Heavy Snow in Washington, February 22 to 24, 2020

Area Forecast Discussion
National Weather Service Seattle WA
735 PM PST Fri Feb 21 2020

.SYNOPSIS...A weak frontal system will bring increasing clouds and
a chance of showers tonight and Saturday. A stronger and wetter
system will bring more rain, gusty winds and mountain snow on
Sunday. Rain and snow will gradually taper Monday, allowing for a
generally benign week of weather next week.

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.SHORT TERM /TONIGHT THROUGH MONDAY/...Starting to see the first
slightest indications of the incoming system both on current
satellite and on radar. Satellite shows clouds associated with the
system just creeping over the far NW tip of the Olympic Peninsula
while current radar shows any prospect of associated precip remains
over the coastal waters...although to be fair...echoes are pretty
light. To emphasize de-emphasizing said echoes...obs over BC show
that only the thicker cloud cover/cooler cloud tops seem to be
supporting any actual observed precip and those remain well north of
the CWA. This would support thoughts of previous shift regarding
potential rainfall chances and amounts associated with this system.

With forecast holding together well...no evening updates
anticipated.  18

From Previous Discussion...We're seeing one last fine day across
western WA with temps in the 50s and skies mostly sunny. But our dry
streak will come to an end as a weak Pacific frontal system moves
inland tonight and Saturday. Most of the showers will be along the
coast and in the mountains as this system passes through. Rainfall
amounts in the interior will be generally light (under 0.10").
Temperatures will be a few degrees cooler on Saturday with highs in
the upper 40s. We'll see a brief break in the weather Saturday night
as this system exits.

A stronger and wetter frontal system is slated for Sunday. This
system will bring more rain, gusty winds and possibly heavy
mountain snow to the area. Snow levels will remain low, around
2500 ft, with accumulating snow expected at the higher passes
(like Stevens and White Pass). Snoqualmie Pass may see a rain/snow
mix at times. Winds will be gusty (out of the S/SW) as the cold
front moves through the interior Sunday afternoon - gusts to 30 to
40 mph expected. Showers will mostly impact the Cascades Sunday
night under post-frontal onshore flow. Will likely see a
convergence zone develop which may bring more heavy snow to the
central Cascades (including Stevens and Snoqualmie Passes).

Showers will linger into Monday as onshore flow continues but it
looks like most of the activity will remain in the Cascades. We
should see drier conditions toward the afternoon and evening as a
ridge moves inland. 33

.LONG TERM /TUESDAY THROUGH FRIDAY/...From Previous
Discussion...High pressure will remain over the Pac NW on Tuesday
for dry and mild weather across western WA. The ridge will
weaken/flatten Tuesday night or Wednesday as a weak frontal system
moves into southern B.C. and clips western WA.
The ridge may rebuild over the region late in the week for another
brief dry spell. 33

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.AVIATION...Westerly flow aloft tonight and Saturday. A weak
front will move into the area tonight before breaking up over the
area on Saturday. The air mass is generally stable with high clouds
in place over most of W WA with mid to low level clouds moving in
with the weak front. Low clouds expected to be widespread by
Saturday morning before starting to scatter out for some locations
Saturday afternoon and evening.

KSEA...High clouds at times this evening. Ceilings will gradually
lower tonight with low clouds developing Saturday morning as a
weak front moves into the area. Southerly wind 4-8 knots.
Schneider/18

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.MARINE...A weak front will reach the area late tonight then fall
apart on Saturday. There could be a few hours of small craft
advisory strength southerly winds tonight and Saturday morning for
the northernmost inner Coastal Waters zone as well as portions of
the Strait of Juan de Fuca and Northern Inland Waters but it looks
pretty marginal. Current obs show only Race Rocks getting close to
SCA criteria while all other obs sites well below thresholds and as
such current thinking remains valid and will opt to leave headlines
as they are.

A stronger frontal system will cross the area Saturday night and
Sunday. This system should bring gale force southerly winds to the
Coastal Waters. Gales force southerly winds are also possible for
the entrances to the Strait and Northern Inland Waters but this
looks more marginal.

Strong onshore flow will follow the front later Sunday and Sunday
night with gale force west winds for the Coastal Waters, the
Strait, and Admiralty Inlet with small craft advisory strength
winds all other waters. Seas over the Coastal Waters will briefly
build to 20-24 feet Sunday afternoon or early Sunday evening. As
there is still a bit of time before this occurs...will allow future
shifts to make the call on any headlines.

Onshore flow will ease on Monday then become light southeasterly
on Tuesday in advance of a weak front. The aforementioned weak
front will move into the area around Tuesday night then fall
apart on Wednesday as it moves inland. Schneider/18

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.HYDROLOGY...No river flooding is expected over the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 10 PM this evening to 9 AM PST
     Saturday for Northern Inland Waters Including The San Juan
     Islands.

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