Scientific Forecaster Discussion
fxus66 ksew 210514
Area forecast discussion
National Weather Service Seattle Washington
919 PM PST Friday Jan 20 2017
Synopsis...a broad upper level trough will remain over the
offshore waters through the weekend. A series of systems rotating
around it will bring showers and breezy conditions at times to
western Washington along with seasonable temperatures. By early next
week, a ridge of high pressure will build into the Pacific northwest
for the possibility of several dry days.
Short term...current radar (pic0) shows the makings of a quiet
evening and overnight period. While there are still echoes
present...those over the north interior look mostly like light
showers/drizzle/sprinkles and echoes over central olympic peninsula
appear to be the last vestiges of a passing occuluded front.
Looking to the models...not much has changed in that regard. Some
lingering stray showers here and there for the start of Saturday but
no real widespread coverage...looks like that will wait until the
late afternoon as a shortwave trough rotates around the main
trough...bringing in another slug of moisture to West Washington. Sunday
morning and early afternoon look to be prime time when it comes to
precipitation as a much more impressive feature passes through the area.
Gradients in the middle morning hours continue to look pretty tight
indicating strong easterly winds (pic1)...but current forecasts show
this lasting only within a narrow 6 hour window...as gradients
loosen by noon on Sunday. Models a touch faster on how quickly the
upper low retreats and descends further south...allowing for dry
conditions in West Washington as early as early Sunday evening.
Ridging will build over the Pacific Monday...bringing continued
Inherited forecast looks good...no need for evening updates. Smr
Long term...from previous discussion...a ridge will begin to
build in over the area late Monday through at least Wednesday.
Beyond that, model solutions diverge with the GFS showing a drier
solution continuing and the European model (ecmwf) showing a few systems moving
through the area bringing showers. The Gem shows precipitation on
Thursday but then a return to dry conditions in its seemingly usual
middle-ground stance. Jsmith
Aviation...an upper level and surface low offshore near 47n/131w
(pic10) will maintain somewhat moist and unstable S flow over West Washington
through Saturday. An upper level shortwave trough will move NE
across West Washington Saturday afternoon (pic13). The air mass is fairly moist
and weakly unstable for scattered showers. Conditions are expected
to remain VFR with ceilings bkn-ovc040-080. An occluded front will move
NE across the area on Sunday.
Ksea...VFR conditions with ceilings broken-ovc040-070 are expected to
continue through Saturday. There will be a few showers at times.
Surface winds will be southeast 4-8 knots. Kam
Marine...a surface (pic11) and upper level low about 350 nm west of the
Washington coast tonight will remain in place before moving northwest on Saturday
(pic12). A couple of surface troughs will wheel around the low and
over the coastal waters, one early Saturday morning and the second
Saturday afternoon and evening. These waves should serve to maintain
Small Craft Advisory winds over the coastal waters and parts of the inland waters.
A deep 960-965 mb surface low will move northward offshore along
130w Saturday night then pause west of the Washington coast on Sunday (pic12).
The associated occluded front will move NE across West Washington on Sunday.
The low is strong enough to produce a gale over the coastal waters.
A gale watch has been issued this evening to address this. At this
point the low looks fairly compact and may be far enough offshore to
limit the inland waters to just Small Craft Advisory winds. The low will depart the
area to the S on Monday. A weak surface ridge will remain over the
area Tuesday through Thursday for a period of weak winds.
A large westerly swell train around 19 feet is expected to reach the
Washington coast after midnight tonight. The highest waves should reach the
area early Saturday morning then gradually subside during the late
morning and afternoon. The swell period is fairly long at 17-19
seconds so waves hitting the beaches should pack a bit of energy.
Buoy 36 has started to pick up the swell train with the latest ob at
19 feet 17 seconds. A high surf advisory remains in effect. Kam
Hydrology...the Skokomish River has fallen below flood stage...so
Flood Warning has been cancelled. No additional flooding is expected
at this time with only light showers in the short term forecast and
then a switch to drier conditions starting Monday and continuing
into much of the week. Jsmith/smr
Washington...high surf advisory from midnight tonight to 6 PM PST Saturday
for central coast-north coast.
Pz...Small Craft Advisory coast through Saturday afternoon.
Gale watch coast late Saturday night through Sunday afternoon.
Small Craft Advisory central and east Strait and north inland
Small Craft Advisory for hazardous seas west Strait.
Small Craft Advisory for rough Grays Harbor bar.