Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 150550
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
950 PM PST Tue Feb 14 2017

.SYNOPSIS...A Pacific frontal system will bring windy weather and
moderate to heavy rain tonight through Wednesday night. A broad
upper trough will follow the front for cooler and somewhat showery
weather Thursday into early next week.

&&

.SHORT TERM...Main focus of this evening`s forecast update was to
address freezing rain in the Cascade Passes. The setup is that we
now have strong and moist southwest flow aloft with lots of warm
advection and lift. A large rain shield has now overspread all of
western Washington. Freezing levels have shot way up, with the 00z
UIL sounding measuring a freezing level of 10,000 feet msl, and
the OTX sounding showing multiple freezing levels up to 11,300
feet msl. In fact, will probably see some overnight temperatures
rise as warmer and moister air advects into the region. Meanwhile,
shallow and cold east wind continues through the Cascade Passes
this evening. 9 pm air temp at Snoqualmie Pass is 30F, and it`s
32F at Stevens Pass. So freezing rain is expected at those two
passes, and have issued a Freezing Rain Advisory there. The
advisory is currently set to expire at 8 am Wed. Will only take a
few degrees of warming to turn over to merely a cold rain.
Easterly cross-Cascade gradients will weaken briefly on Wed
morning, which should also help temps rise just above freezing in
those passes. Re-strengthening of east winds on Wed evening could
bring a return of freezing rain then. Further south at White Pass,
cancelled the Winter Storm Watch. It`s currently 33F at pass
level, and the higher elevation summit ob there shows a warming
westerly component to the wind. So with warming overnight at White
Pass, freezing rain is unlikely there.

Otherwise, rain and wind will be the main concern for the next 36
hours, along with their impacts. Two distinct disturbances will
enhance the rain and wind between now and early Thu AM, though it
may be hard to tell the difference in the two just by looking out
the window. The first one will peak during the overnight hours
tonight, with a slight easing (but not elimination) of the wind
and rain on Wed morning. The second upper disturbance will send a
972`ish mb low center N-NE to the northern end of Vancouver Island
on Thu AM, with a negatively tilted trailing cold front. This will
bring the heaviest rainfall rates of this event to western
Washington on Wed PM, followed by the strongest winds later Wed
night. SE-wind prone areas such as the Coast and North Interior
will probably get advisory-level winds, meaning gusts in excess of
45 mph. S-SW 850 mb winds of 55-70 knots will bring strong
orographic lift and heavy rain. This wind direction at 850 mb more
commonly focuses flooding rain over the Olympic Peninsula, where a
Flood Watch is in effect.

Negative tilt cold front will sweep through early Thu AM. The axis
of richest moisture will get shoved south and east, and gradients
will ease. Still looking for a few showers through Thu PM, but by
Thu afternoon, the weather will seem anticlimactic. A brief period
of flat upper ridging is possible Friday, so Friday should be more
dry than wet.  Haner

.LONG TERM...A broad upper trough will be over the area for the
Saturday through Tuesday period for a general chance of showers.
There are no significant weather systems expected but the pattern
might turn a little wetter early next week. Schneider

&&

.AVIATION...Moist southwesterly flow aloft will continue as a strong
front approaches the coast. Current radar imagery shows rain has
reached most of western Washington. Surface winds will remain
southeasterly.  VFR ceilings will fall to MVFR late tonight or
early Wednesday morning as the front nears. Air mass stable.

KSEA...Discussion above applies. Southeast wind 8-12 KT. CHB

&&

.MARINE...A strong front will reach the coast tonight and move
inland Wednesday. Have already seen gales on the coast and these
gale force winds will spread inland to the east entrance and
northern inland waters later tonight. Small craft advisory level
winds are likely in the central strait and Admiralty Inlet.

A low passing offshore will bring more wind on Thursday. Southeast
gales seem likely over most waters. No headlines for this event
yet but may go for a gale watch in the early morning forecast.
Onshore flow will follow on Friday. Current models bring westerly
swell of 15-20 ft to the coast on Thursday. CHB

&&

.HYDROLOGY...A frontal system will slowly push through the area
Wednesday and Wednesday night with heavy rains and high snow levels.
In general, 3.5-7 inches of rain is likely over the Olympics and
slightly lesser amounts for the Cascades for the 48 hour period
Tuesday night through early Thursday. This will bring a renewed
threat of flooding to some area rivers beginning Wednesday.

The runoff in the river models are quite sensitive to the
temperatures for this storm so there is a fair amount of uncertainty
for the crests. The one exception is for the Skokomish River
where there is high confidence in river levels exceeding flood
stage.

The Skokomish river in Mason county will likely reach flood stage
Wednesday. A Flood Warning is in effect. Other rivers flowing off
the Olympics and possibly the Cascades, especially the north half,
could also flood Wednesday or Thursday. A Flood Watch is in
effect to address this threat.

In addition, the rainfall that occurs tonight through Thursday
should boost the USGS landslide guidance above the threshold at
which landslides typically occur. This means another cycle of
landslides is possible in the next few days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Freezing Rain Advisory until 8 AM PST Wednesday for Cascades of
     Snohomish and King Counties.

     Flood Watch from Wednesday morning through late Wednesday night
     for Admiralty Inlet Area-Cascades of Whatcom and Skagit
     Counties-Central Coast-Eastern Strait of Juan de Fuca-Hood
     Canal Area-Lower Chehalis Valley Area-North Coast-Olympics-
     Western Strait of Juan De Fuca-Western Whatcom County.

PZ...Small Craft Advisory for rough bar until 10 PM PST Wednesday for
     Grays Harbor Bar.

     Gale Warning until 10 AM PST Wednesday 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.

     Gale Warning until 4 PM PST Wednesday for East Entrance U.S.
     Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

     Small Craft Advisory until 4 PM PST Wednesday for Central U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 4 PM PST Wednesday for Admiralty
     Inlet.

&&

$$

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You can see an illustrated version of this discussion at
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