Small Craft Advisory 62, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
949 PM PDT Fri Mar 17 2017

.SYNOPSIS...A strong low pressure system and associated cold front
will bring locally heavy rain and gusty winds to the area overnight
through Saturday morning. Expect scattered post frontal showers
Saturday afternoon. High pressure aloft will prevail on Sunday.
Another low pressure system will approach from the southwest during
the day Monday.

&&

.SHORT TERM...
A plume of moisture emanating from the subtropics was aimed at the
CWA. The satellite-derived total precipitable water values showed
values of over 150% of normal over the CWA. Very strong south-
southwest 850 mb flow (around 65 knots) overnight will result in
enhanced precip rates, especially on the south facing slopes of the
Olympic and Cascade ranges.

Snow levels will also rise overnight; therefore, expect the heavy,
wet snow that is currently falling on Hurricane Ridge and Mount
Baker to change over to rain overnight. These two sites have been
receiving an inch of snow per hour, with amounts now shy of low end
advisory criterion (six inches). Since Mount Baker will stay in the
cold air longer, will consider issuing a Winter Weather Advisory
for the north Cascades.

Expect the cold front to sweep across the CWA during the late night/
early Saturday morning time frame. Gusty winds can be expected with
and behind the front but speeds should remain below advisory
criterion across the lowlands. The steady precip should quickly
taper off to showers behind the front. By Saturday evening, most of
the activity will be over the Cascades and within a Puget Sound
Convergence Zone. Snow levels will plummet into the 1500 to 2500
foot range by Saturday evening; therefore, expect the heavier
showers within the convergence zone to contain small hail. The
showers will diminish Saturday night as heights slowly build over
the region.

An upper level ridge of high pressure will prevail on Sunday for dry
weather with plenty of sunshine. Alas, another low pressure system
will approach the region from the southwest during the day Monday.
The current forecast was dry but this now appeared in jeopardy. It
looks like rain will move into the far southwest part of the CWA
Monday afternoon. Elsewhere, anticipate increasing clouds.

.LONG TERM from the previous discussion...
The forecast is just a broadbrush chance of showers for now with
southwest flow aloft, 500 mb heights in the 543-553 dm range, and
850 mb temps that don't stray too far from 0c for a snow level
around pass level. The euro did not come in today for us, but the
12z GFS showed that Thursday looks like the best chance for a dry
day, and although the front on Friday looks like the strongest
system of the week, with the deepest low passing through the
offshore waters, it also looks like it will splits somewhat--with
the heavier rain moving into California.

&&

.AVIATION...A frontal system will move through Western Washington
tonight and Saturday morning with southwest flow aloft. At the
surface, a 1000 millibar low will make landfall over the north
coast tonight and shift inland Saturday morning. Onshore flow
will prevail behind the low. The air mass is moist and stable.

Low clouds will be prevalent tonight and Saturday morning. The air
mass will dry later Saturday and low clouds should scatter most
areas in the afternoon.

KSEA...MVFR ceilings through Saturday morning. The low clouds
should scatter Saturday afternoon. Easterly wind 4-8 knots will
become southerly 10-15 knots Saturday morning. Schneider

&&

.MARINE...A frontal system will move through the waters tonight
and Saturday morning. An associated 1000 millibar surface low will
make landfall over the north coast later tonight. This is a bit
farther north than previously expected but the overall forecast is
generally on track.

Onshore flow will prevail behind the front Saturday. Westerly gales
are likely in the Central and Eastern Strait. Onshore flow will
rapidly ease Saturday night as weak high pressure moves over
Western Washington. The high pressure will shift inland Sunday and
Monday for a period of low level offshore flow.

A series of weak troughs will move through the waters Monday
night through Wednesday. Schneider

&&

.HYDROLOGY...
The flow has been fluctuating just below flood stage on the White
River at R Street. However, additional heavy rain overnight through
Saturday morning will cause flows to increase, reaching the flood
flow of 5500 cfs Saturday afternoon. Therefore, the Flood Warning
will remain in effect.

The Skokomish River was rising due to the heavy rain that has been
falling, with the river forecast to rise to flood stage stage (17.0
feet) around 7:30 AM Saturday. There is a chance that the river will
reach flood stage a couple of hours earlier. Currently, a Flood
Watch remains in effect for the Skokomish River in Mason County.

Expect modest rises on the other rivers draining off the Olympic and
Cascade range with the additional rainfall. Aside from possibly the
White River at R Street, river flooding is not expected after
Sunday.

Meanwhile, the threat of landslides will remain elevated due to more
rainfall on saturated or nearly saturated soils.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Warnings or Watches are in effect for parts of the CWA.

PZ...Gale Warning in effect from 6 AM to 6 PM Saturday for the
	 central and eastern Strait of Juan de Fuca.

 	 Small Craft Advisories are in effect for the remaining
	 waters.

&&

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html