Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
830 PM PST Tuesday Jan 27 2015
Synopsis...low level moisture will linger across western Washington
tonight and Wednesday for a chance of light rain. An upper ridge
will provide dry and mild weather Thursday through Saturday. A front
will bring rain back to the area Sunday and early next week.
Short term...a weak front dissipated over the area earlier today.
Left over low level moisture is plentiful with low clouds blanketing
the region. There will also be fog and light rain over central
Puget Sound sound tonight in a broad Puget Sound sound convergence zone.
An upper ridge will build over the area on Wednesday. The low levels
will start out moist with clouds in the morning. Low level flow will
switch from onshore to offshore with low levels drying and perhaps a
little sunshine in the afternoon. An upper short wave in the
southwest flow aloft could bring some sprinkles Wednesday afternoon
and night however.
The upper ridge will strengthen over the area on Thursday and remain
in control on Friday with weak low level offshore flow. This should
result in dry weather with some night and morning low clouds or fog
and afternoon sunshine each day.
Highs will be a little above normal for the next three days...with a
gradual warming trend. Schneider
Long term...previous discussion...an upper level ridge will remain
dominant across the Pacific northwest Friday into the weekend with dry weather
continuing across western Washington. Saturday will likely be the warmest
day and may see low 60s again. The ridge will flatten on Sunday as a
warm front moves into b.C. This front may clip the area for light
rain. Rain will increase Sun afternoon and evening as a cold front
moves in. Zonal flow will prevail Monday and Tuesday with upper level
disturbances passing through the area. This keeps a chance of rain
in the forecast through at least midweek. 33
Hydrology...flooding is not expected for the next 7 days.
Aviation...moderate westerly flow aloft will continue over West Washington
tonight along with a broad weak ridge at the surface. An upper level
ridge will build over western Washington on Wednesday giving lighter
A weak dissipating front has left a moist and stable air mass over West
Washington tonight. MVFR ceilings broken-ovc010-025 are widespread with some
isolated IFR ceilings. A weak convergence zone is present between north
Seattle and Everett bringing IFR ceilings/visible in -ra/-dz. With the
surface ridge and weak winds over West Washington...ceilings are expected to lower
overnight with areas of IFR ceilings developing 10z-18z...to go along
with the MVFR ceilings. Ceilings should lift to broken-ovc030-050 or better
after around 20z. Kam
Ksea...a moist stable air mass will maintain MVFR ceilings through 06z.
IFR ceilings are expected to return after 06z. A weak convergence zone
over Seattle earlier this evening made an unplanned shift back north
so ksea ceilings popped up briefly. The air mass is too moist for this
to last. Some guidance indicates visibility getting down to 1/2sm
overnight...as opposed to just IFR ceilings. This is possible but
confidence is not very high. Surface winds will be light and
variable overnight. Kam
Marine...a broad weak surface ridge centered over the Washington coast this
evening will shift over East Washington by Wednesday afternoon...where it will
remain through Saturday. Weak east to NE offshore flow will develop
Wednesday afternoon and remain mostly weak through Saturday. GFS and
European model (ecmwf) are in fair agreement that cold front will reach the area on
Sunday. Increasing southeast flow ahead of the front will probably generate
Small Craft Advisory winds over the coastal waters starting late Saturday.
Onshore flow developed this afternoon behind a weak cold front. Flow
through the central and east Strait reached Small Craft Advisory levels this afternoon
and evening...but the uil-bli pressure gradient is weakening so the
Small Craft Advisory will be allowed to expire at 10 PM as planned. Kam
Pz...Small Craft Advisory central and east Strait.
You can see an illustrated version of this forecast discussion at