Scientific Forecaster Discussion
fxus66 ksew 141042 cca
Area forecast discussion
National Weather Service Seattle Washington
241 am PST Thu Nov 14 2019
Synopsis...an upper level ridge will shift eastward today with
the next front arriving tonight into Friday. Another system will
sag southward across western Washington over the weekend. A potent
system Monday night into Tuesday could drop snow levels down to
some of the higher passes.
Short term /today through Saturday/...an upper ridge axis is
pushing east of the region early this morning. Considerable mid
and high level clouds continue to filter across the area. Low
level flow is offshore. Despite the high level clouds and
offshore flow, deepening nighttime inversions are leading to
widespread shallow fog and low clouds across much of the
interior. The offshore flow is keeping most of the coast stratus
free as of 2 am PST. The low clouds should dissipate for most areas
by midday, but may hang on a little longer in the southwest
Recent model runs have slowed the eastward progress of the next
frontal system somewhat. Nonetheless, rain should reach coastal
areas by mid or late evening then spread into the Puget Sound area
before daybreak on Friday. Rain will turn over to showers by
Friday afternoon and it will become a bit breezy behind the front
as onshore flow increases. Upper ridging will begin building into
the area Friday night with most of the region drying out briefly.
A warm front is expected to brush the coast and north interior on
Saturday as a substantial plume of moisture takes aim at British
Columbia. Initially, the best chances for rain on Saturday will be
concentrated on the northern half of the forecast area, but this
will change as we entered the beginning of the extended forecast
Long term /Sunday through Wednesday/...a frontal boundary will
sag southward across the region on Sunday bringing a fairly wet
day to most of the area. There could be a few hours of moderate
to heavy rainfall on Sunday which may induce some rises on the
rivers flowing off the Olympics and north Cascades (see Hydro
section). Weak upper ridging is expected to give the area another
reprieve from precipitation on Monday. Operational runs of the GFS
and European model (ecmwf) as well as their ensemble anomalies show a considerable
amount of uncertainty with respect to the forecast for Tuesday
and beyond...showing a range of possibilities from split flow to
an open trough. It does appear that temperatures (and snow levels)
will trend downward as we get into next week, but there is low
confidence in anything else. 27
Aviation...high pressure will continue to shift east today as a
approaches the coast and moves inland later today into Friday.
Flow aloft will become southwesterly with surface flow turning
more southerly. Conditions range anywhere from VFR to LIFR this
morning as areas of fog and low clouds have formed once again.
Expect variable conditions across the area through the morning
before improvement back to VFR after 18-20z. Rain will approach
the coast around 6z Friday and spread inland through the evening
and night. Expect ceilings to lower again as this occurs.
Ksea...low clouds and/or fog through about 18z this morning with
IFR/LIFR conditions. Improvement back to VFR as fog/clouds scatter
out. Increasing clouds this afternoon and evening ahead of the
front with high-end MVFR ceilings possible by early Friday
morning. Light northerly winds this morning will turn southerly 4
to 8 kts this afternoon. Ceo
Marine...offshore flow will gradually shift to the south today
ahead of the next frontal system approaching the region. A Small
Craft Advisory remains in effect for easterly winds for the western
Strait of Juan de Fuca through the remainder of the overnight
hours. Otherwise, small craft southerly winds will be likely for
most western Washington waters today and into Friday as the front
moves inland. The only exceptions are the central and eastern
portions of the Strait of Juan de Fuca, where winds are expected
to remain relatively calm. Westerly swell in the coastal waters
will build up to 10-13 feet tonight and into Friday.
Another frontal system will arrive over the weekend, bringing
another, potentially stronger round of advisory level winds to
most of the western Washington waters. Sb
Hydrology...river flooding is unlikely through Saturday. Models
suggest that significant precipitation is expected to move into
British Columbia this weekend. Some moisture associated with this
atmospheric river may impact the Olympics, as well as Whatcom and
Skagit County. Rivers, especially the Nooksack and Skagit, are
expected to rise this weekend, but no flooding concerns are
expected. Another system will move through next Monday into
Tuesday, however the progressive pattern will keep flooding
concerns low into next week. Jd
Pz...Small Craft Advisory from 5 PM this afternoon to 6 am PST Friday
for coastal waters from Point Grenville to Cape Shoalwater
10 to 60 nm-coastal waters from Point Grenville to Cape
Shoalwater out 10 nm.
Small Craft Advisory from 1 PM this afternoon to 4 am PST Friday
for coastal waters from Cape Flattery to James Island 10 to
Small Craft Advisory until 6 am PST Friday for coastal waters
from Cape Flattery to James Island out 10 nm.
Small Craft Advisory from 10 am this morning to 6 am PST Friday
for coastal waters from James Island to Point Grenville 10
to 60 nm-coastal waters from James Island to Point
Grenville out 10 nm.
Small Craft Advisory from 1 am to 10 am PST Friday for East
Entrance U.S. Waters Strait of Juan de Fuca.
Small Craft Advisory from 10 PM this evening to 10 am PST Friday
for northern inland waters including the San Juan Islands.
Small Craft Advisory until 6 am PST early this morning for West
Entrance U.S. Waters Strait of Juan de Fuca.
Small Craft Advisory from 7 PM this evening to 10 am PST Friday
for Admiralty Inlet.
Small Craft Advisory from 7 PM this evening to 1 PM PST Friday
for Puget Sound and Hood Canal.