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Office: SEW

FXUS66 KSEW 151206
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
406 AM PST Fri Dec 15 2017

.SYNOPSIS...A weak cold front will spread light rain across
Western Washington this morning as it moves southeast. An upper
level trough trailing the front will bring a few showers this
afternoon and evening. A warm front will spread more rain and
mountain snow across Western Washington Saturday, then a slow
moving cold front will keep Sunday wet at times. After a minor
lull on Monday, a deepening low center should pass across
southwest B.C. on Tuesday, possibly bringing windy conditions and
heavy mountain snow. Drier high pressure will build across the
Pacific Northwest next Wednesday and Thursday.

&&

.SHORT TERM...A weak cold front will gradually move southeast through
Western Washington this morning. This front will be most notable
for producing the first rain since early last week, albeit light
rain in places. Radar already shows light bands of rain spreading
southeast through Northwest Washington early this morning. The
front will exit by early afternoon. Some onshore flow behind the
front will keep a few showers going into this evening, mainly in
the Cascades and in a diffuse Puget Sound Convergence Zone. In
terms of cooling, the mountains will see the most cooling after
such a long stretch of warm air aloft.

Some flat upper ridging will rebound over the coastal Northwest on
Saturday. This will push a warm front onshore. Rain and mountain
snow will spread onto the coast on Saturday morning then spread
inland later in the day.

As the ridge flattens and retreats to the south on Sunday, an
east-west oriented cold front will slowly sag in from the north.
The tail end of a shortwave in strong west flow aloft will enhance
lift along the front. With the slow movement of the front and
strong west flow aloft, heavier precip amounts are likely with
moderate snow accumulation in the mountains. The front will
eventually reach Southwest Washington on Monday evening before
turning up stationary. Haner

.LONG TERM...The weather on Tuesday bears a watchful eye. A strong
shortwave will travel east along the leftover front just to our
south. Cyclogenesis is expected along the front on Monday between
130W and 140W. Under the influence of strong upper divergence,
the low will rapidly deepen while it tracks east on Tuesday to
central or southern Vancouver Island. The 00z ECMWF deepens the
central pressure by about 20 mb in 24 hours, and the 06z GFS is
not far behind. GEFS ensemble means show similar trend, so
confidence is good at this point. The 00z ECMWF shows the low
continuing to rapidly deepen below 995 mb as it crosses the B.C.
Coast Range and passes our longitude. The continued strengthening
is key, because such lows bring stronger wind and more wind
impacts than lows that are weakening. Again, this setup bears
watching for wind impacts. In addition to wind, heavy mountain
snow appears likely thanks to a sharply defined trailing cold
front that will move across the area concurrent with the large-
scale forcing and lift of a deepening parent low.

Very cold air aloft will follow Tuesday PM's front, with 500 mb
temps falling to -34C on Wed morning. So plenty of post-frontal
convective showers and snow levels down to 2000 feet or less will
follow.

Later Wednesday, an upper ridge axis around 135W will strengthen,
amplify, and spread toward the Pac NW. The air mass should quickly
stabilize. By next Thursday, northwest flow aloft with 500 mb
heights nearing 570 decameters along the coast should lead to dry
weather. All 20 members of the GEFS ensemble system show no precip
at SEA next Thursday, so dried out the forecast for all but the
Mount Rainier area. Haner

&&

.AVIATION...A weak low pressure system and cold front will move over
Western Washington Friday morning. The air will become moist with
areas of light rain or showers developing. Southwest flow aloft will
become northwest later today. Areas of light rain or showers
tapering off by evening except for showers in the PSCZ.

KSEA...Clouds will thicken up with lowering cigs as light rain or
showers develop this morning. A southerly breeze could shift to
northerly around 22z with a weak fropa but variable is a good bet too
through evening as convergence sets up with some shower activity.

&&

.MARINE...A 1020mb low and cold front will move over Western
Washington this morning. Small craft advisory northwest winds
will develop over the coastal waters behind the front and then
spread into the Strait of Juan de Fuca by evening. A warm front will
move through the waters late Saturday night into Sunday. Some small
craft advisory conditions are likely across the waters, especially
Sunday, as flow aloft interacts with the area terrain.

A deepening low will move onto Central Vancouver Island on Tuesday
then move east more or less along the Canadian border. Widespread
small craft advisory conditions are expected with this system and
gale force winds are possible over some waters.

