Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 PM PST Sat Nov 28 2015
Synopsis...a strong upper high will slowly move across southern
British Columbia tonight and Sunday. The high will shift farther
inland Sunday night. This will allow a weak cold front to reach the
coast with some rain on Monday. Active and sometimes stormy
southwest flow aloft will become established starting
Tuesday...continuing through the rest of the week ahead.
Short term...a strong blocking ridge is anchored over the Pacific northwest
with dry and stable weather continuing across western Washington.
Temperatures are already in the 20s to lower 30s across The Lowlands
with patchy freezing fog developing. Freezing fog will expand in
coverage by Sunday morning...with patchy fog possibly lingering into
the afternoon hours. Otherwise expect another dry day on
Sunday with highs in the lower to middle 40s.
The upper level ridge will finally shift east and inland Sunday
night as a cold front moves in from the west. This front will bring
a chance of rain on Monday as it crosses the area. This system looks
fairly weak and rain amounts will be light. The increase in clouds
and moisture will keep temperatures mostly above freezing Monday night.
A stronger/wetter Pacific frontal system will move inland on Tuesday for
an increase in rain. There will also be windy conditions...mainly
coast and north interior. 33
Long term...previous discussion...within the Wednesday-Sat period...Thursday
has currently got my attention...with the emerging but still-small
possibility of a damaging windstorm. But first going
chronologically...Wednesday morning should feature a break in wet weather.
Later Wednesday...our active SW flow aloft will strengthen...and a frontal
boundary well offshore will sharpen up further. This will act as a
good focus for the development of energetic frontal waves. One
frontal wave should move across Vancouver Island on Wednesday
night...pushing some heavy rain across the olympic peninsula and
increasing wind in southeast-wind prone areas. See the Hydro section below.
This wave will shove the main front a little further southeast on Thursday and
will then play a key role in the development and movement of the
more concerning system later Thursday. The 12z European model (ecmwf) and the 12z/18z
GFS runs all point to cyclogenesis along the front somewhere west
of northern cal. Today's 12z European model (ecmwf) is similar to yesterday's 12z European model (ecmwf)
in quickly moving a mature 982-984 mb low just northwest of Tatoosh
Island on Thursday afternoon. Structurally...the model shows a bent-back
occlusion...which is a signature of some of our more damaging western
Washington windstorms. In contrast...last night's 00z European model (ecmwf) and all
recent runs of the GFS move the low onshore well to our south in
Oregon or even northern cal...with much less wind-related impact over
western Washington. At this point...would say there is a one-in-four or one-
in-five chance of a high-impact wind event for western Washington on Thursday or
Thursday night...so stay tuned.
Both GFS and European model (ecmwf) show a transient shortwave ridge providing a
break in active weather next Friday...then both increase SW flow
aloft and wet weather again next Sat.Haner
Hydrology...there is a possibility of minor flooding on the flood-
prone Skokomish River on Thursday. A period of wet weather is currently
forecast on late Wednesday and early Thursday...which could bring 2-3 inches of
rain to the southern Olympic Mountains elsewhere in western
Washington...river flooding appears unlikely for the next 7 days.
Aviation...the strong upper level ridge over the Pacific northwest
will drift slowly east through Sunday night. A weak surface ridge
over southern British Columbia and eastern Washington will maintain
dry low level offshore flow. A very strong inversion is in place
over western Washington making the air mass very stable.
Areas of LIFR fog and stratus are expected to form overnight. The
areas most prone to fog are over the greater Puget Sound sound region...
the SW interior...and along the Strait of Juan Delaware fuca. Any River
Valley is also susceptible. The fog coverage is expected to expand
over what formed this morning and will probably be more persistent.
Early morning temperatures will drop below freezing again...so some
freezing fog is likely.
Ksea...clear skies through most of this evening. Fog is expected to
reach the terminal late tonight. Visibility to 1/2sm is expected and
1/4sm visibility is likely at times. The fog could persist until 20z
or so. Surface winds will be east-southeast 2-6 knots. Kam/chb
Marine...a surface ridge over southern British Columbia and eastern
Washington will keep light easterly offshore flow tonight. On Sunday
a front will approach the region and shift winds to southerly. Small
Craft Advisory strength winds ahead of the front are likely to reach
the outer coastal waters by late Sunday afternoon then spread east.
The front will move inland on Monday with borderline advisory level
winds possible over the north interior waters.
A stronger front will reach the area on Tuesday. Gales are likely
with this system. The GFS European model (ecmwf) and Canadian models agree that the
rest of the week will be an active weather with gales possible at
times. However confidence in the details is low.
The current period of King tides will have the highest tides this
year. Fortunately strong high pressure over the area this weekend
and light winds appear likely to keep the tidal anomalies minimal.
No coastal flooding is forecast during the high tides. Kam/chb
Washington...air stagnation advisory until noon PST Monday for Admiralty
Inlet area-Bellevue and vicinity-Bremerton and vicinity-
east Puget Sound sound lowlands-eastern Strait of Juan Delaware fuca-
Everett and vicinity-Hood Canal area-lower Chehalis valley
area-San Juan County-Seattle and vicinity-southwest
interior-Tacoma area-western Skagit County-western Strait
of Juan Delaware fuca-western Whatcom County.
Pz...Small Craft Advisory coast.
You can see an illustrated version of this discussion at