Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 281647
Area Forecast Discussion
National Weather Service Seattle WA
900 AM PST Mon Nov 28 2016
.SYNOPSIS...Scattered showers and an active Puget Sound convergence
zone over Snohomish and King counties will continue through early
this evening. A front will bring another round of rain and mountain
snow Tuesday night and Wednesday. After a brief dry period Thursday
into Friday, generally wet cool weather appears likely over the
weekend and early next week with low snow leves.
.SHORT TERM...Radar shows an active Puget Sound convergence
zone over a broad area from southern Skagit county south to
north/central King county. Models indicate onshore gradients
weakening this afternoon and tonight as the northwesterly 850 mb
flow turns more north-northwesterly. This may cause the convergence
zone to drift further south over King county and the Seattle metro
area this afternoon before dissipating this evening. Elsewhere,
showers will persist under cold cyclonic flow. The back side of the
jet moves across this afternoon which may cause an uptick in
orographic snow before tapering off this evening. Model QPF suggests
another 1-4 inches could fall today and will update to increase snow
amounts, but below advisory level. The Cascades and passes from
Snoqualmie pass northward should get amounts closer to 4 inches with
snow levels near 2500 feet.
The upper ridge offshore traverses the area tonight and Tuesday with
mostly dry conditions. Low temperatures will be chilly Tuesday
morning in the 30s for outlying areas and patch fog in prone
valleys. Highs will peak near 50 with some partial afternoon
sunshine or sunbreaks.
Yet another system will spread rain and mountain snow into the area
Tuesday night, transitioning to showers Wednesday. This system is
also progressive in nature and QPF amounts are generally a half inch
or less. Snow levels will remain low around 2500 to 3000 feet with
accumulations up to 5 inches possible in the mountains. A few spots
like Mount Baker in the north Cascades could exceed 6 inches which
may require an advisory but nothing issued at this time. Showers
across the lowlands will decrease by Wednesday evening with
shadowing across central/south Puget Sound. However, the convergence
zone could become active again over Shohomish county and possibly
drifting into King county. Mercer
.LONG TERM...From previous discussion...Even the weather is taking
part in cyber week sales apparently...as W WA will get two ridges
for the price of one. An upper level ridge looks to move over the
area late Wednesday night/early Thursday morning and keep conditions
dry for Thursday and into Friday morning. GFS run from 24 hours ago
indicated that even the first half of Friday may prove dry...but
looks like that limited time offer has expired as current GFS run is
leaning toward ECMWF solution of bringing system for Friday south
enough to bring a quicker return to precip over the area. Generally
zonal upper level flow will keep moisture moving over the area and a
shortwave disturbance looks to set up right along the Cascades and
as such...would appear that a damp weekend is in the cards. SMR
.AVIATION...An upper ridge over the northeast Pacific will shift
over Western Washington tonight with north to northwest flow aloft.
At the surface, onshore flow will ease as high pressure over the
offshore waters moves over Western Washington. The air mass is moist
Areas of low and mid clouds this morning. Most of the low stuff
should be gone by afternoon with some areas seeing just scattered
clouds. Low clouds should fill in tonight with some fog as low level
KSEA...Mid clouds will probably scatter today. Low clouds are
expected to form tonight. South wind 5-10 knots. Schneider
.MARINE...High pressure will move over the Coastal Waters today
and then inland tonight with easing onshore flow. West swell will
build to 16 to 19 feet late today and then subside on Tuesday.
A vigorous front will reach the area later Tuesday and move inland
Tuesday night with gales possible for the Coastal Waters, entrances
to the Strait, and Northern Inland Waters.
High pressure will rebuild over the area Wednesday and Thursday.
Another front will reach the waters on Friday. Schneider
.HYDROLOGY...All rivers have fallen below flood stage. Any
additional rain through Saturday should not be enough to produce
flooding on any area river. A stronger storm may bring a decent
amount of rain Sunday and Monday with lowering snow levels. The
Skokomish river would probably be the only river with a slight
chance of flooding late Sunday or Monday.
PZ...Small Craft Advisory until 6 PM PST Tuesday 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-West Entrance U.S. Waters Strait Of
Juan De Fuca.
Small Craft Advisory for rough bar until 7 AM PST Tuesday for
Grays Harbor Bar.
Small Craft Advisory until 1 AM PST Tuesday for Central U.S.
Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
Strait Of Juan De Fuca.
Small Craft Advisory until 4 PM PST this afternoon for Admiralty
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