&&

.HYDROLOGY...River flooding is not expected in the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 AM this morning to 3 AM PST Saturday
     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 from 4 PM this afternoon to 3 AM PST
     Saturday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca-West
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle



Office: OTX FXUS66 KOTX 151055 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 255 AM PST Fri Dec 15 2017 .SYNOPSIS... Snow will spread over much of the Inland Northwest today. The heaviest accumulations will be over southeast Washington and the southern Idaho Panhandle. It will be drier for Saturday before another weak weather system brings light snow on Sunday. A stronger and wetter system arrives next Tuesday into Wednesday. && .DISCUSSION... Today through Saturday Night: Localized light snow falling from the low clouds this morning, most notably over the West Plains of Spokane. Pavement was already cold so this dusting of snow shouldn't impact roads much. The main frontal band of snow is currently over NW Washington at 2:30am. It will move into the Cascades by sunrise. At 10am, the band should extend from Sandpoint to Ellensburg. This should mean a start time in the Spokane metro area just before noon. By this time, road pavement temperatures will probably around 35F, so initially the snow will melt on the roads. But if it snows hard enough, there will be road accumulations. North of Highway 2 it should be cold enough for the roads to accumulate snow. A mixed bag of precipitation types (snow, sleet, freezing rain) is possible from Wenatchee to Moses Lake and southward, mainly this morning and early afternoon. This could lead to slick roads and side walks but won't be enough ice to cause tree or power line problems. The front will slowly exit over southeast WA and the southern Panhandle tonight. This is the favored area for snow accumulations with this flow pattern. As such, winter weather advisories have been issued for the Palouse, central Panhandle, L-C valley, and the Camas Prairie. Saturday should be dry but cold and cloudy. RJ Sunday through Thursday: Active weather pattern continues. The ridge is rather flattened and easily over-topped by disturbances running over it and pushing moisture through it Sunday through Tuesday morning. With this in mind pops will still paint a good possibility of valley rain and mountain snow with a rain shadow expanding in the lee of the Cascades into the Columbia Basin and points further east at times Sunday through Tuesday morning. Models depict a shortwave digging on the back of a baroclinic band with moderate amounts of precipitable water running through it Tuesday and pass the resulting low pressure system aid its associated frontal features through Eastern Washington and Northern Idaho in the Tuesday/Wednesday time interval. This is followed up with ridge axis positioning well off the coast allowing for a cold northerly flow Thursday which appears void of significant shortwaves in the latest models. As far as how this plays out with temperatures it should allow for a warming trend maxing out on Tuesday as the low pressure system works through followed by a cooling trend brought about by the northerly flow that follows after. Winds could be breezy Tuesday due to the low pressure systems surface low positioning in Southern British Columbia bringing about a tight south to north pressure gradient followed by a shift to breezy southwest winds Wednesday when the back edge of the same low pressure system passes through. /Pelatti && .AVIATION... 12Z TAFS: Stratus and fog in the lower elevations will be overrun by a frontal system today. The front will be aligned from Sandpoint to Ellensburg by 18Z, and exiting the southern Panhandle overnight. KEAT and KMWH could see a mixed-bag of precipitation types. For the remainder of the TAFs it should be just snow with IFR conditions this afternoon and evening. RJ && .PRELIMINARY POINT TEMPS/POPS... Spokane 32 25 30 25 34 31 / 100 10 0 20 40 40 Coeur d'Alene 33 27 30 23 34 31 / 100 20 30 20 60 70 Pullman 32 27 30 23 34 32 / 100 60 40 10 30 70 Lewiston 35 30 36 25 38 35 / 80 80 20 10 20 50 Colville 33 23 33 26 32 29 / 40 10 0 30 40 30 Sandpoint 32 27 32 24 33 30 / 80 40 20 20 60 80 Kellogg 31 25 28 21 31 29 / 100 80 50 10 50 90 Moses Lake 31 27 35 27 36 30 / 40 0 0 10 20 10 Wenatchee 35 30 37 28 35 30 / 20 0 0 20 30 20 Omak 33 26 34 27 32 28 / 10 0 0 30 30 20 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...Air Stagnation Advisory until 10 AM PST this morning for Central Panhandle Mountains-Coeur d'Alene Area-Idaho Palouse-Lewis and Southern Nez Perce Counties-Lewiston Area-Northern Panhandle. Winter Weather Advisory from 1 PM this afternoon to 4 AM PST Saturday for Central Panhandle Mountains-Idaho Palouse- Lewiston Area. Winter Weather Advisory from 1 PM this afternoon to 7 AM PST Saturday for Lewis and Southern Nez Perce Counties. WA...Air Stagnation Advisory until 10 AM PST this morning for East Slopes Northern Cascades-Lower Garfield and Asotin Counties- Moses Lake Area-Northeast Blue Mountains-Northeast Mountains-Okanogan Highlands-Okanogan Valley-Spokane Area- Upper Columbia Basin-Washington Palouse-Waterville Plateau- Wenatchee Area. Winter Weather Advisory from 1 PM this afternoon to 4 AM PST Saturday for Lower Garfield and Asotin Counties-Washington Palouse. && $$