Heavy Snow in Washington, November 18 to 19, 2020

Area Forecast Discussion
National Weather Service Seattle WA
915 AM PST Wed Nov 18 2020

.UPDATE...The only change to the forecast this morning was to
expand the slight chance for a thunderstorm this afternoon and
evening to include much of the local area.

Morning water vapor loop continues to show a large scale trough
extending from the central Gulf of Alaska southward and eastward
through the far NE Pacific and Pacific Northwest. A weak
shortwave trough/area of mid level vorticity is noted near the
Oregon Coast within the large scale flow. Latest surface analysis
shows a weak reflection of this wave along with a weak trough.
These features will move across the southern half of WA through
the day today, with mid level height falls accompanying its
passage. This will help aid in larger scale lift, which is
already evident in radar imagery as the next round of stratiform
rain is pushing across the area.

Widespread rain will continue through the rest of this morning and
then give way to scattered showers and isolated thunderstorms this
afternoon and evening. Latest SPC mesoanalysis is already
indicating some instability offshore and along the Coast, and
much steeper mid level lapse rates across the entire area compared
to 24 hours ago (given better larger scale troughing influence).
This is expected to be sufficient enough for a few thunderstorms
to develop. The main threats with any thunderstorm today will be
small hail, lightning, and localized heavy rainfall.

The previous discussion is available below with an updated
aviation, marine, and hydro section.

Kovacik

&&

.PREV DISCUSSION... /issued 304 AM PST Wed Nov 18 2020/

.SYNOPSIS...Active weather continues today as several more
frontal systems progress through the region into the late week
period. Precipitation will not be as widespread getting into
Thursday and Friday, but the general unsettled pattern remains in
place. Another stretch of active and wet weather begins Sunday and
carries into next week.

&&

.SHORT TERM /TODAY THROUGH FRIDAY/...Broad cyclonic flow is
setting up over the North Pacific and Gulf of Alaska over the next
couple of days. Shortwave impulses ejected off of the parent
cyclone will be responsible for the progressive systems moving
into the area today and into Thursday. Precipitation will be less
widespread and approach the area from south to north. Rain has
already returned to the Olympic Peninsula at this hour, with rain
likely to arrive around the Puget Sound later in the morning.
Temperatures remain around seasonal average. Snow levels will drop
in the wake of the cold front today, yielding a few more inches
of possible snow accumulation at Stevens and Snoqualmie Pass.

Left a slight chance of thunderstorms in the forecast, primarily
for the coast and southwest interior. The airmass remains
marginally unstable, with a decent amount of shear just above
ground level for potential thunderstorms to work with.
Temperatures at the 500mb level are cold, and may support small
hail in the strongest cells. Gusty winds and lightning may also be
embedded in the thunderstorms.

More progressive systems will move across the area for Thursday
and Friday with scattered showers and minimal impacts expected.
Stronger cyclonic flow will continue to develop over the Bering
Sea during that time.

.LONG TERM /SATURDAY THROUGH TUESDAY/...Longwave troughing
becomes more established in the long term timeframe over the Gulf
of Alaska. This will result in the development and progression of
a shortwave ridge downstream, which will impact W WA on Saturday
for a mostly clear day between weather systems.

By early Sunday morning, the next frontal system will be arriving
onshore and clear the area by Sunday evening. More moisture is hot
on its heels, ushering in more wet weather as W WA may be
positioned in the general storm track/Polar Jet trajectory into
early next week. Resultant systems may pick up on some degree of
subtropical to tropical Pacific moisture, therefore the potential
for them to register more as atmospheric river events will be
further evaluated in the days to come.

Kristell

.AVIATION...Southwesterly flow aloft will remain in place over W WA
today and into Thursday as several weak weather systems move across
the area. Rain this morning will become more showery this afternoon
into Thursday with isolated thunderstorms across the Olympic
Peninsula and around the Sound this afternoon and evening. Coverage
remains too low for inclusion into the TAFs at this time but will
continue to monitor near-term trends. Conditions will remain a mix
of VFR to IFR through the day, dipping down in heavier showers. May
see a bit of improvement tonight as showers scatter out. Winds will
be considerably lighter than yesterday with a few gusts to 20-25 kts
later today.

KSEA...Low-end VFR this morning may drop down to MVFR as showers
continue this morning into the afternoon. Isolated thunderstorms
remain a possibility in the afternoon/evening hours but confidence
in coverage near the terminals remains low so have left out of the
TAF for now. As showers taper off, will likely see improvement back
to VFR. Southerly winds 7 to 12 kts with gusts to 20 kts this
afternoon/evening. Winds under 10 kts overnight into Thursday
morning.

Carpenter

&&

.MARINE...Brief break in the winds this morning but they will
pick back up during the day today. Small craft advisories remain in
place into Thursday across all waters. Westerly swell will gradually
subside today, dropping below 10 feet this afternoon and remaining
there through the end of the week. The next weather system of
consequence will arrive by the end of the weekend with stronger
winds and larger swell.

Carpenter

&&

.HYDROLOGY...The Skokomish near Potlatch remains in flood stage as
river levels are cresting this morning. Looking further upstream,
the combined forks of the Skokomish has crested and is gradually
decreasing in levels there. This should spell a gradual decrease for
the Skokomish at Potlatch later this morning, especially as showers
become more scattered over that area as the day progresses.

Elsewhere, no river flooding is expected into the weekend. Model
guidance suggests continued active weather is expected over W WA
coupled with a potentially very wet system early next week may allow
for local river rises but this will need to be monitored to see if
forecast solutions remain consistent. JD/18

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 4 AM PST Thursday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville Out 10
     Nm-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 until 4 PM PST this afternoon for Grays
     Harbor Bar.

     Small Craft Advisory until 1 AM PST Thursday for Admiralty Inlet-
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands-Puget Sound and Hood Canal-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

&&

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