Scientific Forecaster Discussion

000 fxus66 kotx 171219 afdotx Area forecast discussion National Weather Service Spokane Washington 419 am PST Mon Dec 17 2018 Synopsis... an active and relatively mild weather pattern will continue this week. A wet and vigorous storm will arrive Monday night into Tuesday night with heavy mountain snow and valley rain. Another wet system will bring more rain and mountain snow Wednesday night into Thursday night. Drier and cooler weather is forecast for Friday into Saturday, followed by snow chances over the weekend. && Discussion... ..heavy mountain snow...Soaking rains...and gusty winds Tuesday... Monday: today will feature a break in our wet weather pattern. Bands of rain and mountain snow are expected this morning over the Idaho Panhandle. Snow levels are expected to be in the 4000-4500ft range. An inch or two of slushy accumulation will be possible this morning at Lookout Pass before precipitation decreases to light showers this afternoon. Temperatures today will be mild by mid December standards with afternoon readings in the 40s (about 10 degrees above average). Our next round of precipitation will arrive this evening as rain and mountain snow spread across the Cascades after sunset. We will be keeping a close eye on temperatures in the Methow Valley today. At this time, it looks like this valley will be in the lower to mid 30s this evening setting up a difficult forecast. At this time, we are calling for a mix of rain and snow for the communities of Methow and Carlton. Twisp, Winthrop, and Mazama have a shot of wet accumulating snow with 2 to 5 inches possible by Tuesday morning. Tuesday: a deep low pressure system will move inland over northern British Columbia on Tuesday. As this powerful storm arrives, a deep plume of moisture will be focused over Washington and north Idaho Tuesday into Tuesday night. Look for moderate to heavy rain in The Lowlands, heavy mountain snow, and gusty winds. *Lowland rain: elevations below 4000 feet in the Idaho Panhandle will likely receive an inch or more of rain on Tue. Amounts of a half inch to an inch will be common from Colville to Spokane to Pullman and Ritzville. Creeks and small streams will experience rises and localized field flooding will be possible across the Panhandle, the Palouse, and in portions of northeast Washington. Paradise creek at Moscow Idaho will need to be monitored closely. Rises on the Palouse river and hangman creek can also be expected. *Heavy mountain snow: two to three feet of new snow is expected above 3000 feet for Cascades west of Leavenworth, Lake Chelan, and Mazama. Stevens Pass will experience the brunt of this storm from early Tuesday morning through Tuesday night. Snow levels will be in the 4000-4500 range for the Idaho Panhandle where 1 to 2 feet of new snow will fall at the ski resorts Tuesday and Tuesday night. Motorists with plans to travel over Lookout Pass (4700 ft) on I-90 should monitor the forecast and check the Idaho transportation 511 site for Road conditions. Snow at the pass will likely be wet and heavy with this mild and wet storm. *Gusty winds: southerly winds will increase during the day Tuesday likely peaking in the afternoon and early evening. At this time, it looks like the frontal occlusion will arrive late in the afternoon with sustained winds of 20 to 25 mph across the foothills of the Blue Mountains, Palouse, and West Plains. Gusts up to 40 mph will be possible. Wednesday through sunday: active weather continues with more mild weather for the work week, then cooling off for the weekend. The strong atmospheric river weakens for Wednesday as an upper level ridge builds over the inland northwest. Gusty winds are possible Wednesday, especially at the ridge tops, as the jet stream pushes north across the region. Meanwhile, a surface low deepens in the eastern Pacific with a warm front pushing inland and sweeping northward up the Cascades. This feature seems to have slowed down its arrival slightly from previous models. Clouds will increase with light precipitation developing over central Washington by late Wednesday afternoon and evening. Aided by another slug of Pacific moisture and a renewed atmospheric river, the warm front pushes toward the Canadian border into north Idaho Wednesday night into Thursday morning with good isentropic lift and steady precipitation. Aside from the sheltered Cascade valleys like the Methow valleys, snow levels start out near 3k ft Wednesday and rise to 4-5k ft by Thursday as the region lies in the warm sector with low level southerly flow. The GFS seems a bit more aggressive, but all models agree on a strong cold front sweeping through the region Thursday afternoon and evening with a band of precipitation. This will be followed by falling snow levels to the valley floors and breezy southwest to westerly winds with gusts of 20 to 30 mph across the Columbia Basin and 30 to 40 mph gusts on the exposed ridge tops. Overall, could see another potential significant mountain snow event for the higher Cascades and selkirks with another foot of snow possible above 5k ft late Wednesday night into Thursday evening. Will see a brief break in precipitation Friday into Saturday with shortwave ridging and high pressure over the region. Patchy fog is possible. It will be colder, but still above normal for mid December with daytime highs in the 30s to lower 40s and lighter winds. Unsettled weather returns for late in the weekend with an upper trough digging off the British Columbia coast and another plume of Pacific moisture heading inland with a round of precipitation. Models diverge on the timing, although they do indicate colder air and lower snow levels at the onset with the precipitation starting as snow, then gradually mixing to rain across the Columbia Basin. This may be enough to give parts of the inland northwest a dusting of snow before Christmas. /Rfox. && Aviation... 12z tafs: showers will produce periods of light rain at Pullman and Lewiston through 16z-17z before pushing through the Idaho Panhandle. A ragged low stratus deck will be possible in the 14-18z time frame at kgeg, ksff, kcoe, and kpuw as winds shift to the south-southwest around sunrise. Areas of fog around keph and kmwh may reduce visibility to a half mile for a few hours this morning. A very wet frontal system will arrive tonight bringing several hours of rain. Strong winds ahead of the front will increase the threat of low level wind shear at many taf sites tonight. /Gkoch && Preliminary point temps/pops... Spokane 43 37 47 36 42 34 / 10 100 100 40 10 40 Coeur D'Alene 42 36 45 36 41 33 / 30 100 100 80 20 40 Pullman 45 38 47 38 45 35 / 30 100 100 80 20 20 Lewiston 49 40 52 42 50 38 / 40 80 100 70 10 10 Colville 42 36 43 32 43 30 / 20 100 100 40 10 60 Sandpoint 40 35 41 36 40 34 / 50 100 100 90 40 50 Kellogg 40 34 41 34 40 34 / 90 80 100 100 70 40 Moses Lake 46 38 51 35 47 35 / 10 100 80 10 10 40 Wenatchee 41 34 46 34 44 35 / 10 100 70 10 10 50 Omak 40 35 44 32 42 33 / 10 100 80 20 10 60 && Otx watches/warnings/advisories... Idaho...Winter Storm Watch from this evening through late Tuesday night above 4000 feet for central Panhandle mountains-northern Panhandle. Washington...Winter Storm Watch from this evening through late Tuesday night above 4000 feet for northeast mountains. Winter Storm Watch from this evening through late Tuesday night above 3500 feet for Okanogan Highlands. Winter Storm Watch from this evening through late Tuesday night above 3000 feet for east slopes northern Cascades.

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