuesday night may need to be lowered a bit. Still expect lots of
clouds as the weakening front moves into the area and dies right
over western Washington Tuesday through Wednesday.
Current forecasts look good for now. No morning update is needed.
Long term...from the previous long term discussion...a few showers
will linger into Wednesday with abundant residual moisture over the
area. The models have trended drier overall with much less quantitative precipitation forecast. Will
keep the chance probability of precipitation in place for now. Both the GFS/European model (ecmwf) continue
to show a stronger ridge on Thursday which would give dry weather.
The forecast was adjusted to remove probability of precipitation over the interior. The
ridge will persist will persist Friday and Saturday for continued
dry and mild weather. Mercer
Hydrology...flooding is not expected for the next 7 days.
Aviation...western Washington is at the northern periphery of strong upper
level high pressure today. Moderate to strong west-SW flow aloft. The
air mass will remain very stable. The air mass in the lowest 1000
feet is quite moist...with dry conditions above. As a result of the
moist layer near the surface...some areas of fog and low clouds
developed this morning prior to sunrise...with the greatest coverage
over the south sound region. A pilot weather report near ktiw at 1629z reported
cloud tops of 1500 feet. Considering the shallowness of the
fog/clouds and 5-15 knots of wind aloft entraining dry air into the top
of the moist layer...expecting fog and low clouds to burn off fairly
quickly this morning. Tonight under clear skies...expect fog to
re-develop...with greater coverage and more widespread low visibilities
developing over The Lowlands. Fog and low clouds may be stubborn to
burn off over the south sound region on Monday.
Ksea...expecting steady burnoff of low-level clouds for the rest of
this Sunday morning...with a VFR afternoon and evening merely
featuring scattered middle-high level clouds above 15 thousand feet.
Late tonight...fog is likely to develop at the terminal...possibly
dense. On Monday...the development of a gentle south breeze will
first help to lift the fog into a low stratus late in the
morning...with burnoff expected on Monday afternoon. Haner
Marine...surface high pressure will shift east of the Cascades this
morning with the low level flow becoming light offshore. Pressure
gradients this morning have been trending more northerly and
easterly. The light offshore flow pattern will continue into Monday.
A weakening front will arrive on Tuesday with surface high pressure
rebuilding over the area Wednesday and Thursday.
Small craft advisories for hazardous seas for the outer coastal
waters will continue into this evening. Haner
Pz...Small Craft Advisory hazardous seas outer coastal waters.
You can see an illustrated version of this forecast discussion at