Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
925 am PST Friday Mar 7 2014
Synopsis...high pressure aloft will bring drier weather and sun
breaks today. Another strong frontal system will affect western
Washington this weekend with rain...high snow levels...and locally
windy conditions. An upper trough will move through on Monday for
cooler showery weather...and a strong upper ridge will provide dry
and mild weather Tuesday and Wednesday.
Short term...radar shows a few weak echoes across western Washington this
morning...but in general expect a break in the weather today as an
upper level ridge moves over the Pacific northwest. We may see a few breaks in
the clouds this afternoon for partly sunny skies. Temperatures will remain
mild with highs in the 50s.
The upper level ridge will slide farther inland tonight as our next
wet Pacific system moves in. Models show another warm front clipping
western Washington Sat morning...followed by the cold front Sat afternoon
and evening. Models show another 2-4 inches of precipitation in the
mountains with high snow levels /over 6000 ft/. The rivers are
already running high from heavy rain this week and a few more inches
will keep them running high...with some flooding possible once
again. Will likely need another Flood Watch. Heavy precipitation will taper
down by Sun afternoon.
A weak upper level trough will Cut Through the area Sun night with a
few more showers across the region...although heavy showers are not
expected. Snow levels will drop back down to 3000-4000 feet. 33
Long term...previous discussion...a substantial upper ridge will
start to build into western Washington on Monday. By Tuesday...500 mb heights will
exceed 5700 meters...then a thermal pressure trough will build up
the coast on Wednesday morning...according to both the European model (ecmwf) and GFS. This
will lead to dry and sunny weather on Tuesday and Wednesday...with the
potential for well above normal temperatures on Wednesday...especially if the
thermal pressure trough can linger near the coast well into Wednesday
afternoon. Models diverge by next Thursday...the 00z European model (ecmwf) brings a
progressive shortwave by to our north...with a cold front and return
to onshore flow. The 00z GFS passes a closed low to our south...and
the 18z GFS maintains a Flat Ridge with high heights in place.
Hydrology...precipitation has been decreasing over western
Washington since a front moved through the area Thursday. Additional
precipitation through tonight will be hydrologically insignificant.
The upper reaches of western Washington rivers have crested...and
crests will move downstream to the mouths of the rivers by late
today. The exception to that is -- as always -- the Chehalis which
is a uniquely long river in a low-elevation basin. But it looks like
the Chehalis river near Grand Mound will crest just under flood
stage. It will be very close and will keep the Flood Warning going
for now. If the lower Chehalis does flood...it will be relatively
minor. The same GOES for the Chehalis at Porter.
Numerous rivers crested within 1 or 2 feet of flood stage...but only
Two Points have flooded -- the Skokomish River near Potlatch and the
Snoqualmie river near Carnation. Flood warnings will remain in
effect for those sites with the updated flood statement by noon
Another period of significant rainfall is likely this weekend with
the snow level around 7000 to 8000 feet. At this point it looks like
rainfall in the mountains will be 2 to 4 inches...mainly from midday
Saturday to Sunday afternoon. That could easily renew flooding on
the Skokomish River. In addition...the nwrfc hydrologic guidance
indicates that the Snoqualmie river near Carnation and some of the
other rivers that almost flooded recently could see similar crests
late Sunday or Sunday night. May need another Flood Watch. 33
Aviation...an upper ridge will prevail over the region today. This
ridge will move east tonight...allowing an upper trough to approach the
Pacific northwest. Light westerly flow aloft will strengthen tonight. Light
low level or southerly onshore flow will become southeasterly tonight.
There was patchy fog over the area this morning. This fog was
reducing visibilities into the 3-5sm range. Look for the fog to lift by
midday. Meanwhile...ceilings should slowly lift today. By 0000 UTC...
anticipate VFR conditions across much of the area...although there
will be localized ceilings in the 2-3k feet range...especially over the
Ksea...VFR conditions are anticipated today...although expect
occasional ceilings near 2k feet through this morning or about 2000 UTC.
weak high pressure over the area will result in light onshore or
southerly flow. Southeast flow will strengthen tonight in response to
falling pressure over the coastal waters. Anticipate a strong frontal
system to approach the area during the day Saturday before moving
across the area Saturday night. Gale force winds can be expected
across parts of the area...particularly during the late morning
through afternoon time frame.
Washington...flood watches are in effect for parts of the County Warning Area.
Pz...Gale Warning in effect for the coastal waters and West
Entrance to the Strait of Juan Delaware fuca.
Gale watch in effect from Saturday morning through Saturday
afternoon for the East Entrance to the Strait of Juan Delaware
fuca and the northern inland waters.
For the graphical afd...visit